J. Exp. Mar. Biol. Ecol., 148 (1991) 121-134
121
© 1991 Elsevier Science Publishers B.V. 0022-0981/91/$03.50
JEMBE 01580
Drag-induced deformation" a functional feeding strategy in
two species of gorgonians
Su Sponaugle and Michael LaBarbera
Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois, USA
(Received 8 August 1990; revision received 10 December 1990; accepted 3 January 1991)
Abstract: The Jamaican gorgonians Pseudopterogcrgia acerosa (Pallas) and P. americana (Gmelin) exhibit
whole colony flexibility in regions of moderate to high water movement, Juvenile colonies were tested under
laboratory conditions to delineate the functional consequences of fexibility. Comparison of the drag forces
exerted on flexible and arS~cially stiffened colonies in different flows to the forces required to detach
colonies in the field suggested that the reduction in drag afforded by flexible colony deformation is not critical
to preventing dislodgement. A further consequence of colony deformatiori was the reduction in the range
of flow velocities encountered by the polyps relative to mainstream current velocity. Colonies appeared to
feed most successfully when exposed to intermediate mainstream velocities of 10-15 c m . s - !. Therefore,
the reduction of flow velocities at the level of the polyp broadens the range of ambient velocities over which
successful feeding can occur, and as such, may be the most important consequence of flexibility.
Key words: Cnidaria; Drag; Flexibility; Gorgonacean; Suspension-feeding
INTRODUCTION
Suspension-feeders are those organisms obtaining nutrition through the capture of
food particles suspended in fluid media. Two broad categories of suspension-feeders
can be distinguished based on the source of energy used to move fluids past the
suspension feeding apparatus (Jergensen, 1966). Active suspension-feeders use metabolic energy to drive currents through their feeding structures. In contrast, passive suspension-feeders invest no metabolic energy in the movement of fluids, instead depending
entirely on ambient currents to move fluids through their feeding structures.
Many gorgonians (Cnidaria: Gorgonacea) are passive suspension-feeders. These
colonies of small polyps assume a variety of morphologies and distributions in tropical
reef communities: they are totally dependent on ambient currents to drive particle-laden
water through the colony for both feeding and removal of waste and reproductive
products. Several studies have established general morphologic~l and orientational
trends occurring in gorgonians and other passive suspension-feeders in response to
current regimes. Organisms occurring in areas of turbulent or unpredictable current flow
Correspondence address: S. Sponaugle, Marine Sciences Research Center, State University of New York
at Stony Brook, Stony Brook, NY 11794-5000, USA.
122
S. SPONAUGLE AND M. LABARBERA
tend to exhibit bushy, "radial" morphologies, which allow the interception of oncoming
flow from any current direction. Organisms occurring in uni- or bidirectional currents,
however, tend to have flattened, planar morphologies, and are oriented with the plane
perpendicular to the flow direction (Riedl, 1966, 1971; Riedl & Forestner, 1968). In
addition to gorgonians (Laborel, 1960; Theodor, 1963; Grigg, 1972; Kinzie, 1973;
Schuhmacher, 1973; Velimirov, 1973; Leversee, 1976), these morphological and orientational trends have been observed in octocorals (Rees, 1972), hydroids (Svoboda,
1970), crinoids (Magnus, 1964; Meyer, 1973), and ophiuroids (Warner & Woodley,
1975). Highly predictable and strongly unidirectional currents often permit a less frequently encountered morphology: that of a dish-shaped feeding structure oriented with
the concave side facing into the flow (Theodor, 1963; Grigg, 1972; Macurda & Meyer,
1974; Muzik & Wainwright, 1977; Warner, 1977).
Pseudopterogorgia acerosa (Pallas) and P. americana (Gmelin)occur in shallow, open
sand and rubble channels in the coral reefs along the northern coast of Jamaica. Current
flow is oscillatory and colonies can be subjected to heavy surge. Colony morphologies
are generally planar and oriented normal to the prevailing current direction. Laborel
(1960) and Barham & Davies (1968) have suggested that this perpendicular orientation
is an adaptation for greater feeding efficiency. Supporting this view, Leversee (1976)
measured feeding efficiencies of Leptogorgia colonies in several orientations, and concluded that an orientation normal to ambient currents enhances food particle capture
for planar colonies.
