Deep-Sea Research II 47 (2000) 2999}3026
Scavenger assemblages under di!ering trophic
conditions: a case study in the deep Arabian Sea
Felix Jan{en *, Tina Treude, Ursula Witte
Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany
Geomar Research Center, Wischhofstr. 1-3, 24148 Kiel, Germany
Received 2 October 1999; received in revised form 11 December 1999; accepted 20 December 1999
Abstract
Baited cameras and traps were deployed at four stations in the deep Arabian Sea to investigate
the composition of the necrophagous fauna and to evaluate whether regional di!erences in
trophic conditions are re#ected by di!ering scavenger assemblages. The ophidiid "sh Barathrites
iris, the large lysianassoid amphipod Eurythenes gryllus, the aristeid prawn Plesiopenaeus armatus,
and zoarcid "shes of the genus Pachycara were abundant at the bait at all stations. The ophidiid
Holcomycteronus aequatorius, the liparid "sh Paraliparis sp., and galatheid crabs of the genus
Munidopsis occurred in considerable numbers at single sites. Trap catches further contained
lysianassoid amphipods of the genera Paralicella, Abyssorchomene and Paracallisoma. In contrast
to scavenger assemblages of the Atlantic and Paci"c Ocean, macrourid "shes were virtually
absent at the bait. E. gryllus and B. iris consumed the main proportion of the bait, while
consumption was at most moderate in all other taxa. Feeding strategies of the respective taxa are
inferred from their behavior at the bait and discussed with regard to the pro"t that can be drawn
from food falls.
Di!erences between stations were pronounced with respect to species dominating bait consumption. E. gryllus appeared in highest numbers at the bait in the productive northern and
central Arabian Sea where a relatively high availability of food items is expected to sustain high
population densities. High numbers of B. iris in the least productive southern part indicate their
ability to persist under food-poor conditions and may correspond to a high dependency on food
falls. E. gryllus and B. iris both occurred in smaller numbers in the particularly productive western
Arabian Sea. This may re#ect a reduced dependency on food falls, due to an access to alternative
food sources, rather than small population densities. Smaller numbers of E. gryllus and B. iris
resulted in slower bait consumption and gave Pachycara spp. the opportunity to contribute
considerably to bait consumption. The relation between scavenger assemblages and trophic
conditions is discussed with respect to results obtained under di!ering trophic regimes in the
Atlantic and Paci"c Ocean. 2000 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Tel.: #49-421-2028-830; fax: #49-421-2028-690.
E-mail address: [email protected] (F. Jan{en).
0967-0645/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 5 6 - 4
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F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
1. Introduction
Large-food falls (sinking carcasses of larger pelagic organisms) reach the deep-sea
#oor as a local and highly concentrated organic input (Rowe and Staresinic, 1979;
Stockton and DeLaca, 1982). Numerous investigations with baited cameras in diverse
deep-sea areas have revealed the presence of a diversity of large motile fauna that is
promptly attracted to carrion (Dayton and Hessler, 1972; Desbruyères et al., 1985;
Hessler et al., 1972; Isaacs and Schwartzlose, 1975; Jones et al., 1998; Smith, 1985;
Wilson and Smith, 1984). These scavengers have been found to rapidly consume and
disperse carcasses, thus largely excluding a direct bene"t to the local fauna. At abyssal
depths scavenging communities are dominated mostly by large lysianassoid amphipods and macrourid "shes, both highly e$cient necrophages that are thought to
be ubiquitous (Christiansen et al., 1990; Ingram and Hessler, 1983; Priede et al., 1991;
Smith et al., 1979).
The scavenger community of the deep Arabian Sea has only recently become the
subject of investigations with baited cameras (Witte, 1999). The megafauna that was
attracted to a shark carcass at abyssal depth displayed several unique characteristics.
Large lysianassoid amphipods and macrourid "shes were rare or even absent. Instead,
zoarcid "shes (genus Pachycara) and aristeid prawns (genus Plesiopenaeus), both
thought to be only facultative necrophages (Thurston et al., 1995), predominated at
the bait numerically as well as in terms of bait consumption. The resulting bait
consumption rate was particularly low, which led to the suggestion that carcasses
might remain partly undispersed by the motile megafauna thus being available for the
local sediment community (Witte, 1999). As these "ndings stem from one single
station in the western Arabian Sea, it has remained uncertain whether they are valid
for the whole ocean basin or are a local phenomenon.
In several studies with baited cameras and traps comparing the bait-attending
fauna of di!erent areas within the North Atlantic and Paci"c, the scavenger communities have been found to di!er in terms of species composition and numbers of
individuals attracted (Armstrong et al., 1992; Christiansen, 1996; Priede and Merrett,
1998; Priede et al., 1990; Smith and Baldwin, 1984; Thurston et al., 1995). It has been
hypothesized that these di!erences are connected to di!erences in productivity of the
overlying waters and the resulting export #ux to the deep-sea. Proposed linkages of
scavenger assemblages to the trophic regime include availability of food falls and
alternative food sources a!ecting population densities of the scavengers and the
proportion drawn to the bait as well as competition and antagonistic behavior among
the di!erent species (Christiansen, 1996; Smith and Baldwin, 1984; Thurston et al.,
1995).
The Arabian Sea is an ideal area to investigate the impact of the trophic regime on
scavenger community structure as the monsoonal climate leads to pronounced
di!erences in productivity and vertical particle #ux within the same basin and climatic
zone (Brock et al., 1991; Haake et al., 1993; Honjo et al., 1999; Rixen et al., 1996).
Annual primary production (calculated from long term satellite data of Antoine et al.
(1996) for &20 km areas surrounding the stations of this study) covers a range of
154}238 g C m\ yr\. Values generally decrease from the western, across the
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3001
northern and central Arabian Sea, to lowest values in the southern part. This regional
pattern is found even more pronounced in the deep-sea in a number of parameters
including vertical particulate organic carbon (POC) #uxes, sediment chlorophyll
concentrations and benthic carbon remineralization as well as biomass of deep
zooplankton and macrofauna (Koppelmann and Weikert, 1997; Pfannkuche et al.,
2000; Witte, 2000; Witte and Pfannkuche, 2000). Besides di!erences in total annual
supply, POC #uxes to the deep Arabian Sea display a pronounced seasonality with
variations that are among the largest recorded in the open ocean (Ittekkot et al., 1996).
However, seasonality also varies within the area, with the deep western Arabian Sea
experiencing the strongest seasonal food pulses while particle #uxes in the southern
part display no signi"cant temporal variation (Haake et al., 1993; Honjo et al., 1999;
Rixen et al., 1996).
The aim of this study is a basin-wide investigation of the scavenger community of
the deep Arabian Sea by means of baited cameras and traps. Main focuses are an
inventory of the bait-attending taxa, including observations of their feeding behavior
and an investigation of spatial di!erences in scavenger community structure. The
results are discussed with regard to feeding strategies of the di!erent taxa and to the
signi"cance of the trophic regime in controlling the composition of the scavenger
community.
