Journal of Experimental Marine Biology and Ecology
290 (2003) 197 – 210
www.elsevier.com/locate/jembe
Tissue-specific palatability and chemical defenses
against macropredators and pathogens in the
common articulate brachiopod Liothyrella uva
from the Antarctic Peninsula
Andrew R. Mahon a,1, Charles D. Amsler a,*, James B. McClintock a,
Margaret O. Amsler a, Bill J. Baker b
a
Department of Biology, University of Alabama at Birmingham, Birmingham, AL 35294-1170, USA
b
Department of Chemistry, University of South Florida, Tampa, FL 33620, USA
Received 10 October 2002; received in revised form 6 December 2002; accepted 30 January 2003
Abstract
The punctate terebratulid brachiopod Liothyrella uva is the most common brachiopod species in
Antarctica. Whole brachiopods, either live or freeze dried and ground into a powder and suspended in
alginate, were unpalatable to the sympatric macropredators Odontaster validus (an abundant, omnivorous sea star) and Notothenia coriiceps (an abundant, omnivorous, epibenthic fish). The unpalatability
of these ground tissues coupled with that of lipophilic extracts of whole L. uva presented in alginate
pellets to O. validus, suggests an involvement of chemical defenses. Several isolated brachiopod tissues
were also unpalatable to O. validus after being freeze dried, ground and suspended in alginate, but only
the pedicle was unpalatable in such preparations to both O. validus and N. coriiceps. This observation is
consistent with the Optimal Defense Theory since the pedicle is the only tissue not protected inside the
brachiopod shell. There was, however, no correlation between the energetic content and unpalatability
of any of the individual tissues. Organic extracts of tissues involved in feeding (lophophore and
intestine – stomach) had relatively strong antimicrobial activity when assayed against several strains of
Antarctic bacteria. However, the lophophore was palatable to both macropredators, suggesting
nonoverlapping chemical defenses are involved in protection against predators and pathogens.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Antarctica; Brachiopods; Chemical defenses; Liothyrella uva; Optimal Defense Theory; Predation
* Corresponding author. Tel.: +1-205-975-5622; fax: +1-205-975-6097.
E-mail address: [email protected] (C.D. Amsler).
1
Present address: Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529, USA.
0022-0981/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-0981(03)00075-3
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A.R. Mahon et al. / J. Exp. Mar. Biol. Ecol. 290 (2003) 197–210
1. Introduction
Evidence from the fossil record indicates that articulate brachiopods dominated
macrobenthic assemblages during the Paleozoic but then declined considerably during
the Mesozoic (Thayer, 1986). This decline in dominance may have been the result of
increased diversification of predators (Stanley, 1977, 1979), a hypothesis supported by
direct evidence of increased predation damage in extinct clades of articulates during the
Mesozoic (Alexander, 1986). Today, only three of the original six articulate clades remain
(Terebratulida, Rhynchonellida, Thecideids). One possible explanation for patterns of
extinction among articulate clades is that increased predation pressure may have selected
for those species that were unpalatable as all extant clades have unpalatable representatives
(Thayer and Allmon, 1990). While direct demonstration of chemical defenses are
generally lacking in articulates (Thayer, 1985; Thayer and Allmon, 1990; but see
McClintock et al., 1993), it is likely that the demonstrative unpalatability of brachiopod
soft tissues is related to defensive chemistry.
Articulates are somewhat unique with respect to their defensive attributes in that a lack
of tissue palatability is combined with a shelled armature (Thayer and Allmon, 1990).
Chemical defenses commonly characterize groups of marine invertebrates such as
sponges, soft corals and nudibranchs that lack significant physical defenses such as shells
(Paul, 1992; Pawlik, 1993; McClintock and Baker, 2001) . Nonetheless, there are some
examples of shelled marine invertebrates that rely in part on defensive chemicals for their
protection. Pawlik et al. (1986) describe a novel triterpene used to defend the intertidal
limpet Colisella limatula, and Kvitek et al. (1991) found that diet-derived neurotoxins are
used to defend the burrowing soft sediment bivalve Saxidomus giganteus. Clearly, theories
examining patterns of evolution in articulate brachiopods based on their predator– prey
dynamics will allow a more complete picture of the palatability, and more specifically,
chemical defenses that may distinguish extant species.