While a perpendicular orientation to current flow may maximize both mechanical
stability and food intake, it also exposes a maximum surface area to the current,
inducing large drag forces. Wave-induced drag force dislodges colonies and appeared
to be the predominant cause of mortality in the fan-shaped colonies of Gorgonia ventalina
(Birkeland, 1974). Dislodged specimens were found attached to remnant substratum,
suggesting that detachment is due to weaknesses in the substratum rather than in the
colony or the colony-substratum interface. The increase of drag force with surface area
was therefore suggested to place an upper limit to colony size.
Thus, gorgonians may face several design conflicts between minimizing drag forces
to prevent dislodgement znd increasing the surface area exposed to the flow for maximal
food capture and exchange. The apparent conflict may be sidestepped in many species
of gorgonians through their capacity to significantly deform when exposed to current
and wave forces. Flexibility ameliorates the drag problem in trees (Vogel, 1984), algae
(Koehl & Wainwright, 1977), anemones (Koehl, 1977a), zooanthids (Koehl, 1977b),
and hydroids (HarveU & LaBarbera, 1985) by enabling shape modification in currents.
Organisms are bent so that their long axis becomes almost parallel to the oncoming flow,
reducing prqiected surface area normal to the flow and decreasing the wake size.
Bending may also move the organism closer to the substratum where it may encounter
reduced flow speeds (Wainv~'right& Koehl, 1976). Compression or stacking of branches
into more compact shapes with lower drag coefficients may also occur (Wainwright &
Koehl, 1976; Vogel, 1984; Denny et al., 1985).
DEFORMATION AND FEEDING IN GORGONIANS
123
Flexibility may also affect colony feeding success. Capture and retention of food
particles is dependent on the current velocities and flow patterns at the level of the
feeding structures (Rubenstein & Koehl, 1977). The hydrodynamics of suspension
feeding suggests that there may be a relatively narrow range of flow velocities over which
polyp feeding is efficient (LaBarbera, 1984); therefore, reducing local velocity ranges
may enhance feeding success. Harvell & LaBarbera (1985) suggested that the
maintenance of reduced and less variable local velocities in a hydroid is an important
consequence of flexibility; however, the direct effect of flexibility on feeding success was
not measured.
This study was designed to investigate the functional consequences of flexibility in
juvenile colonies of P. acerosa and P. americana. Both species exhibit whole colony
deformation in response to current flow. To determine the relative importance of the
consequences of flexibility, laboratory and field measurements were made to quantify
the feeding success and the drag forces encountered by colonies in different current
velocitie,~.
METHODS AND MATERIALS
Field work and laboratory analyses were conducted at the research facilities of
Discovery Bay Marine Laboratory, Discovery Bay, Jamaica, in 1985. In situ observations and specimen collections were made at several sites at depths of 8-15 m in the
vicinity of Discovery Bay. Juvenile colonies were collected by prying the holdfasts off
the substratum or by collecting the attached substratum with the animal. Colonies were
immediately transferred to laboratory sea tables without exposure to air and kept in a
O.15-m 3 glass aquarium with circulating sea water. Quick-release stainless steel hose
clamps clipped onto colonies just above the holdfast served as anchors to maintain
colony posture without inhibiting water circulation around the colony. Daylight
illumination in the laboratory was sufficient to induce polyp expansion. Species identifications were based on colony, polyp and spicule morphology (Bayer, 1961).
All laboratory manipulations were conducted in a 15 x 15-cm cross-section, recirculating flow tank (Vogel & LaBarbera, 1978). Although these gorgonians can occur in
areas of high bidirectional surge, the period of oscillation in nature is generally quite long,
and measurements in unidirectional flow may approximate natural conditions. Potential
differences between colony performance in unidirectional flow and under natural
oscillatory flow are discussed below.
To estimate feeding rates of individual P. acerosa colonies, measured concentrations
of Artemia nauplii were introduced downstream from a colony at various current
velocities to ensure even dispersal prior to contact with a colony. Initial concentrations
of Artemia were determined by counting individual nauplii, vital stained with rhodamine
dye, in three 25-ml samples of a suspension of Artemia in salt water. Each sample was
strained through nylon plankton netting and counted on a superimposed grid under
10 x magnification. Filtered nauplii were added to the flow tank to produce concentra-
124
S. SPONAUGLE AND M. LABARBERA
tions of 2.5-4.0 nauplii. 1- 1. Individual captures by gorgonian polyps could be readily
observed as the stained nauplii were visible both in the water and in the polyp gastrovascular cavity. The number of captures during several 5-min intervals was recorded
for each flow velocity over a period of 20 min, at which point the tank was flushed out.