2. Materials and methods
The experiments were carried out during cruise no. 129 of R.V. Sonne
(31 January}8 March 1998) at four stations (Fig. 1). Three stations formed a meridional transect of approximately 1100 km length from the northernmost station
(NAST), past the central station (CAST) to the southernmost station (SAST). The
fourth station (WAST) was situated approx. 450 km west and somewhat north of
CAST. A detailed description is given in Pfannkuche and Lochte (2000).
Data on the scavenging megafauna, meaning animals that are large enough to be
visible in photographs (Kaufmann and Smith, 1997) were obtained with the `Observation Systema by means of baited time-lapse cameras. Smaller animals or scavenging
macrofauna were sampled with baited funnel traps mounted on the `Trap Systema.
Deployment data are given in Table 1. Additionally, a benthic trawl was towed at all
stations.
2.1. Sampling methods
2.1.1. Observation system
A stereo time-lapse camera system consisting of two cameras and a #ash (type 372
and 382, Benthos, North Falmouth, USA) was mounted on an autonomous lander
frame equipped with glass spheres for buoyancy and ballast weights (Priede et al.,
1998). The cameras were oriented vertically towards the sea #oor and took stereo
photographs (Kodak Ektachrome 200, 35 mm;30 m, approx. 800 exposures) covering an area of 1.25 m. The deployments lasted between 1.4 and 4.4 d. A total of 2018
3002
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Fig. 1. Sampling sites in the Arabian Sea.
photographs were taken at time intervals that were preset to cover the respective
deployment durations with the 800 exposures available. A camera failure led to a 78-h
gap in the observation at CAST. At WAST a fault in processing one of the "lms
restricted 3D examination and length measurements to the "rst and last 11 h of the
deployment.
Freshly thawed tuna (Thunnus sp.), weighing 2100}5300 g, without head and caudal
"n was used as bait. The skin of the upper side was detached to facilitate odor
dispersal and access of necrophages. The bait was tied to the center of a net (mesh size
4 mm) that was stretched on a steel frame lying on the sea #oor during deployments.
A second net, released above the bait at the end of the deployments, caught the
scavengers present. The bait was weighed prior to deployment and after retrieval. To
estimate the soft tissue weight, i.e. the amount of bait available to scavengers, bait
weights were reduced by 3% of the initial weight (approximate weight of the skeleton).
2.1.2. Trap system
Four funnel traps and two large "sh traps were attached to an autonomous lander
equipped with glass spheres and ballast weights. The 0.7 m-long funnel traps were
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3003
Table 1
Deployment data of the Observation System and the Trap System.
NAST
CAST
SAST
WAST
Depth
20300N
65335E
3190 m
14325N
64334E
3950 m
10302N
65300E
4420 m
16313N
60316E
4050 m
Observation System
Bottom time
Frame interval
Number of frames
57.7 h
5.2 min
672
104.3 h
9.6 min
170
84.1 h
9.6 min
527
32.9 h
3.0 min
649
Trap System
No. of trap set
Bottom time
1.
59 h
Position
1.
43 h
2.
50 h
1.
32 h
2.
35 h
Covered time period only 26.6 h due to camera failure from 17.4 to 95.1 h after touchdown.
made of plastic tube (diameter 0.1 m) and partitioned into two chambers of 0.35 m
length by a 335 lm mesh netting. While bait was put in only one of the chambers, bait
odor could penetrate into the neighboring chamber. Openings in the wall of the
plastic tube covered by the same netting aided in the dispersal of bait odor outside the
trap. The funnel entrances to the chambers were situated at both ends of the tube and
narrowed to an inner opening 30 mm in diameter. Tubes of thin latex were "tted onto
the inner end of the funnels to keep scavengers from leaving once they entered the
chamber. The four funnel traps were mounted horizontally at 0.3, 0.5, 0.7 and 0.9 m
above bottom (mab). 80 g of tuna or red snapper (Lutjanus sp.) served as bait in each
trap. Both "sh traps, each 0.8 m;0.5 m;0.8 m in size, were made of 6 mm;6 mm
wire netting and mounted side by side at 0.2 mab. Each trap had two funnel
entrances (lateral and from below) narrowing to an inner diameter of 140 and 80 mm,
respectively. 1300 g of tuna were used as bait in each trap.
2.2. Analytical procedures
2.2.1. Stereo photographs
The photographs were examined with a stereo viewer. All individuals with heads
visible in the photographs were counted. It was noted whether individuals were
swimming, in contact with the bait, or with the bottom. Amphipods were considered
only if their length exceeded about 40 mm, as smaller individuals were hardly discernible. A tell-tale ribbon "xed ca. 0.2 mab and a compass, in view of the camera (Fig. 4),
were used to determine current direction in each photograph.
The calibration of the stereo camera system as required for photogrammetric length
measurements of the fauna attracted was done by means of bundle adjustment
(Wester}Ebbinghaus, 1985) using digitized photographs (Kodak PhotoCD) of a
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F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
control-point frame taken in situ. Lengths of not more than three "sh or decapods and
"ve amphipods per photograph were determined in randomly chosen, digitized
photographs. 3-D coordinates of characteristic morphological structures (e.g. snout,
"ns and exoskeleton margins) were determined with the photogrammetric software
`PHAUSTa (Invers, Essen, Germany). Lengths of straight lines (crustacean carapaces)
or b-spline curves ("shes and prawn's curved pleons) connecting the coordinates were
measured using the CAD software `Microstation 95a (Bentley Systems, Exton, USA).
Coordinates of "shes were projected onto a plane to eliminate dorsal curvature.
Partial lengths, measured in incompletely visible individuals were converted into total
lengths using isometric regressions derived from catches and photogrammetric
measurements. The length of a straight line connecting the dorsomedian boundaries
of "ve consecutive segments was measured in the amphipod Eurythenes gryllus
(Lichtenstein 1822), which was rarely seen completely owing to the curved feeding
posture (Fig. 4a). This length proved to be only slightly a!ected by the degree of
curvature in repeated measurements of a trapped specimen (length 114 mm). Ratios of
the respective "ve segment lengths to total length, derived from the same individual,
were used to convert photogrammetric measurements.
The progression of bait depletion was estimated by "ve persons in photographs
of every sixth hour. The amount of remaining soft tissue was compared with the
"rst picture (100%) and the last picture (proportion of soft tissue weight left at
retrieval). Soft tissue removal until the end of the "rst day (i.e. before a substantial bait
shortage set in) was used to compare the bait consumption rates at the respective
deployments.
2.2.2. Samples
On board the animals captured were "xed and preserved in 4% formaldehyde in sea
water (borax bu!ered), except for some amphipods that were frozen after being
photographed for identi"cation. Three percent gluteraldehyde in sea water was used
to "x and preserve intestines removed from "shes.
In the laboratory, lengths of all specimens were measured. Wet weights were
determined in megafauna organisms and in small amphipods of the genus Paralicella.
Stomach contents of "shes were transferred into sorting #uid (Steedman, 1976), sorted
(bait, amphipods, minute crustaceans, amorphous fraction) and weighed.
The scavengers caught with the funnel traps were counted separately for each trap
chamber.