The Antarctic benthos is characterized by abundant and diverse marine communities
(Dayton et al., 1974, 1994; Starmans, 1977; Dayton, 1990; Clarke, 1992, 1996; Brey et al.,
1994; Gutt and Starmans, 1998; Gray, 2001). Articulate brachiopods are no exception to
this pattern, with densities in both sub-Antarctic and Antarctic environments that range to
over 3000 individuals per meter square (Foster, 1974; Peck et al., 2001). Below the
shallow depths impacted by anchor ice and ice scour (generally down to 25 –30-m depth;
Dayton et al., 1970), the rich Antarctic benthic communities are dominated by sessile
suspension feeders including sponges, soft corals and articulate brachiopods. These deeper
communities are considered ‘‘biologically accommodated’’ as predation and competition
structure the benthos (Dayton et al., 1974; see also Dayton, 1990; Gutt and Starmans,
1998). The combination of a benthic fauna isolated for at least 20 million years (Dayton et
al., 1994), and the constraints of predation and competition has selected for chemical
defenses in Antarctic marine plants and a wide assortment of sessile and sluggish marine
invertebrates (reviewed by McClintock and Baker, 1997a, 1998; Amsler et al., 2000,
2001a,b). While Antarctic predators such as fish and sea stars are capable of consuming
articulate brachiopods, evidence suggests that they are rarely included in diets (Brand,
1974; Temnikov et al., 1976; Dearborn, 1977; Sloan, 1980; Eastman, 1994; McClintock,
1994).
A.R. Mahon et al. / J. Exp. Mar. Biol. Ecol. 290 (2003) 197–210
199
The most well-studied Antarctic brachiopod is the punctate terebratulid Liothyrella uva
which occurs in abundance down to 300-m depth (Bellisio et al., 1972; Dell, 1972; De
Castillanos, 1973; Foster, 1974; Arnaud, 1977; Picken, 1985a,b; Peck et al., 1986, 1997;
Voss, 1988; Peck and Robinson, 1994). Near Palmer Station on the Antarctic Peninsula,
populations of L. uva may comprise up to 6% of the biomass at 20 – 30-m depth
(Zamorano, 1983). McClintock et al. (1993) investigated aspects of the morphometry,
biochemical and energetic composition of whole soft body tissues and shells, and
chemically based behavioral tube foot retraction responses of sea star predators exposed
to whole tissue filtrates of L. uva. They also performed laboratory feeding assays using
agar pellets containing a krill feeding stimulant and finely ground lyophilized whole-body
tissues of L. uva employing two allopatric fish predators. Six sea star species and one of
the two fish species were deterred by brachiopod tissue filtrates or pellets containing finely
ground whole brachiopod tissues, respectively.
The present study significantly extends the evaluation of chemical defenses in the
common articulate L. uva by examining chemical deterrence in somatic [lophophore,
intestine– stomach (including digestive diverticula), pedicle] and reproductive (male and
female gonads) body components using feeding deterrence bioassays employing ecologically relevant sympatric sea star and fish predators. These data are evaluated within the
context of the Optimal Defense Theory (ODT) (Rhoads, 1979; reviewed by Cronin, 2001),
which assumes that resources are limited and predicts that chemical defenses should be
allocated to those tissues most vulnerable to predation or most likely to maximize fitness.
Moreover, in order to extend the evaluation of chemical deterrents to functions other than
that of defending against macropredators, the present study examines the antimicrobial
activity of somatic and reproductive tissues of L. uva using sympatric marine bacteria
cultured from the surfaces of benthic Antarctic marine invertebrates. This provides an
evaluation of the presence of bioactive compounds that may serve as antifoulants or inhibit
bacterial infection.
2. Materials and methods
The brachiopod L. uva and the sea star Odontaster validus were collected subtidally
from several sites within 3.5 km of Palmer Station on Anvers Island, Antarctica (64j46VS,
64j04VW; cf. Amsler et al., 1995). Antarctic rockfish, Notothenia coriiceps, were
collected by hook and line from the shoreline at Palmer Station. Animals used in most
assays were collected in March and April 2000 but brachiopods and fish used in one assay
(ground whole brachiopod, see below) were collected in November 2001 (brachiopods)
and December 2001 through January 2002 (fish). Sea stars and fish as well as those
brachiopods that were utilized live were maintained at ambient temperatures ( 1 to 1 jC)
in flow through filtered seawater tables until use.