Replicate tests were made several hours apart to allow complete clearing of Artemia
from polyp guts. Data from colonies feeding in different current velocities were grouped
into three velocity ranges, 0-5, 10-15 and 2 0 - 2 5 c m . s -~. Feeding success was
standardized to the number of nauplii captured per polyp to account for variability in
colony size. Analysis of feeding success was based on the mean number of nauplii
captured during the first 5 min since subsequent capture success was dependent on the
earlier depletion of nauplii.
Flow velocities within 2 mm of the polyp were measured for both P. acerosa and
P. americana using a thermistor flow probe with a spatial resolution of 0.5 mm
(LaBarbera & Vogel, 1976). The probe was positioned with a micromanipulator to
determine the local flow velocity at the tentacles of an average polyp (mid-colony,
mid-branch) for each ambient velocity in which feeding experiments were performed.
Ambient current velocity was determined by timing particle travel over a known
distance, and was subsequently confirmed by positioning the flow probe in the
mainstream. Throughout the study the colonies were anchored in the center of the flow
tank using a lead weight attached to their holdfasts. Reduced major axes (RMA) (Laws
& Archie, 1981; LaBarbera, 1989) slopes were calculated from a least squares
regression to take into account the error associated with both variables. Slopes were
compared with a standard t test (Chatterjee & Price, 1977).
Drag forces acting on both species in various flow velocities were measured with a
foil strain gauge bonded to an aluminum beam which was attached to the colony by a
lever arm (Harvell & LaBarbera, 1985). The colony was bonded to the lever arm (a
17-cm long, 3-mm diameter solid steel cylinder) with thermoplastic glue and suspended
upside down through a slit in a Plexiglas sheet lying atthe water surface along the entire
length of the tank. The slit permitted uninhibited deflection of the colony under drag
forces, while the Plexiglas surface mimicked substratum effects on flow. Measurements
were recorded for living, flexible colonies and for the same colonies with their flexural
stiffness enhanced through drying in a desiccation oven, attachment of wire supports
behind the central stalks, and application of several coats of polyurethane ("stiffened"
colonies). Living flexible colonies were tested in three states of polyp expansion: fully
expanded, partially expanded, and retracted. Reduced major axes slopes were
calculated from least squares linear regressions and compared with a standard t test as
above.
Projected colony areas perpendicular to the flow in different current speeds were
photographed with a 35-mm camera fitted with a 200-mm macrotelephoto lens, and
later traced and measured with a computer-aided digitizer. Coefficients of drag were
calculated from the drag measurements and cross-section areas, based on the equation:
drag = I/2pCDSU 2,
DEFORMATION AND FEEDING IN GORGONIANS
125
where t9 = density, CD = drag coefficient, S = projected area and U = velocity (Vogel,
1981 ). In addition, "E values" (Vogel, 1984, 1989) were calculated from the RMA slopes
to describe the relationship between the drag coefficient and speed. Typically, the more
negative the E value, the greater the relative reduction of drag with increasing speed.
At an E value of - 2.0, drag becomes effectively independent of speed (Vogel. 1984).
To quantify the importance of drag forces on gorgonian colonies in nature, the forces
necessary for detachment were measured in the field. A spring gauge, which was
mechanically modified to record the maximum force exerted, was attached with a nylon
string noose just above the holdfasts of a total of 15 colonies (P. acerosa = 8;
P. americana = 7). Force was applied through the gauge and string parallel to the colony
axis until the colony detached, and this critical force recorded. Colony size was
estimated by the product of maximum colony height and width for each colony.
Flow patterns through and around colonies of both P. acerosa and P. americana in
various currents were observed by the continuous release of fluorescein dye from
catheter tubing positioned upstream from the colony. Observations of flow patterns
were also made in situ by releasing fluorescein dye from a large syringe held upstream
of the colonies.
RESULTS
Artemia capture success by P. acerosa varied as a function of ambient current velocity
(Fig. 1). Feeding rate appeared to be highest during moderate mainstream flow velocities (10-15 cm. s- ~), and reduced at low (0-5 cm. s- ~) and high (20-25 cm. s- l)
current velocities (Table I).