3. Results
3.1. Composition of the scavenger community and taxonomic notes
Scavengers that were captured or found frequently in the photographs belonged
to eleven di!erent taxa (Table 2). Trawl and trap catches facilitated reliable
identi"cation, but assignments to photographed individuals remained di$cult. This
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3005
Table 2
Frequently photographed and captured taxa. The Column &sampling' indicates how samples used for
taxonomy were taken: trawled with benthic trawls, trapped with the Trap System, photographed or captured
with the Observation System
Family/Superfamily Sampling
Megafauna
Fishes
cf. Barathrites iris (Zugmayer 1911)
cf. Holcomycteronus aequatorius
(Smith and Radcli!e 1913)
cf. Pachycara spp.
cf. Paraliparis spp.
Ophidiidae
Ophidiidae
Photographed, trapped
Photographed, trawled
Zoarcidae
Liparidae
Photographed, captured
Photographed
Crustaceans
cf. Plesiopenaeus armatus (Bate 1881)
cf. Eurythenes gryllus (Lichtenstein 1822)
cf. Munidopsis sp. A
cf. Munidopsis subsquamosa (Henderson 1885)
Aristeidae
Lysianassoidea
Galatheidae
Galatheidae
Photographed,
Photographed,
Photographed,
Photographed,
Lysianassoidea
Lysianassoidea
Lysianassoidea
Lysianassoidea
Trapped
Trapped, captured
Trapped
Trapped
trawled
trapped
captured
trapped, captured
Macrofauna
Paralicella spp.
Eurythenes gryllus
Abyssorchomene abyssorum (Stebbing 1888)
Paracallisoma spp.
Samples were trawled in 1995 in the same area by B. Christiansen.
should be kept in mind as the pre"x cf. for a probable but uncertain identi"cation is
used only in tables and "gures.
The zoarcid "shes of the genus Pachycara included at least three species, two of
which are new while the third one is P. shcherbachevi Anderson 1989. The record of
Paraliparis sp. is the "rst evidence of liparid "shes from the deep Indian Ocean at low
latitudes (N. Chernova, pers. comm.). The galatheid crab Munidopsis sp. A is a new
species closely related to M. sundi Sivertsen and Holthuis, 1956 of the deep southern
North Atlantic. Specimens of Paralicella spp. and Paracallisoma spp. deviated from
given descriptions and probably represent new subspecies or even new species.
Some taxa occurred occasionally in the photographs. A single macrourid "sh,
probably Coryphaenoides (Nematonurus) armatus (Hector 1875), showed up in three of
the photographs at CAST. Other rarely seen "shes included a synodontid and a few
eel-like "shes (most of them probably synaphobranchids). Single specimens of the
dorippid crab genus Ethusina (carapace length approx. 20 mm) were photographed at
CAST and WAST and captured with the Observation System at CAST. Ethusina sp.
covered its back with a spindle-shaped hexactinellid sponge of the genus Holascus.
Similar masking behavior has been observed in the Peru Basin at 4100 m where
3006
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
dorippids were found to carry asteroids and holothurians (H. Bluhm, pers. comm.).
A few ophiuroids occurred occasionally in the photographs but never approached the
bait. The catches of the Observation System additionally contained four small
gastropods (shell height approx. 10 mm) of the carnivorous families Turridae (genus
Gymnobela), Epitoniidae and Marginellidae.
3.2. Behavior at the bait
Megafauna. The ophidiid "sh Barathrites iris, the lysianassoid amphipod Eurythenes
gryllus, the aristeid prawn Plesiopenaeus armatus, and zoarcid "shes of the genus
Pachycara were found at all stations (Table 3). These taxa dominated the scavenging
megafauna numerically and showed a characteristic temporal succession at the bait.
The "rst individuals of B. iris, P. armatus and E. gryllus arrived no later than 3.5 h, and
maximum numbers of individuals were mostly attained within 24 h (all times are given
relative to the moment of touchdown at the sea #oor). Soon after the end of the "rst
day, numbers generally declined and specimens were only found occasionally after the
bait became scarce around the middle of the second day (Fig. 2). In contrast,
maximum numbers of Pachycara spp. were not attained until approx. 30 h. Taking
residence in the vicinity of the remains of the bait, Pachycara spp. were found in
considerable numbers until the end of the deployments. In the following, some details
concerning the behavior of these taxa as well as the less common ones are given.
Behavioral characteristics are inferred from deployments where the respective taxa
were particularly abundant.
Barathrites iris occurred mainly over short periods of time when several specimens
were present in most photographs (Fig. 2). Permanently changing numbers of individuals indicated short residence times, but it was impossible to rule out repeated
arrival. Nearly, 80% of all individuals photographed were in contact with the bait
(Table 4) and were certainly feeding, as bait depletion was obvious.
The feeding of Eurythenes gryllus, which attained higher numbers of individuals
than any other taxon, also led to an obvious decrease of the bait. The individuals seemed to stay at the bait continuously as numbers of individuals #uctuated
only slightly, and almost all specimens were found in contact to the bait (Fig. 2,
Table 4).
Plesiopenaeus armatus was always the "rst species to arrive (Table 3). Specimens
seemed to move continuously: they were frequently found swimming and never
occupied the same place in consecutive photographs. Consequently numbers of
individuals were incessantly #uctuating (Fig. 2). Individuals with distinctive markings
(see below) were recognized several times. Bait consumption was low, as no loss of bait
was visible in periods when P. armatus was the only scavenger present and only some
40% of all individuals were in contact with the bait (Table 4). Injured antennal scales
and uropods were observed frequently, and one individual, dead from unknown
causes, was recorded from 13 h on at CAST.
Specimens of Pachycara spp. were always found in close contact with the bottom.
Often it took hours before individuals that appeared in view of the cameras got close
to the bait. Once there, individuals hardly moved at all. Continuous increase and
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3007
Fig. 2. Temporal patterns of numbers of individuals of the most abundant megafaunal scavengers.
Examples were chosen from the two deployments, where photographs covered the longest period of
time. The choice of either NAST or SAST depended on where the taxon in question appeared in higher
numbers of individuals. In E. gryllus, Paraliparis sp. and M. subsquamosa data of every second photograph
are shown. The pattern of M. subsquamosa is given as an example for the general behavior of both
galatheids.
low #uctuations in numbers of individuals (Fig. 2) indicated long residence times.
Mean residence in the vicinity of the bait was 42.2 h (s.d.$10.0 h) in four recognizable
individuals traced at SAST. Approximately 50% of all individuals photographed were
in contact with the bait (Table 4), while even less seemed to be in a feeding posture
since the snouts were often raised. Stomach contents of individuals caught with the
Observation System at CAST and WAST were investigated. Mean stomach content
wet weight as a proportion of "sh weight was 9.3% (s.d.$2.2%, n"8) at CAST and
9.2 h/84.1 h
3664
13 (48.5 h)
7.8/7.0
3.2 h/45.1 h
442
3 (9.5 h)
0.9/0.7
cf. Paraliparis sp.