A portion of the collected brachiopods were sorted as male, female or juvenile, and
used live in bioassays. Males and females examined were reproductively active at the time
of collection as evidenced by the presence of spermatozoa and oocytes in the gonads, as
well as brooded embryos, gastrulae and three-lobed larvae in the lophophores of some
females greater than 30 mm in length. ‘‘Juveniles’’ were defined as V 15 mm in length
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based on the report of Meidlinger et al. (1998). Additional adult animals were separated by
sex and frozen whole at 70 jC before being freeze dried. Other adult brachiopods were
dissected and the lophophore, intestine– stomach (including digestive diverticula), pedicle
and gonads (ovaries or testes) were separated, weighed and then freeze dried. The tissues
were used for feeding bioassays, for extraction and use in microbial bioassays, or for tissue
energetic studies. These tissues were chosen because they comprise the major internal
components on a biomass basis. The small adductor and diductor muscles were included in
whole-body homogenates and extractions but not used as isolated tissues.
2.1. Feeding bioassays
Whole, live L. uva were used in some feeding bioassays. In others, either whole, freezedried L. uva excluding the shells or separated, freeze-dried tissues, including the
lophophore, pedicle, intestine –stomach, male reproductive tissues and female reproductive tissues were individually macerated and suspended in a 2% alginate solution. Each
homogenate suspension was formed into artificial food pellets or disks for feeding
bioassays. This was done by dripping the homogenate suspension into a 1 M CaCl2
solution where it solidified into pellets or by slowly pouring 1 M CaCl2 over the
homogenate in a petri dish where it was subsequently cut into disks. Both pellets and
disks were prepared at a final homogenate concentration of 5% tissue in the 2% alginate.
Ground, freeze-dried Antarctic krill (Euphausia superba) at a concentration of 5% tissue
in 2% alginate served as feeding controls (McClintock and Baker, 1997b). This level of
tissue homogenate in the artificial foods was chosen as it has proven effective in previous
bioassays with Antarctic predators (e.g., McClintock et al., 1993; McClintock and Baker,
1997b) and falls well within the range used in artificial foods in studies of feeding
preferences in a variety of other systems (e.g., Duffy and Hay, 1991; Hay et al., 1994;
Pawlik et al., 1995; Bolser and Hay, 1998; Nagle et al., 1998; Kelman et al., 1999).
Hydrophobic extracts for feeding bioassays were prepared by extracting whole, freezedried adults in three changes (24 h each change) of 1:1 CH2Cl2/methanol. The extracts
from the individual changes of each solvent mixture were combined, filtered through glass
wool and the solvents removed by evaporation under reduced pressure. Extract yields were
calculated on a wet weight basis and added to 5% krill at their natural wet weight
equivalent concentration.
A bioassay utilizing the common, omnivorous sea star O. validus was based on methods
previously described by McClintock and Baker (1997b). After collection, O. validus were
placed in a 2-m diameter circular holding tank (3200 l) equipped with running ambient
seawater. They were allowed to acclimate for at least 96 h before any feeding assays were
started. Sea stars commonly climb the sides of the aquarium up to air – water interface and
then extend one or two arms out directly under the water surface. In the bioassays,
individuals at the surface were offered a live whole brachiopod or an experimental pellet.
Experimental pellets consisted of either ground whole brachiopod (without shell) in
alginate, a hydrophobic extract of whole brachiopod added to ground krill in alginate, or
an individual tissue ground in alginate. The pellet was placed within the ambulacral groove
of a single arm, equidistant between the arm tip and oral opening. Acceptance was recorded
when the animal moved the brachiopod or pellet to the oral opening. Rejection was
A.R. Mahon et al. / J. Exp. Mar. Biol. Ecol. 290 (2003) 197–210
201
recorded when the animal dropped the brachiopod or pellet, or moved it away from the
mouth out of the ambulacral groove or towards the arm tip. Twenty minutes after the animal
either accepted or rejected the experimental pellet or tissue, it was given a control pellet
consisting of 5% krill in 2% alginate (McClintock and Baker, 1997b). Sample size was 10 –
13 individual sea stars for each treatment except whole brachiopods ground in alginate
where the total sample was 37 individual sea stars. No animal was used more than once.