I
m
/
A\
==
w
==
10
O
E
Z
ot
o
10
15
20
25
30
Current Velocity (10 .2 m.s "I)
Fig. i. Feeding of a typical P. acerosa colony oll Artemia nauplii. The points represent the number ofnauplii
captured p,,:r polyp for a colony feeding during a five minute period in each of several current velocities.
Rate of Artemia capture was generally highest at intermediate mainstream current velocities.
126
S. SPONAUGLE AND M. LABARBERA
TABLE I
Mean number ofArtemina nauplii + 1 SD captured during a 5-min period by P. acerosa colonies feeding in
a range of current velocities. Velocities were grouped into low (0-5 cm. s - :), intermediate (10-15 cm. s - :)
and high (20-25 cm. s - ~) velocities.
Current velocity (cm. s - :)
Mean number of Artemia captured/polyp ( x 10 --~)
Number of colonies tested
0-5
10-15
8.2 + 0.1
15,3 + 6.9
7
2
20-25
6.4 + 2.1
4
40
E. acerosa
A
W
30
"~
20
/
0
0
o
_...,,.--~--~
0
I
8
0
10
16
20
I
I
28
SO
I
,
36
J
40
Current Velocity (I0 a m.s':)
40
P. americana
E
30
20
ol
rj
o
10
0
mm
0
~
0
, ~
8
,0
10
..
I'5
15
_ ,
20
!
,
26
30
36
•
40
Current Velocity (10 .2 m.$ "t)
Fig. 2. Influence of mainstream current velocity on flow velocity at the level of a midbranch polyp for
P. acerosa and P. americana. The lines drawn are reduced major axes regressions ofthe data; for P. acerosa,
the data above and below mainstream velocities of ~, 20 era. s - ~ were analysed separately. Note that for
both species at mainstream velocities < 20 cm. s- ~+ flow velocities at the polyp are approximately proportiGnal to the cube root of mainstream velocity; at velocities > 2 0 c m . s - t , velocity at the polyp in
P. acerosa becomes directly proportional to mainstream velocity.
D E F O R M A T I O N AND F E E D I N G IN G O R G O N I A N S
127
Local flow velocities measured at the position of an average polyp were significantly
reduced from ambient current velocities (Fig. 2). The reduced major axes (RMA)
slope + 1 SD for flow velocity at the polyp vs. mainstream velocity for P. acerosa was
0.654 + 0.056 ( r = 0.86). However, an obvious break in the data occurred at
20 cm. s - ! ambient flow velocity. Flow velocities at the polyp increased more rapidly
beyond this point, so in addition to analyzing the data as a single slope, the data were
split into two groups and separate slopes calculated. Analysed separately, the first group
of data (mainstream velocity from 0 to 20 cm. s - ! ) followed a RMA slope of
0.290 + 0.032 (r = 0.78) (significantly different from a slope of 0 and 1 (P < 0.0005);
and the second group (mainstream velocity from 20 to 35 c m . s - 1 ) a slope of
1.176 + 0.154 (r= 0.92) (not significantly different from a slope of 1; P > 0.25). For
P. acerosa, therefore, velocities at the polyps increased directly with current velocity
beyond 20 cm. s - !. Flow velocity at the polyps for P. americana increased more consisen,,y with increasing mainstream velocity. No obvious change in slope occurred at any
measured ambient velocity, so a single slope was calculated. The RMA slope of
0.313 _+ 0.053 (r = 0.54) is significantly different from both a slope of 0 (local velocities
independent of ambient velocities; P = 0.0025) and 1 (local velocities directly proportional to ambient velocities; P < 0.0005).
Since hydrodynamic theory predicts a power law relationship between drag forces
and current velocity in this flow regime, log transformations were used to calculate the
reduced major axes slopes for drag forces vs. current velocity for the colonies. Slopes
therefore reflect the exponent of current velocity to which drag forces were proportional.
The data were split and analysed separately where a break in the trend of the points
T..x8 H:. II
Reduced major axes slopes + 1 SD (r) for log-transformed drag forces as a function of current velocities.
Data were obtained for colonies with fully expanded polyps, partially expanded polyps, retracted polyps,
and those artificially stiffened. Where appropriate, data were split and analysed separately (see Fig. 3).
r = Pearson product-moment correlation coefficient.