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
10.3 h/104.3 h
826
*13 (91.5 h)
(7.7/4.9)
7.4 h/57.7 h
1914
9 (34.6 h)
3.3/2.9
2.1 h/39.8 h
309
11 (33.1 h)
1.3/0.6
4510 g
180 g (4.0%)
98.7%
cf. Pachycara spp.
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
3.4 h/*17.4 h
51
*3 (13.5 h)
(0.6/0.3)
5140 g
110 g (2.1%)
97.7%
SAST
3.9 h/83.8 h
837
8 (48.4 h)
1.7/1.6
1.1 h/17.8 h
15
2 (9.3 h)
0.1/0.02
3200 g
60 g (1.9%)
97.2%
CAST
cf. Holcomycteronus aequatorius
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
cf. Barathrites iris
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
Fishes
Species data
Initial bait weight (soft tissue)
Post deployment bait weight (proport. of initial weight)
Propotion of photographs showing megafauna
Community data
NAST
Table 3
Species occurrence and bait weight (soft tissue) in all Observation System deployments
3.3 h/32.9 h
9144
28 (32.7)
15.6/14.1
13.3 h/21.8 h
53
1 (13.3 h)
0.3/0.1
1.2 h/24.9 h
168
5 (4.1 h)
0.4/0.3
2040 g
230 g (11.3%)
98.6%
WAST
3008
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3.5 h/53.8 h
5416
34 (23.5 h)
9.2/8.1
10.2 h/57.7 h
949
6 (42.7 h)
1.7/1.4
26.4 h/57.7 h
1409
8 (53.8 h)
3.8/2.1
cf. Eurythenes gryllus
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
cf. Munidopsis sp. A
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
cf. Munidopsis subsquamosa
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
Parentheses and * symbols at CAST indicate uncertainties due to camera failure.
0.6 h/38.7 h
568
5 (10.7 h)
1.3/0.9
cf. Plesiopenaeus armatus
First/last occurrence
Total no. of individuals in all photographs
Max. no. of individuals (at time after touchdown)
Mean no. of ind. while present/in all photographs
Crustaceans
0.7 h/*17.4 h
2455
*47 (13.2 h)
(23.8/14.4)
0.7 h/103.7 h
369
*12 (12.7 h)
(2.3/2.2)
62.2 h/73.1 h
36
1 (62.2 h)
0.5/0.1
3.3 h/37.6 h
46
2 (5.6 h)
0.2/0.1
0.9 h/81.9 h
312
6 (12.6 h)
0.6/0.6
1.2 h/18.1 h
84
3 (1.6 h)
0.3/0.1
0.3 h/32.8 h
853
9 (1.9 h)
1.3/1.3
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3009
3010
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Table 4
Proportions of individuals close enough to the bait to feed (pooled for all stations)
cf.
cf.
cf.
cf.
cf.
cf.
cf.
cf.
Barathrites iris
Holcomycteronus aequatorius
Pachycara spp.
Paraliparis sp.
Plesiopenaeus armatus
Eurythenes gryllus
Munidopsis sp. A
Munidopsis subsquamosa
Proportion of total individuals
with bait contact (%)
n
79.7
0.0
52.7
43.0
41.9
98.1
77.5
39.0
543
891
15,548
442
2102
8001
949
1445
7.2% ($3.1%, n"9) at WAST. Meals consisted predominantly of bait and small
lysianassoid amphipods (length about 5}20 mm). At CAST the mean proportion of
the diet comprised of amphipods exceeded that of bait (71.5%$24.6% and
15.4%$27.4%, respectively) while the opposite was true at WAST (7.4%$9.2%
and 82.1%$10.6%).
The ophidiid "sh Holcomycteronus aequatorius was never observed feeding on
the bait (Table 4). All individuals were found hovering approx. 0.5 mab, barely moving
relative to the bottom. Groups of several specimens stayed continuously for periods
of up to one day (Fig. 2), but it was not possible to decide whether these individuals
returned or if they were replaced by others. Individuals orientated directly upstream. At SAST their heading was used to determine the current direction after
the tell-tale ribbon was torn o! at 34 h. H. aequatorius was found in considerable
numbers only at SAST (Table 3), rarely at WAST, but it was absent at both other
stations.
The liparid "sh Paraliparis sp. was found in small numbers (1}3 individuals) over
a period of nearly two days at NAST (Table 3). The records were most likely repeated
counts of only a few individuals, as numbers were fairly stable over long periods of
time (Fig. 2). Bait consumption by these small individuals (approximately 150 mm
total length) probably was of minor importance.
The behavior of both Munidopsis species resembled that of the zoarcid "shes in that
the galatheid crabs moved slowly and individuals stayed in the vicinity of the bait for
at least several hours. Numbers of individuals did not rise until the second day by
which time most of the bait was dispersed (Fig. 2). Specimens were still present in the
end of the deployment. Both species occurred in considerable numbers only at NAST
(Table 3).
Macrofauna. While no direct observations were made, the vertical distribution of
funnel trap catches provides an insight into the behavior of macrofaunal amphipods.
All species showed a clear preference for the lowest trap (0.3 mab) (Table 5). The
relative frequency of individuals captured in the lowest trap averaged over all
deployments was 73.3% in A. abyssorum, 87.3% in Paralicella spp., 86.7% in
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3011
Table 5
Catch numbers of scavenging amphipods from the funnel traps
No. of trap set
NAST
CAST
1
1
Altitude above
bottom (m)
0.3
0.5
0.7
0.9
1
1
Eurythenes gryllus
0.3
0.5
0.7
0.9
26
3
0.3
0.5
0.7
0.9
7
1
0.3
0.5
0.7
0.9
3
Paracallisoma spp.
2
1
2
Number of individuals captured
Paralicella spp.
Abyssorchomene abyssorum
SAST
4
650
64
45
1
132
2
1
7
2
2
2
2
2
1
1
1
E. gryllus, and 100% in Paracallisoma spp. Catch numbers decreased steadily with
distance from the bottom. Apart from one single Paralicella spp., no specimens were
captured in the highest trap (0.9 mab).
3.3. Spatial variability of the scavenger community
Megafauna. The stations varied both in faunal composition and in the frequency of
occurrence of the respective taxa (Table 3, Fig. 3). Strongest similarities existed
between the deployments at NAST and CAST, where E. gryllus occurred in very high
numbers, clearly dominating bait consumption, while B. iris was rare (Fig. 4a). The
opposite was true at SAST where B. iris seemed almost exclusively responsible for bait
consumption and numbers of E. gryllus were very low (Fig. 4b). At WAST dominance
at the bait was less conspicuous. E. gryllus was rare but, compared to SAST, numbers
of B. iris were also small. On the other hand Pachycara spp. attained particularly high
numbers (Fig. 4c), and contributed considerably to bait consumption, as soft tissue
was still available on arrival and individuals were gradually eating into the #esh.
NAST was unique with regard to high abundances of Munidopsis spp. at the remains
of the bait (Fig. 4d), while numbers of Pachycara spp. were comparatively low and
dropped to zero at the end of the "rst day.
3012
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Fig. 3. One-hour averages of numbers of individuals vs. bottom time of the most abundant megafaunal
scavengers. The lowest diagram gives visual estimates of soft tissue removal relative to initial weight. The
legend shows patterns of curves and deployment durations. The thin dotted line indicates the period of the
camera failure at CAST.