Differences between tissue pellets and corresponding controls were determined using a
two-tailed Fisher’s Exact Test of Independence (Sokal and Rohlf, 1995).
The Antarctic rockfish N. coriiceps was also used for feeding bioassays. Fish were held
in 1 2 m seawater tables equipped with flowing seawater approximately 0.25 m in depth.
Each table was divided into three compartments using fine mesh dividers with a single fish
in each compartment. Two separate experimental series were run. In one series, all fish
were fed one of each of the following tissue disk types in a random order per trial:
lophophore, pedicle, intestine– stomach, male reproductive tissues, female reproductive
tissues and a control disk composed of freeze-dried krill. Tissue preparations were
presented in random order and no fish was given an experimental disk type more than
once throughout the experimental series. Assays were performed every 12 h until the
experiments were concluded. Five minutes after being presented with the experimental
disk, the fish was given a disk consisting of 5% krill in 2% alginate. If the fish did not eat
the postexperimental control disk in a given trial, those results were discarded. Based on
the results of these bioassays, we decided to also test disks prepared from whole L. uva
excluding the shells (collections in November 2001). These were assayed as above but
with a different group of 12 N. coriiceps. Acceptance of a given disk was recorded when
the fish ate a pellet and did not regurgitate it. Rejection was recorded when the fish took
the disk into its mouth and subsequently spit it out. Significance of difference between
tissue disks and corresponding controls was determined using a two-tailed Fisher’s Exact
Test of Independence (Sokal and Rohlf, 1995).
2.2. Microbial bioassays
Antimicrobial assays utilized four strains of psychotrophic marine bacteria that have
been described previously (DeMarino et al., 1997). Strains McM13.3, McM18.1 and
McM32.2 were isolated from the surfaces of Antarctic marine macroinvertebrates and
strain McM11.5 was isolated from the near-shore Antarctic water column. All were
capable of growth at both
1.0 and 20.0 jC. Voucher specimens of the bacteria are
available in the Department of Biology, University of Alabama at Birmingham and in the
Department of Chemistry, University of South Florida.
Isolated brachiopod tissues were weighed wet, freeze dried and then extracted in three
changes (24 h each change) of 1:1 CH2Cl2/methanol, resulting in hydrophobic extracts. This
was followed by three changes (24 h each change) of 1:1 methanol/water, resulting in
hydrophilic extracts. The extracts from the individual changes of each solvent mixture were
combined, filtered through glass wool and the solvents removed by evaporation under
reduced pressure. Extract yields were calculated on a wet weight basis. The extracts were
resuspended in either methanol (hydrophobic extracts) or 1:1 methanol/water (hydrophilic
extracts) at 1 ml per g of wet tissue originally extracted. Paper antimicrobial assay disks
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(BBL Microbiology Systems 31039) were prepared by placing 20 Al of these extract
solutions or of solvent only onto each disk. This volume essentially saturates a disk.
Consequently, this method results in an extract concentration that approximates tissue
concentrations on wet weight and volumetric bases. For bioassays, the bacteria were spread
onto Difco marine Agar 2216 (Difco Laboratories). Extract-containing disks were placed
onto the culture plates and the cells allowed to grow for 2 days (strains McM13.3, McM18.1
and McM32.2) or 3 days (strain McM11.5) at 20 ( F 1.5) jC. Antimicrobial activity was
defined as visible inhibition of cell growth in a region surrounding the paper disk.
2.3. Tissue energetics
Freeze-dried brachiopod tissues were returned to the University of Alabama at
Birmingham for calorimetric analysis. The lophophore, pedicle, female reproductive
tissues, male reproductive tissues and intestine –stomach tissues were each pressed into
small pellets and calorically analyzed with a Parr 1421 Semimicro Calorimeter. Three
determinations were made for each tissue. Energy content (kJ/g dry wt.) was calculated
against a benzoic acid standard and included a fuse correction. Trials were repeated three
times for each tissue measured. Differences in the energy content of tissues was
determined by one-way analysis of variance and by a Ryan– Einot – Gabriel– Welsch
(REGWQ) post hoc test using SPSS software (SPSS, Chicago, IL).