All data
State of colony/polyps
Split data
First group
(0-13.3 c m . s - ~ )
Second group
(13.3 c m . s - ~ +)
P. acerosa
Flexible/fully expanded
Flexible/partly expanded
Flexible/retracted
Stiffened
0.848
0.780
0.968
!.194
+
+
+
+
0.092
0.095
0.097
0.029
(0.90)
(0.93)
(0.92)
(0.99)
1.742 + 0.112 (0.99)
1.204 + 0.065 (0.99)
1.533 + 0.120 (0.98)
0.270 + 0.026 (0.96)
0.233 + 0.036 (0.92)
0.309 + 0.048 (0.89)
0.792
0.902
0.786
1.592
+
+
+
+
0.087
0.064
0.059
0.046
(0.91)
(0.98)
(0.98)
(0.99)
1.634 + 0.124 (0.99)
0.411 + 0.055 (0.92)
P. americana
Flexible/fully expanded
Flexible/partly expanded
Flexible/retracted
Stiffened
128
S SPONAUGLE AND M. LABARBERA
-'.5
_P.c~c_¢.rs_oa
27
_/
1
!
,'-
:
(
.-~.
0
1.5
Flexible
i
:
~
1
:
I
•
1.7
1.9
2.1
2.3
2.5
2.7
2.9
Log Velocity(10"3m.s"1)
P ' ~
•
C"
~ :
C,
1.5
Z
Q
lml
~a
~a
I,,,
,.a
0.5
?
;
i/,
i
to.~ S,i,,e..
Flexible
*
I
O;
1.5
;
l
1.7
1.9
-- ....
2.1
|
;
I
2.3
2.5
2.7
..
2.9
Log Velocity(10"3m.s"1)
Fig. 3. Drag force as a function of current velocity for P. acerosa and P. americana in several states of
flexibility. The lines plotted were fit using reduced major axis regressions of log-transformed data (see
Table I I).
DEFORMATION AND FEEDING IN GORGONIANS
129
indicated the existence of two distinct slopes (Fig. 3). For both species, consideration
of the grouped data produced slopes for flexible colonies that were one-half to twothirds those for stiffened colonies (Table II). With the exception of the first group of data
for fully expanded P. acerosa colonies which was barely nonsignificant (P = 0.06), all
slopes were significantly different from 2 (P < 0.0005-0.05). Therefore, the drag forces
exerted on flexible and stiffened colonies were not proportional to the square of velocity,
as the drag equation would predict for a rigid object. Slopes for the restrained colonies
were < 2 most likely because colonies could not be kept perfectly rigid. Slopes were also
tested to determine whether drag was directly proportional to velocity. Where data were
split for flexible colonies, initial slopes were all significantly > 1 (P < 0.001-0.005), with
the exception of partially expanded P. acerosa (P > 0.10), and corresponded closely
with slopes for the stiffened colonies. Slopes beyond the 13.3 c m . s - ~ velocity
0.6
a
36
30
0.5
qt~
[:
.~-
2(;
0.4
E
C,~
0
N
0
20
0.3
1(;
L_
10
0
S
0.1
6
0
0
I
l
I
0.2
0.4
0.6
0
0.8
Current Velocity.(m,s"I)
_P. americana
q2e
2O
1.6
[=
mm
~J
15
O
m
O
"<
E¢J
0
E
0.6
s
--G- Drag ooeffiolenl
3
Colony k , n
O'
0
l
I,
I
0.1
0,2
0.3
0
0.4
C u r r e n t Velocity. (m.s "a)
Fig. 4. Drag coefficient and projected colony area as i'unctions of mainstream current velocity for P. acerosa
and P. americana. The lines shown were fit to the data by eye. Reduced major axis regressions of projected
area vs. mainstream velocity produced a slope of - 8.05 for P. acerosa and - 10.34 for P. americana. The
dramatic change in drag coefficient over the range of mainstream current velocities investigated cannot be
attributed, therefore, to changes in projected area of the colonies.
130
S. S P O N A U G L E A N D M. LABARBERA
breakpoint were significantly < 1 (P < 0.0005), and as much as six times smaller than
initial slopes.