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3013
Fig. 4. Examples of megafauna assemblages photographed with the Observation System: (a) cf. Eurythenes
gryllus; (b) cf. Barathrites iris, one individual of cf. Pachycara spp. can be seen at the bottom left of the bait;
(c) cf. Pachycara spp.; (d) cf. Munidopsis spp. and cf. Pachycara spp. The tell-tale ribbon and compass are
best seen in photograph (d).
The soft tissue depletion with time (Fig. 3) was similar at all deployments. As
numbers of scavengers rose, rapid consumption set in and lasted until the major part
(60}80%) of the soft tissue was consumed. Particularly rapid bait consumption over
short periods of time at SAST and WAST was related to peak abundances of B. iris.
At NAST and CAST, where E. gryllus was abundant, the consumption was more
steady and longer lasting. The transition into an enduring period of slow consumption
coincided with declining numbers of B. iris, E. gryllus, and P. armatus. Consumption
estimates for the "rst day ranged between 1730 and 4320 g d\ (Table 6). The lowest
consumption rate was found at WAST, where both B. iris and E. gryllus occurred only
in moderate numbers.
Macrofauna. No small amphipods were trapped with the funnel traps at NAST
(Table 5). Each of the two trap sets at SAST and CAST were combined, as they were
deployed in the same area separated only by a period of 10 and 20 h, respectively. The
total number of amphipods captured at SAST was almost 20 times that of CAST. At
SAST almost all captured individuals belonged to the genus Paralicella (97.9%),
whereas E. gryllus dominated the catch at CAST (72.3%) (Table 5). Few A. abyssorum
and Paracallisoma spp. were caught at CAST and SAST.
3014
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Table 6
Bait consumption during the "rst 24 h on the basis of visual estimates
Initial soft tissue weight
Proportion left after 24 h (s.d.)
Bait consumption
NAST
CAST
SAST
WAST
3200 g
16.0% ($8.5%)
2690 g d\
5140 g
4510 g
2040 g
16.0%
28.6% ($13.0%) 15.0% ($0.0%)
4320 g d\ 3220 g d\
1730 g d\
Proportion of soft tissue left at 24 h was assumed to equal value at NAST as consumption was similar
until camera failure at 17.4 h (see Fig. 3).
3.4. Lengths and meal size
Photogrammetrically measured lengths of the scavenging megafauna as well as
lengths of small amphipods caught with the funnel traps are given in Table 7. Lengths
of trapped E. gryllus were much smaller than those measured in the photographs. This
might have been because of trap avoidance by the larger specimens and due to the fact
that measurements of small individuals were impossible in the photographs. However,
taken together the lengths of captured and photographed individuals reveal that
E. gryllus is a necrophage over a broad size range. The same is true for B. iris. In
E. gryllus a similar range has been reported from baited trap studies in the Atlantic
and Paci"c Ocean (e.g., Ingram and Hessler, 1987; Christiansen, 1996) whereas these
are the "rst length measurements of scavenging B. iris. Narrower size ranges were
found in decapod crustaceans. Lengths of P. armatus (n"34), trawled in autumn 1995
at WAST (data of Christiansen and Martin, 2000), are not signi"cantly di!erent from
lengths measured in this study (n"50) (Mann}Whitney U-test, a"0.05). The length
range of individuals trawled in 1995 (135}256 mm) is also comparable to that of this
study. It is thus likely that P. armatus is a necrophage throughout the size range
occurring in the area.
Specimens of Paralicella spp. trapped with access to bait tended to weigh more than
specimens trapped with bait protected but di!erences were not signi"cant (analysis of
covariance of log transformed data, a"0.05). Relative meal sizes calculated accord
ing to Hargrave et al. (1994) increased with body length from 2% (5 mm) to 24%
(17 mm) (Fig. 5). For the smallest individuals ((5 mm) negative meal sizes were
predicted. The calculated meal sizes are probably underestimates since specimens with
access to bait appeared greatly swollen, and an increase of body volume by up to "ve
times in fed Paralicella has been reported by Shulenberger and Hessler (1974). The
underestimates of meal sizes including the prediction of negative values in specimens
(5 mm may have been a result of an increase of body length with increasing gut
expansion. As a result the compared weights of fed and unfed specimens would have
belonged to animals of di!erent size.
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3015
Table 7
Lengths of megafaunal (photogrammetry) and macrofaunal taxa (direct measurements)
Data origin
n
Length (mm)
Mean
s.d.
Range
$107.5
$103.2
$47.6
$21.1
$12.8
$14.8
$6.7
150}816
154}599
149}362
136}253
61}137
55}109
43}71
$2.6
$8.1
$2.7
$2.3
3.2}17.0
11.4}56.1
2.6}13.2
4.2}9.3
Megafauna (photographs)
cf.
cf.
cf.
cf.
cf.
cf.
cf.
Barathrites iris
Holcomycteronus aequatorius
Pachycara spp.
Plesiopenaeus armatus
Eurythenes gryllus
Munidopsis sp. A
Munidopsis subsquamosa
SAST, WAST
SAST
NAST, SAST, WAST
NAST, SAST, WAST
NAST
NAST
NAST
118
51
150
150
106
53
53
308.8
363.9
247.8
200.4
102.9
83.9
60.0
CAST,
CAST,
CAST,
CAST,
897
45
15
4
6.7
22.6
6.0
7.6
Macrofauna (funnel trap catches)
Paralicella spp.
Eurythenes gryllus
Abyssorchomene abyssorum
Paracallisoma spp.
SAST
SAST
SAST
SAST
Fishes: total length; P. armatus: eye-stalk insertion to tip of telson; large E. gryllus: tip of head to tip of
telson; Munidopsis spp.: carapace length including rostrum; megafaunal amphipods: tip of head to tip of
uropods.
3.5. Current pattern
Current direction data derived from the tell-tale ribbon have been supplemented by
direction and velocity measurements obtained every tenth minute by an Aanderaa
current meter (Savonius rotor type) moored contemporaneously at 530 mab not more
than 5 km apart from the bait deployments at NAST, CAST and SAST (P. SchaK fer,
unpublished data). The current directions showed a semi-diurnal periodicity at all
stations indicating a tidal regime. Since the current directions recorded with the
current meter were virtually the same than those drawn from the tell-tale ribbon, it
was assumed that a comparison of the current velocities 530 mab would, at least
qualitatively, re#ect conditions close to the bottom. Mean current velocities (assuming
a value of 0.05 cm s\ for currents below current meter stall-speed of 1.1 cm s\) did
not di!er signi"cantly (Kruskal}Wallis H-test, a"0.05), with mean (maximum)
velocities of 2.7 (8.4), 2.9 (7.5), and 2.8 (6.3) cm s\ at NAST, CAST and SAST,
respectively. At SAST a comparison between velocities 530 mab and camera obtained
particle speeds 1 mab (B. Springer, unpublished data) was possible. Mean particle
speeds covering half an hour (3.0 cm s\, s.d.$1.8 cm s\, n"16) were not signi"cantly di!erent from mean current velocities measured 530 mab at the same time
(3.1 cm s\, s.d.$1.1 cm s\, n"12) (Mann-Whitney U-test, a"0.05). Thus currents 530 mab and near bottom might well correspond also in terms of absolute
velocities.