3. Results
3.1. Feeding bioassay
The sea star O. validus rejected whole male, female and juvenile L. uva and a
hydrophobic extract of L. uva added to the krill feeding stimulant (Fig. 1). However,
lophophore tissue was accepted in all trials (Fig. 1).
Fig. 1. Percent acceptance by the sea star O. validus of whole L. uva individuals or L. uva lophophores (LOP)
(dark bars) and hydrophobic extracts of L. uva (hatched bar) in comparison to corresponding controls (open bars).
Asterisks indicate significant differences from controls (Fisher’s Exact Test, p < 0.05).
A.R. Mahon et al. / J. Exp. Mar. Biol. Ecol. 290 (2003) 197–210
203
Fig. 2. Percent acceptance by the sea star O. validus of specific tissues of L. uva homogenized and suspended
in alginate (filled bars) in comparison to corresponding controls (open bars). Asterisks indicate significant
differences from controls (Fisher’s Exact Test, p < 0.05). MRT = male reproductive tissue, FRT = female
reproductive tissue, PED = pedicle, INT = intestine – stomach, LOP = lophophore, WHO = whole ground
individual.
Homogenized L. uva tissues suspended in alginate caused a differential response in the
sea star and fish feeding bioassays. Odonaster validus rejected ground whole brachiopods,
male reproductive tissues, the stomach – intestine and the pedicle (Fig. 2). In the first N.
coriiceps bioassay, the only individual ground tissue rejected by the fish was the brachiopod
pedicle (Fig. 3). In the second N. coriiceps bioassay with all soft tissues combined, the fish
significantly rejected the ground brachiopod disks (Fisher’s Exact Test, p = 0.0006). Only 1
of 12 experimental disks was accepted (8.3%) compared to 10 of 12 controls (83%).
3.2. Microbial analysis
Antimicrobial activity was greatest in extracts of the lophophore and of the intestine
with extracts of female reproductive tissues having somewhat less antimicrobial activity
(Table 1). Hydrophobic extracts from the lophophore, intestine – digestive diverticula and
female reproductive tissues and hydrophilic extracts from the lophophore and intestine –
stomach had growth inhibition effects on two of the four strains of bacteria tested,
Fig. 3. Percent acceptance by the fish N. coriiceps of specific tissues of L. uva (filled bars) and hydrophobic
extract of entire individuals (hatched bar) in comparison to control disks (white bar). Asterisks indicate significant
differences from controls (Fisher’s Exact Test, p < 0.05). See Fig. 2 for definitions of abbreviations.
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A.R. Mahon et al. / J. Exp. Mar. Biol. Ecol. 290 (2003) 197–210
Table 1
Results of antimicrobial bioassay
Strain
Extract
Tissue type
Lophophore
Intestine – stomach
Female rep.
Male rep.
Pedicle
McM11.5
HB
HP
HB
HP
HB
HP
HB
HP
HB
HP
+++
0
0
++
0
0
0
++
1.68
5.58
+++
0
+
++
0
0
+
0
3.07
9.36
++
0
+
0
0
0
+
0
3.69
9.92
0
0
0
0
0
0
0
0
3.36
12.6
0
0
0
0
0
0
0
0
1.70
5.06
McM13.3
McM18.1
McM32.2
Extract mass
‘‘0’’= no effect. ‘‘ + ’’= growth inhibition halo present but < 1 mm wide. ‘‘ + + ’’= growth inhibition halo 1 mm
wide. ‘‘+ + + ’’= growth inhibition halo 2 mm wide. Extract mass indicates mg of dry extract added to
antimicrobial paper assay disk (see text). HP: hydrophilic extract. HB: hydrophobic extract.
respectively (Table 1). Neither male reproductive tissue extracts nor pedicle extracts had
any visible effect on bacterial growth (Table 1). Likewise, control disks prepared with
either solvent alone did not inhibit growth in any bacterial strain.
3.3. Tissue energetics
There were significant differences between specific tissues from L. uva. The brachiopod
pedicle was significantly lower in caloric content than the intestine – stomach, male
reproductive tissue and the lophophore (Fig. 4). Female reproductive tissue was intermediate in caloric content and not significantly different from any of the other tissues
examined (Fig. 4).