A marked decrease in the drag coefficient with increases in current velocity (Fig. 4)
is evident for both P. acerosa and P. americana. However, projected colony area
remained relatively constant, exhibiting only a gradual decrease (Fig. 4) with increasing
flow velocities. Drag reduction during colony flexion therefore appears to be less of a
function of changing projected area than of changes in flow pattern. Calculated E values
for live, flexible colonies with fully expanded polyps ranged from - 0 . 2 6 for P. acerosa
and - 0.37 for P. americana in flow velocities < 13.3 cm. s - ~ (split data); to - 1.73 for
P. acerosa and - 1.59 for P. americana in currents > 13.3 cm. s - ~.
Detachment strengths for juvenile colonies tested in the field ranged from 13.3 to
106.8 N, and were dependent on colony size (Fig. 5). The average force necessary to
dislodge the colonies was ~63 N for P. acerosa (11 x 7 cm in height and width) and
50 N for P. americana (9.5 x 10cm). These forces correspond to forces that an
inflexible, restrained colony would encounter in unidirectional currents of 128 m . s (P. acerosa) or 27 m . s - ~ (P. americana). In theory, flexible colonies would only be
dislodged in currents of 3 x 10 ~ 2 m . s - ~ (P. acerosa) and 5 x 1 0 8 m . s (P. americana). For both species, almost all failures occurred at the holdfast-substratum interface; 50~o of those failing at the interface lost sections of the outer edge of the
holdfast during detachment.
A difference in flexural stiffness was evident in observations of colony behavior in
different flows. P. americana flexed more readily than P. acerosa and tended to compress peripheral branches inward during flexion. Dye streams revealed that flow through
the colony was laminar for both species until 5 cm. s - ~ for P. acerosa and 3.4 cm. s for P. americana, whereupon vortices began forming downstream of the branches. As
120 r
m
v
100
O
8O
~
6O
~
4o
eo
2o !
~
~
*
0
P. acerosa
o
I
I
i
I
o
0.5
1
1.5
Colony Size (10 .2 m 2)
Fig. 5. Detachnient strength as a function of colony size for P. acerosa and P. americana. The product of
maximum height and width was used as a metric of colony size. The lines shown were fit to the data by
eye.
DEFORMATION AND FEEDING IN GORGONIANS
131
the colonies flexed and compressed, more water appeared to pass over and around the
colony than through it. Colony branches were often compressed together to the point
of contact. During feeding runs at these higher velocities, most captures were made by
polyps on branch tips. In situ injections of dye revealed that water flow permeated
through the colony during strong surges, often carrying the dye through the colony a
second time on the reverse surge.
DISCUSSION
MECHANICS OF FLEXION
Most of the drag measurements made on flexible colonies produced data that could
be analyzed as two distinct slopes; data divided by eye clearly fell to either side of a
mainstream velocity of 13.3 c m . s - ~. Since initial slopes closely approximated slopes
of the stiffened colony and the following slopes were substantially lower, colony flexion
appears to become more important for drag reduction at velocities > 13.3 c m . s With a maximal E value of -1.73, a flexible colony in cLJrrents > 13.3 c m . s approaches near independence (E = - 2 ) of current speed. 1,arge reductions in the
coefficient of drag with increasing flow velocities suggest that changes in shape, rather
than changes in projected area are primarily responsible for the reduction of drag forces
in flexible colonies. Marked changes in flow patterns ~und the colonies at mainstream
velocities > 13.3 cm. s-~ are probably a result of the compression of individual
branches during colony flexion. The interaction of flow patterns beyond this point are
probably very complex, and not simply a function of the degree of flexibility. Although
P. americana deforms more readily (i.e., begins to deform at lower current velocities),
and tends to compress lateral branches closer than P. acerosa, P. americana has the
higher slope and less negative E v~due, suggesting that colonies experience less of a
I'' h
reduction in drag at nig,,er
flow velocities.