3016
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Fig. 5. Absolute (dots) and relative (circles) meal size (MS) of Paralicella spp. vs. body length (BL). The
relationship between BL (tip of head to tip of uropods) and absolute MS is described by MS"7.93;10\
BL (r"0.99). The function was calculated from di!erences in body wet weight (WW) between
specimens with and without access to bait according to Hargrave et al. (1994). The power curves "tted to BL
and WW are WW"3.81;10\ BL (r"0.87, n"454, with access to bait) and WW"4.84;10\
BL (r"0.88, n"352, without access to bait).
4. Discussion
4.1. Consumption rates and community composition
The scavenging community of the deep Arabian Sea was found to quickly discover
and consume bait as has been reported from other deep-sea areas (Dayton and
Hessler, 1972; Hessler et al., 1972; Isaacs and Schwartzlose, 1975; Jones et al., 1998;
Rowe and Staresinic, 1979; Smith, 1985). Community consumption rates of
1.7}4.3 kg d\ are similar to those found in the central North Paci"c, where 1}5 kg
"sh bait have been reported to last 24}48 h (Ingram and Hessler, 1983 and references
therein). In the North Atlantic, on the other hand, bait consumption seems to be
considerably faster (Jones et al., 1998; Rowe and Staresinic, 1979). In this study the
lowest consumption rate of 1.7 kg d\ was found in the western Arabian Sea, at the
same site where a rate of 0.5 kg shark carcass per day has been reported by Witte
(1999). The low rate in the previous study was probably due to the absence of B. iris.
This might be attributed to the use of shark as bait, as a preference for tuna compared
to shark has been reported for ophidiids (Jannasch, 1978). Given the consumption
rates found in this study, it is unlikely that signi"cant proportions of carcasses
reaching the deep Arabian Sea remain undispersed by motile scavengers as previously
suggested by Witte (1999).
With the exception of the ophidiid H. aequatorius, all taxa of the scavenging fauna
frequently observed or captured in this study have been reported to attend bait in
other deep-sea areas (Armstrong et al., 1992; Ingram and Hessler, 1983; Jones et al.,
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3017
1998; Lampitt et al., 1983; Shulenberger and Hessler, 1974; Thurston, 1990; Thurston
et al., 1995). A surprising feature of the scavenger assemblages of the Arabian Sea was
the almost complete absence of macrourids. These "shes appear routinely in substantial numbers at bait deployments in other areas (e.g. Isaacs and Schwartzlose, 1975;
Smith et al., 1979; Wilson and Smith, 1984) and have been trawled and attracted to
bait at bathyal depths in the Arabian Sea as well (Shcherbachev and Iwamoto, 1995;
Witte, 1999). Throughout the northern hemisphere Coryphaenoides (Nematonurus)
armatus and C. (N.) yaquinae Iwamoto and Stein, 1974 dominate the abyssal scavenging "sh fauna numerically and in terms of bait consumption (Priede et al., 1991). The
assignment of the single macrourid at CAST to C. armatus suggests that these e$cient
scavengers are present in the deep Arabian Sea as well. The rareness at the bait in this
study probably re#ects low population densities of macrourids in the Arabian Sea at
abyssal depths as not a single macrourid was found in benthic trawl catches at CAST
and WAST (Christiansen and Martin, 2000).
4.2. Feeding strategies
Concerning bait consumption, the four dominant megafaunal taxa are clearly
divided into two groups. The large lysianassoid amphipod E. gryllus and the ophidiid
"sh B. iris were the main bait consumers, while bait consumption by the zoarcid "sh
genus Pachycara and the aristeid prawn P. armatus was only modest.
Main consumers. The behavior of B. iris and E. gryllus displayed strong similarities.
Both arrived early and in high numbers at an almost untouched bait and ingested
large amounts of bait within short periods of time. This behavior maximizes the
energy gain potentially drawn from food falls as a smaller proportion is lost to other
scavengers. Furthermore, staying times required to feed to satiation are reduced,
which raises the number of food items that can be exploited per unit of time. These
advantages, on the other hand, require extra energy expenditures, as early arrival is
favored by active search for carcasses and, once a scent trail is encountered, by high
swimming speeds (Sainte-Marie and Hargrave, 1987; Smith and Baldwin, 1982).
Such an active foraging strategy has been predicted, on theoretical grounds, to
maximize net energy gain in large motile scavengers (Jumars and Gallagher, 1982).
Further, it has been suggested that these scavengers should be specialist carrion
feeders. While E. gryllus is thought to match this concept with regard to morphology,
physiology and behavior (Dahl, 1979; George, 1979; Ingram and Hessler, 1983;
Sainte-Marie, 1992), the scavenging ability of B. iris is a new "nding. In previous
studies with baited cameras no pronounced bait consumption has been reported
(Jones et al., 1998; Rowe et al., 1986). Armstrong et al. (1992) explicitly stated that
B. iris did not ingest any bait. In all these studies macrourids were attracted together
with B. iris. It has been reported for other ophidioid "shes that they seemed to be
discouraged by the presence of feeding macrourids (Isaacs and Schwartzlose, 1975).
The almost complete absence of necrophagous macrourids in the deep Arabian Sea
thus might enable B. iris to take their place as the predominant scavenging "sh.
Modest consumers. The feeding behavior of Pachycara spp. and P. armatus di!ered
completely. Pachycara spp. remained bottom-bound and stayed for days near the bait,
3018
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
hardly moving over long periods of time, while P. armatus seemed to move and swim
continuously. It is not certain whether the sluggish behavior of Pachycara spp. in the
vicinity of the bait also indicates a slow approach towards the bait, but late arrival and
gradually rising numbers of individuals support this idea. This points to a feeding
strategy where energetic expenditures necessary to reach food falls are lowered
through reduced locomotion. Besides extra expenditures for high swimming speeds,
this might even reduce routine respiration compared to that of active "shes (Somero,
1982). At the same time a comparatively small pro"t from food falls seems inherent to
this strategy as at least smaller carcasses would be largely consumed prior to arrival.
The stomach contents of Pachycara spp. revealed that zoarcids did not rely completely
on the availability of bait, but also fed on small bait-attending amphipods. After the
104 h deployment of the Observation System at CAST there were still 94 small
E. gryllus (16}44 mm body length) collected from the remains of the bait. Thus even
a late arrival at an almost consumed carcass is advantageous for Pachycara spp.
Predation in addition to necrophagy "ts the prediction of Jumars and Gallagher
(1982) that slow moving, bottom bound species should be generalist feeders rather
than specialized scavengers.
Early arrival as well as pronounced locomotion in the aristeid prawn P. armatus
seem to correspond to active foraging as proposed for the main consumers. However,
as bait consumption was small, a specialization on carrion feeding does not seem
likely. Phytodetritus found in the guts of P. armatus from the North Atlantic gives
further evidence that this species is a facultative necrophage (Thurston et al., 1995).