Fig. 4. Calorimetric data from the specific tissues of L. uva. Means F 1 S.E.M. Analysis of variance indicated
significant differences between tissues ( F4,14 = 5.844, p = 0.011). Post hoc test (REGWQ, p < 0.05) results
identifying differences between means are symbolized by letters above bars. Means with different letters are
significantly different. See Fig. 2 for definitions of abbreviations.
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205
4. Discussion
These data confirm our previous report (McClintock et al., 1993) that L. uva is
unpalatable to sea stars and fish. Moreover, these data extend the previous report by
directly demonstrating lack of palatability in feeding assays with sympatric predators.
Although bioassays based on sea star tube foot retraction correlate well with known sea
star feeding preferences on sponges in nature (reviewed by Amsler et al., 2000, 2001a,b;
McClintock et al., 2000), such a correlation has not been made for other invertebrate prey.
The present study, however, provides one data point that may be used to help make such
correlations. The use of allopatric predators in feeding deterrence assays was not
uncommon in the past and was necessary for logistical reasons with fish predators in
our previous study, but lacks the direct ecological relevance afforded by assays with
sympatric, potential predators.
Sea stars demonstrated strong rejection of whole live brachiopods regardless of their
sex or age (feeding data in present study), while more qualitative observations revealed
that fish (N. coriiceps) also repeatedly spit out whole adult brachiopods (C.D. Amsler,
personal observations). Fish rejected ground, whole-body tissue embedded in alginate
disks, but the lipophilic extract added to a feeding stimulant in alginate was not rejected.
Feeding deterrence in fish could be the result of hydrophilic compounds but, due to
logistical constraints, we were not able to test a hydrophilic extract with fish. The basis for
these rejection responses to whole brachiopods by sea stars is definitively chemical in
nature, at least in part, as lipophilic extracts of whole-body tissues embedded in alginate
pellets were rejected. Because half or more of the internal tissues of L. uva may be
composed of calcareous support spicules and other inorganic matter (McClintock et al.,
1993; Peck, 1993), physical defenses and/or overall low nutritional value could also play a
role in unpalatability as has been suggested by Peck (1993, 2001). Calcified cell walls are
known to act synergistically with chemical defenses in reducing herbivory on some marine
macroalgae (e.g., Hay et al., 1994; Schupp and Paul, 1994). In animals, calcareous
sclerites produced by gorgonian corals can act synergistically with chemical defenses
against predation but are often ineffective defenses by themselves and are rarely the sole
defensive mechanism (van Alstyne et al., 1994; Koh et al., 2000; Puglisi et al., 2002).
Siliceous spicules produced by sponges appear to have no defensive roles against a variety
of predator types including sea stars (Chanas and Pawlik, 1995, 1996; Waddell and
Pawlik, 2000a,b). The potential for physical defenses in L. uva that might act in addition to
the observed chemical defenses against sea stars remains to be determined. It is of note that
the structural integrity of L. uva spicules would have been destroyed by our homogenization procedure but that similar treatment usually did not effect the defensive properties
of gorgonian sclerites (Puglisi et al., 2002).
Our direct measurements of the caloric contents of L. uva soft tissues (13 –19 kJ/g dry
weight) are somewhat higher than overall contents indirectly estimated from tissue
biochemistry by McClintock et al. (1993) and Peck (1993) (9 and 12 kJ/g, respectively).
This could potentially be due to differences in the individuals sampled but more likely is
because of differences in the technique (cf. Paine, 1971). Regardless, although lower than
most invertebrates (Brey et al., 1988), even the lower estimates of L. uva caloric contents
are comparable to or greater than the contents of organisms in groups such as sponges,
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echinoderms, ascidians and macroalgae (Paine, 1964; McClintock, 1986; Brey et al.,
1988) that are commonly preyed upon by Antarctic sea stars including O. validus
(McClintock, 1994).
The presence of chemical defenses in tissues of a shelled animal is probably less
surprising in polar waters than in more temperate seas. Polar molluscs commonly have thin
and poorly calcified shells (Nicol, 1967; Vermeij, 1978), perhaps due to the relatively high
energetic cost of precipitating calcium carbonate in very cold water (Clarke, 1993). L. uva
is similar to polar molluscs in having a very thin shell and must be handled carefully
during collection to prevent breakage (A. Mahon, C. Amsler, personal observations).