SIGNIFICANCE OF FLEXIBILITY: MORTALITY
Although colony deformation results in a substantial reduction in drag forces, this
may not be the most important functional consequence of flexibility. Flexion does not
appear to be necessary to prevent the dislodgement of juvenile gorgonian colonies. The
forces necessary to detach artificially stiffened colonies were substantially greater than
flow-induced forces encountered naturally, suggesting that drag reduction may not be
the primary selective benefit of flexibility for juvenile gorgonians. Detachment strengths
were, however, dependent on colony size, and while larger colonies had correspondingly
greater holdfast areas and detachment strengths, drag-induced mortality may be more
significant for adult colonies. Birkeland (1974) found that 800,0 of the mortality in the
sea fan Gorgonia ventalina was due to drag-induced dislodgement. However, much of
this dislodgement was due to substratum and not colony weakness. Mortality levels for
132
S. SPONAUGLE AND M. LABARBERA
P. acerosa and P. americana are unknown, and because these colonies exhibit markedly
different morphologies from anastomosed fans and occur in different regions of the reef,
comparisons with Birkeland's data are difficult. It is conceivable, however, that a
proportion of colonies with a high probability of dislodgement once existed and were
selectively removed during Hurricane Allen in 1981. An additional component contributing to the drag encountered naturally probably occurs in areas of bidirectional surge,
where the rapid acceleration and deceleration of water may result in colony detachment
at lower current velocities.
The impact of Hurricane Allen on the north coast of Jamaica demonstrated that
flexibility may reduce intracolony damage (Woodley et al., 1981). Fracture of gorgonian
branches occurred less frequently than the breakage of rigid, Millepora-encrusted gorgonian branches (Woodley et al., 1981). The energy necessary to break a flexible
organism of a given strength is greater than that for a rigid organism of the same strength
(Wainwright & Koehl, 1976). Therefore, an additional consequence of flexibility may
be the reduction of intracolony breakage. The force required to break a branch of a
hypothetically rigid gorgonian is unknown, however, and therefore only relative comparisons can be made.
SIGNIFICANCE OF FLEXIBILITY: PARTICLE CAPTURE
The maintenance of reduced and less variable local flow velocities at the polyp level
appears to be an important consequence of the deformation of gorgonian colonies.
Colony flexion reorients polyps relative to the oncoming flow and shields some polyps
through branch deflection and compression. As the number ofbranches within a colony
increases, the potentia! for polyp shielding increases; thus feeding behavior and local
velocities should differ between large colonies and small colonies for given ambient
velocities. In addition to whole colony flexion, individual polyp flexion functions in
reducing the flow velocities encountered by tke polyp tentacles (Sponaugle, 1991). The
relative contributions of whole colony and polyp flexion to flow velocity reduction varies
with mainstream velocity, with whole colony flexion accounting for relatively greater
velocity reduction at the level of the polyps during higher ambient velocities (see
Sponaugle, 1991).
The importance of flow reduction is indirectly reflected in the feeding success of
P. acerosa measured in the laboratory. The enhanced feeding success at intermediate
flow velocities is typical of some suspension-feeders (e.g, Leonard, 1989), and supports
a prediction based on two theories. First, with increased ambient flow velocities, a
greater number of Artemia should be transported across the colony. At higher flows,
however, mechanical deformation of the polyps should increace the difficulty of capturing and retaining moving particles. Based on these two processes, feeding should be
most successful at an intermediate range of ambient flow velocities. Flexibility, by
damping the influence of ambient flow velocities, should broaden the range of ambient
velocities over which successful feeding could occur. In this study, P. acerosa appeared
DEFORMATION AND FEEDING IN GORGONIANS
133
to have the highest capture rates at intermediate flow velocities, indirectly supporting
the theory that food capture is more difficult at higher flow velocities, and that reduction
of flow velocities at the polyp level is important. Additionally, in areas of bidirectional
flow, flexibility would permit swaying with the surge, which may result in even lower
flow velocities encountered by the polyps (Wainwright & Koehl, 1976). Feeding, therefore, may be further enhanced in oscillatory flow (e.g., Hunter, 1989).
Flexion in gorgonians may therefore not only be a solution to the conflict between
exposing maximum surface area for suspension-feeding while maintaining low drag, but
also between existence in a high flow environment for maximum particle flux and
successful particle capture at intermediate flow velocities. By broadening the range of
mainstream velocities over which successful feeding can occur, the reduction of velocity
fluctuations at the level of the polyp may be the most important consequence of
flexibility.
ACKNOWLEDG EM ENTS
We thank R. K. Cowen, K. Muzik, and two anonymous reviewers for comments on
an earlier dr~ft of the manuscript, Discovery Bay Marine Laboratory for making their
facilities available, and B. Fouke for field and laboratory assistance. Financial support
was provided by a grant to S. Sponaugle from the Richter Fund of the University of
Chicago and NSF Grant 84-06731 to M. LaBarbera.
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