The early arrival may re#ect high population densities rather than active search for
food falls. This view is supported by high estimates of population density derived from
trawl catches (Christiansen and Martin, 2000).
The ophidiid Holcomycteronus aequatorius was never seen to approach the bait.
However, it cannot be excluded that H. aequatorius also preys on small necrophagous
amphipods. The same might be true for Paraliparis sp. as reported for Paraliparis
bathybius (Lampitt et al., 1983).
Macrofaunal Scavengers. Lysianassoid amphipods with body lengths (20 mm are
mostly captured near the bottom (Christiansen, 1996; Ingram and Hessler, 1983).
Ingram and Hessler (1983) introduced the term `demersal guilda for small species that
occur mainly within 1 mab. In this study all genera were almost exclusively captured
in the lowest trap. This reveals an uneven distribution even within the lowermost
meter of the water column and suggests that specimens approached the bait close to
the bottom. Since nothing is known about the behavior prior to bait deployment, it is
not sure whether amphipods adopted a `sit and waita strategy buried within the
sediment, or if they were foraging actively. However, since lysianassoids are known to
approach carcasses against the current (Busdosh et al., 1982; Thurston, 1979), staying
in the lowermost centimeters of the water column where current velocities are reduced
(Caldwell and Chriss, 1979) would clearly reduce energetic expenditures. Another
explanation for the high catches in the lowermost trap may be that specimens con"ne
their search for carrion to the immediate vicinity of the bottom where natural food
falls end up. E. gryllus is usually assigned to the `pelagic guilda, consisting of
lysianassoids that are found primarily at distances of tens to hundreds of meters above
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3019
the sea #oor (Ingram and Hessler, 1983). Since in this study juvenile E. gryllus were
also most abundant in the lowest trap, it has to be considered if they should be
regarded as members of the demersal guild.
Relative meal sizes of Paralicella showed a clear trend to increase with body length.
This also has been reported for E. gryllus (Hargrave et al., 1994). The larger relative
meal sizes resulted in higher estimates of sustenance times for larger specimens, which
in turn enables them to cover greater distances in search of food falls. In Paralicella
large specimens with relatively large gut capacities might thus be specialized in the
utilization of food falls as it has been assumed for the whole genus on morphological
grounds (Dahl, 1979; Sainte-Marie, 1992; Shulenberger and Barnard, 1976). Small specimens, on the other hand, that need to feed more frequently might be facultative
necrophages that survive periods without food falls by feeding on alternative food sources,
e.g. detritus. This assumption is supported by sediment found in the guts of Paralicella
caperesca smaller than 7 mm (Smith and Baldwin, 1982 and references therein).
4.3. Spatial pattern of community composition and its relation to the trophic regime
Discussion of regional patterns implies that the characteristics of the scavenger
assemblages can be inferred from single deployments, i.e. that the results are reproducible. Strong similarities of the results at WAST compared to that of the previous study
of Witte (1999) suggest that this assumption is reasonable. In both years Pachycara
spp. and P. armatus occurred in similar temporal patterns and absolute numbers
(Fig. 6). Although di!erences can be found in abundances of E. gryllus it is obvious
that in both years WAST was characterized by small numbers of individuals. Additionally, similar current regimes during the respective deployments are required for
comparisons, as current speeds greatly a!ect the occurrence of scavengers at bait
(Desbruyères et al., 1985; Sainte-Marie and Hargrave, 1987). This requirement seems
ful"lled, as current speeds did not di!er signi"cantly and a diurnal periodicity was
found at all stations.
Main consumers. The di!erences in the dominance of E. gryllus and B. iris along the
meridional transect paralleled the trophic regime. E. gryllus almost monopolized the
bait at the productive stations NAST and CAST, while B. iris occurred in highest
numbers under the regime of lower production in the southern Arabian Sea. The
highly productive western Arabian Sea occupied a special position as both main
consumers were of minor importance. The tendency of E. gryllus to occur in higher
numbers in more productive waters was similarly observed in trap studies in the
North Paci"c and Atlantic Ocean. Along a transect extending from eutrophic waters
o! California to the central North Paci"c, Smith and Baldwin (1984) found higher
catch rates in productive waters at the western boundary of the California Current
compared to the oligotrophic open Paci"c. A similar pattern has been reported from
the eastern North Atlantic, comparing the temperate West European Basin to the
subtropical Madeira Abyssal Plain where the deep-sea receives less organic input
(Christiansen, 1996). While mandibular structure and strip-feeding of E. gryllus are
thought to point to obligate necrophagy (M. Thurston, pers. comm.), it has been
suggested that pelagic as well as benthic prey, e.g. "sh, squid or benthic detritivores,
3020
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
Fig. 6. Three-hour averages and ranges of numbers of individuals at WAST. Comparison of data from the
present study (dots) with a previous study one year earlier (Witte, 1999) (circles).
might be alternative food sources (Christiansen, 1996; Sainte-Marie, 1992; Smith and
Baldwin, 1984; Templeman, 1967). An enhanced organic input into the deep-sea
probably yields more potential prey. It is thus proposed, that high numbers of
E. gryllus at the bait at NAST and CAST re#ect high population densities that are
sustained by a higher availability of prey organisms as compared to SAST. If E. gryllus
is an obligate necrophage an enhanced supply of food falls at NAST and CAST would
be required to explain higher densities. However this remains speculation, as it is still
unknown if the distribution of food falls is related to productivity (Christiansen, 1996;
Stockton and DeLaca, 1982).
The low numbers of E. gryllus at the bait at the most productive station WAST
contradict a simple relationship between food input, population density, and numbers
of individuals attracted to bait. A similar situation is found in the North Atlantic and
Paci"c where the trend to higher catch rates in productive waters is not extended to
particularly food-rich sites (Christiansen, 1996; Smith and Baldwin, 1984). It has been
suggested that population densities of E. gryllus in highly productive areas might be
reduced through predation and competition for the same food items by particularly
abundant demersal "shes (Christiansen, 1996). This is contradicted by low biomass of
benthopelagic "shes at WAST (Christiansen and Martin, 2000). With regard to high
productivity and vertical #uxes at WAST, it is possible that the numbers of E. gryllus
attracted to the bait are reduced due to a high abundance of simultaneously available
food items. Such a dependence of the reaction to carrion upon food supply agrees with
the "nding that only starved lysianassoids exposed to bait odor show an elevation of
metabolism that is thought to re#ect the initiation of foraging (Smith and Baldwin,
1982). Smith and Baldwin (1984) and Baldwin and Smith (1987) further stated that the
attraction of E. gryllus to bait may change seasonally. To explain lower catch rates in
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3021
spring and summer, i.e. in times of organic enrichment through sedimentation of the
spring bloom, they suggested that E. gryllus shifts away from necrophagy when
alternative food sources are particularly abundant. This may have also been the case
at WAST where temporal variability in vertical #uxes and sediment community
oxygen consumption is most pronounced (Haake et al., 1993; Witte and Pfannkuche,
2000). Since the nature of alternative food sources as well as the time-lag between
organic enrichment and biomass increase of these food sources are both unknown,
further evidence is needed to con"rm this theory.