The present study of L. uva provides an opportunity to evaluate the tenets of the ODT
(Rhoads, 1979; reviewed by Cronin, 2001) which assumes that resources are limiting and
predicts that chemical and/or physical defenses should be localized in tissues that are most
exposed to predators and/or that maximize fitness. Discrete brachiopod body tissues
‘‘exposed’’ to sea star and fish predators in the natural environment include the
lophophore, a branched filter feeding appendage that is repeatedly extended beyond the
confines of the shell valves then rapidly withdrawn, and the pedicle, a stalk-like structure
comprised of tough connective tissue that functions to affix the individual to the substrata.
In the present study, sea stars and fish readily accepted whole lophophores and freezedried ground lophophore embedded in artificial foods. In contrast, ground pedicle
embedded in artificial foods was unpalatable to both sea stars and fish. These results
conform to the predictions of the ODT in that the likelihood of sea stars or fish
successfully attacking and consuming the rapidly retractable lophophore is low, while
the constantly exposed pedicle is highly vulnerable. Therefore, in the context of ODT,
investing defensive resources in the pedicle protects not only the most exposed and
vulnerable body component, but is an allocation pattern most likely to maximize fitness, as
consumption of the pedicle would result in the dislodgement of the individual from the
substrata and subsequent probable mortality.
There was little variation in our measurements of the energetic content of tissues
comprising discrete body components in L. uva. The pedicle contained the least energy
per unit mass compared to the lophophore, reproductively active gametic tissues
(gametogenesis is asynchronous in L. uva, Meidlinger et al., 1998) and the gut
(intestine –stomach). The intact pedicle has a relatively small mass (unpublished data)
when compared to other body components (Meidlinger et al., 1998) and would, thus,
represent a smaller investment when considering the total allocation of energy to
discrete body components. Even if as discussed above, the magnitude of our energetic
content measurements is slightly high, the relative differences between components
should be correct. In summary, there is no evidence for an increased investment of
defenses to those tissues highest in energy content as has been suggested in some
marine and terrestrial plants (Simms, 1992; Zangrel and Bazzaz, 1992; Cronin, 2001).
Moreover, the allocation of defenses for the purpose of preventing predation upon
internal body components seems unlikely as even a partially eaten individual is likely
to die. Thus, the lack of palatability observed in sea stars rejecting food pellets
containing homogenates of gametic tissues (weakly unpalatable) and intestine (strongly
unpalatable) is more likely explained by other selective factors such as chemical
defenses against pathogens.
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207
Our analysis of the antimicrobial activity (growth inhibition) of organic extracts of
the body components of L. uva exposed to four strains of sympatric Antarctic marine
bacteria detected potent antibacterial activity in both hydrophilic and hydrophobic
extracts of the lophophore and gut (intestine– stomach). Weaker antimicrobial activity
was detected in the gonad (ovary) tissues. The presence of potent antimicrobial activity
in the lophophore and gut provides compelling evidence that the internal organs least
defended against predators, particularly the lophophore, possess bioactive secondary
metabolites that provide defense against pathogens. Both the lophophore and gut are
among those tissues most exposed to water column borne pathogens and, therefore, most
likely to require antimicrobial defenses. Our data indicate that there is not a ‘‘crossreaction’’ of bioactive compounds whereby the same defensive secondary metabolites
serve multiple functions (e.g., see Paul, 1992; Schmitt et al., 1995 and references
therein), but rather L. uva produces secondary metabolites that specifically provide for
either defense against pathogens or macropredators, and that are allocated in such a
functional manner to defend the most appropriate body components. We believe this is
one of the few studies to date to demonstrate such secondary metabolite target
specificity in a marine invertebrate.
Acknowledgements
We are grateful to Drs. K. Iken, M. Powell and R. Angus for laboratory or statistical
assistance, and to the employees and subcontractors of Raytheon Polar Services and
Antarctic Support Associates for logistical support in Antarctica. This research was
facilitated by the generous support of the Office of Polar Programs of the National Science
Foundation to CDA and JBM (OPP-9814538) and to BJB (OPP-9901076). [SS]
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