The dominance of B. iris at the most food-poor station, SAST, is in accordance with
"ndings from the subtropical Madeira Abyssal Plain, where Armstrong et al. (1992)
found higher numbers of B. iris attracted to bait than in more productive temperate
waters. This agrees with the general prevalence of ophidiids in oligotrophic tropical
regions (Carter, 1984). However, an avoidance of neighboring sites of higher food
availability does not seem to be a reasonable explanation for lower numbers of
individuals at NAST, CAST and WAST. From morphological characteristics, it has
been inferred that B. iris probably feeds on benthic prey (Carter, 1984). It is possible
that B. iris appears less numerous at the more food-rich stations as a result of
a reduced dependency on food falls in the presence of alternative food sources. An
alternative explanation for low numbers at NAST and CAST may be that B. iris was
discouraged by the high numbers of feeding E. gryllus, as it has been reported for an
anoplopomatid "sh by Isaacs and Schwartzlose (1975). This view is supported by
reports of aggressive behavior in large scavenging amphipods (Sainte-Marie, 1992 and
references therein).
Modest consumers. Early arrival and rapidly increasing numbers of individuals of
Pachycara spp. and P. armatus at WAST both indicate high population densities
(Priede et al., 1990). High densities of Pachycara spp. might be sustained by a high
availability of prey organisms. In addition, food falls may be an important food source
at WAST where a slower consumption by B. iris and E. gryllus probably facilitates the
access of Pachycara to carcasses. In P. armatus a high pro"t from food falls at WAST is
unlikely, as individuals have been found leaving the bait particularly early. As the
disappearance of P. armatus coincided with rising numbers of Pachycara spp., it may
be that the prawns avoided the zoarcids. While antagonistic behavior cannot be ruled
out, no evidence of predation was observed, and Pachycara spp. were never observed
feeding on the dead prawn at CAST. Early declining numbers of P. armatus may also
be attributed to easier access to alternative food sources at WAST.
Macrofaunal scavengers. The di!erences in the catches of macrofaunal scavenging
amphipods also paralleled the trophic regime. Catch numbers were highest in the
food-poor southern Arabian Sea suggesting that a high proportion of the populations
were attracted to the bait due to a particularly high demand for food falls. As it has
been discussed for large E. gryllus at WAST, lower numbers of individuals captured at
CAST and the absence of lysianassoids in the deployments at NAST may indicate
a reduced a$nity to the bait because of a higher abundance of food items at these
more productive sites. Mandibular structures of all trapped genera except Abyssorchomene point to a specialization for necrophagy (Dahl, 1979; Thurston, 1979).
However, sediment in the guts of small Paralicella caperesca (Smith and Baldwin,
3022
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
1982 and references therein) provides evidence that food sources like detritus that are
closer related to primary production than food falls, can be exploited as well. Since no
traps were deployed in the western Arabian Sea it is not known whether the trend to
reduced catches in productive waters is extended to the most productive site. However, as nearly 40 small E. gryllus were recovered with the Observation System at
WAST, it seems that the attraction of small scavenging amphipods to bait depends
also on factors other than the organic input into the deep-sea. The dominance of
Paralicella spp. in the funnel trap catches in the southern Arabian Sea corresponds to
results of Christiansen (1996) from the eastern North Atlantic. Here, too, proportions
of Paralicella in trap catches were highest at the food poor Madeira Abyssal Plain.
This was related to the ability of Paralicella to withstand long starvation periods.
5. Conclusions
The scavenging community of the deep Arabian Sea was found to quickly discover
and consume bait. Consumption rates were similar to those reported from the North
Atlantic and Paci"c Ocean. A minor importance of e$cient scavengers, as reported by
Witte (1999), seems to be restricted to the western Arabian Sea and is probably tied to
the particularly high productivity in this area.
Nearly, all bait-attending taxa found within the deep Arabian Sea have been
reported from bait deployments in the Atlantic and Paci"c Ocean. However, the
scavenger assemblages of the deep Arabian Sea displayed some pronounced di!erences to those reported from other areas. Bait-attending macrourids, dominating the
scavenging "sh fauna of the whole deep northern Atlantic and Paci"c Ocean, were
virtually lacking. The ophidiid "sh B. iris, on the other hand, displayed pronounced
carcass consumption. This is contrary to previous observations and probably linked
to the absence of macrourids. Early arrival and rapid bait consumption by B. iris and
the large lysianassoid amphipod E. gryllus imply a feeding strategy in which high
energetic expenditures for locomotion are outweighed by an ingestion of large amounts
of carrion. The opposite strategy is suggested for Pachycara spp., since specimens moved
slowly and mostly arrived at a largely consumed bait. However, they exploited an
additional food source by predation on small bait-attending amphipods.
Di!erent scavenger species dominated bait consumption at the di!erent sites. These
regional characteristics are likely to be related to di!erences in productivity of the
overlying waters and the resulting export #uxes to the deep-sea. High population
densities sustained by an enhanced supply of food items under productive waters may
account for high numbers of large E. gryllus at the central and northern sites. Another
possible link between the trophic regime and numbers of attracted individuals is that
the proportion of a population that is drawn to the bait may correspond to their
dependence on food falls. High numbers of B. iris at the least productive southern site
are attributed to a high dependence on food falls, as alternative food sources are
expected to be scarce. The opposite may be true at the particularly productive western
site, where both B. iris and E. gryllus appeared in low numbers. The same connection
between the availability of suitable food items and reduced attraction to the bait may
F. Jan}en et al. / Deep-Sea Research II 47 (2000) 2999}3026
3023
also explain low catch numbers of small lysianassoids in the productive central and
northern Arabian Sea.
Acknowledgements
Thanks are due to captain D. Kaltho! and the crew of R.V. Sonne. We are
particularly indebted to A. Cremer and W. Queisser for their excellent technical support
and M.L. MuK ller for aid in photogrammetric measurements. The calibration of the
camera system was done by H.J. Przybilla. Taxonomic identi"cations were kindly
carried out by N. Chernova, B. Christiansen, T. Iwamoto, P.D.R. M+ller, J.G. Nielsen,
U. Zajonz, and H. Zetzsche ("shes), A. Crosnier and M. TuK rkay (decapod crustaceans),
H.G. Andres (amphipods), H. Palm (parasitic copepods), R. Janssen (gastropods), and
O.S. Tendal (hexactinellid sponges). G. Schulze was very helpful in establishing the
contact to "sh taxonomists. Special thanks are due to B. Christiansen for additional
megafauna data. Current and particle speed data were kindly put at our disposal by P.
SchaK fer and B. Springer, respectively. The manuscript was improved by reviews provided by I.G. Priede, M. Thurston and an anonymous reviewer. Additional corrections
were made by M. Seaman. This work was funded by the Bundesministerium fuK r
Bildung, Wissenschaft, Forschung und Technologie (FoK rderkennzeichen 03G0129A
and 03F0117A) and is a publication of the joint research program BIGSET.
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