Feeding Differences between Small and Large
Individuals of the Sea Anemone Metridium Senile
Jennifer E. Purcell
Hopkins Marine Station
of Stanford University
June 7, 1975
Running title: Feeding Differences in Metridium
Direct all proofs and correspondence to:
Feeding Differences in Metridium
Metridium senile is an abundant anemone of circum-
polar distribution. On central and southern Californian
shores it occurs in large numbers in protected bays and
harbors (Hand, 1955). Much of the earlier work on M.
senile is summarized in Stephenson (1935), and the num-
erous papers of Batham and Pantin (e.g. 1950, 1950)
cover many aspects of structure and function.
Previous studies include some work on the feeding
behavior in Metridium. Parker (1898, 1905, 1917) des-
cribed reactions to food, ciliary currents and their
reversal on the lips and pharynx, and fatigue in the
feeding response due to repeated stimuli, but made no
investigation of the natural diet. Stephenson (1935),
echoing the opinions of Elmhirst (1925) and Pax (1928),
assumed that Metridium are zooplankton-feeders. This
conclusion, reached also by Hand (1955), is based solely
on the following laboratory and experimental observations.
1) Metridium accepts macroscopic food less readily than
other anemones. 2) They live for years in flowing
seawater aquaria with no food other than the plankton
in the water. 3) Large individuals reject or respond
sluggishly to macrofood. Elmhirst noted that large
Metridium reacted reluctantly even to copepods and crab
larvae in the lab. Small individuals accepted pieces
of mussel and fish readily, but seemed not to depend
Feeding Differences in Metridium
on larger food. Hand attributes this "change in food
habits" with body size to the intertidal distribution
of the animals; small anemones live near shore where
the cresting waves carry larger food to them,, while
the larger anemones, living in deeper water receive a
constant supply of plankton.
No studies have yet been conducted to determine
the natural diet of Metridium of any size, or to sys-
tematically explore the feeding habits to test whether
the feeding preferences of small and large animals in-
dicated above are real and consistent differences.
This study was undertaken to discover the natural foods
of large and small individuals of Metridium.
Habitat of the Study Animals
The research was conducted at the Hopkins Marine
Station of Stanford University in Pacific Grove, Cal-
ifornia during April and May 1975. Animals were collected
from Monterey Wharf #2. There is a rough distribution
in size with smaller anemones tending to be higher on
the pilings and closer to shore. The very large
Metridium (20-35 cm in column height) occur only at the
end of the wharf within a few feet of the bottom. Small
individuals (less than 6 cm) intermingle with the
Feeding Differences in Metridium
intermediate sizes (10-15 cm), and are often densely
arranged on the pilings. The largest animals tend to be
more solitary. However, all of the smaller and inter-
mediate sized specimens used in the present work were
collected from areas immediately around large individuals.
Morphology of the Tentacular Crown
A description of the oral disc and tentacles of
the different sized individuals of Metridium is essential
to understanding their feeding behavior (Fig. la, b, and c).
The smallest animals (less than 3 cm in column height)
have an unlobed disc bearing tentacles about 1 cm in
length, arranged in only a few rows. From this size
through the intermediate sizes the disc becomes larger
and progressively more lobed; tentacles remain the same
size but many more rows of tentacles are added.
There is a hiatus in the continuum of both complexity
and size between the intermediate and the large animals.
In the largest Metridium the disc is elaborately lobed
with six to eight petal-like extensions. The tentacles
remain about 1 cm in length but are very numerous es¬
pecially at the margins of the disc and its lobes.
Several rows of larger tentacles surround the mouth and
extend partially down the center of each lobe. The
Feeding Differences in Metridium
opposing edges of each disc lobe are usually held fairly
close together, forming a solid thicket of tentacles
on top and a conical space below between tentacles and
disc in each lobe.
Feeding Mechanisms in Metridium
Reactions to different sizes and types of food were
observed in the various sizes of Metridium. Smaller
food was dispensed by a Pasteur pipette at a distance
such that the prey did not swim away but also was not
swept into the tentacles by the force of ejection.
Larger foods were presented with forceps. Foods pre-
sented included Artemia larvae of two sizes (1 mm and
1.5-2 mm long), Artemia adults (1 cm), the copepod
Tigriopus californicus (1.5 mm), and pieces of the squid
Loligo opalescens and the mussel Mytilus californianus
(1 cme). Responses were observed to food placed on the
tentacles at various distances from the mouth of the
anemone. The reactions of anemones 3-15 cm in height
were similar, and these animals are treated as one group,
Newly hatched Artemia larvae (1 mm) were not caught
by the small Metridium, but swam among the tentacles
and were seen to touch them with no apparent effect on
either animal. Slightly larger Artemia larvae (1.5-2 mm)
Feeding Differences in Metridium
were killed and caught readily with one tentacle, as
were Tigriopus of the same size. The method of transfer
of food to the mouth differed according to the distance
of the food-catching tentacle from the mouth. Tentacles
near the mouth carried the prey by ciliary current to
the tentacle tip, then bent letting the food fall into
the mouth. Tentacles on the edges of the oral disc
bent medially until the prey touched another tentacle
where it attached. It was then released from the first
tentacle and the process repeated until the food reached
an inner row of tentacles; ciliary currents then carried
it off the tentacle tip into the mouth.
Large Metridium responded differently to Artemia
larvae and Tigriopus. These foods were always caught
and held with one tentacle. The prey remained attached
to the tentacle, no movement toward the mouth occurring.
Shortly after capture, the anemone closed completely.
For small Metridium, the capture of active Artemia
adults required the participation of a localized group
of tentacles. The portion of the anemone's disc involved
in capture increased the larger the prey was in relation
to the size of the anemone. After food capture the
contracted group of tentacles relaxed and the prey re-
mained attached to one tentacle. The prey was then trans-
ferred by being passed medially from tentacle to tentacle
Feeding Differences in Metridium
and finally dropped into the mouth.
When adult Artemia were fed to large Metridium
the response was restricted to the few tentacles involved
in capture,, or to a fraction of a lobe of the oral disc.
The opposing margins of a lobe pressed together enclosing
the prey. The tentacles then relaxed and and the food
was passed among adjacent tentacles toward the mouth.
The prey was transferred to the larger central tentacles
which deposited the food in the mouth. Often the Artemia
was moved randomly and sometimes it was discarded from
the edge of the disc.
Small Metridium fed squid and Mytilus pieces, caught
them with a group of tentacles. The entire group of
tentacles then moved the food toward the mouth, the
oral disc contracting until the food reached the lips.
Food transfer did not seem to involve ciliary currents
of the tentacles.
When squid and Mytilus were fed to large individuals,
the opposite sides of the disc lobe stimulated converged
to enclose the food. The tentacles immediately surrounding
the food began to wave, and a mucus coating with bubbles
was produced around the food. Food and mucus were
passes between tentacles into the cavity formed by the
sides of the disc lobe where it was hidden from view.
The food appeared again near the base of the lobe and
Feeding Differences in Metridium
from there slid rapidly into the mouth. The sliding
may have involved both gravity and ciliary action.
In general, the small Metridium do not respond to
the smallest food, perhaps due to insufficient mechan-
ical stimulation of the tentacles. They efficiently
consume larger food, the extent of response increasing
with the size and degree of activity of prey. Large
anemones can easily capture numerous small food animals,
Much more movement and time are needed to handle larger
food. The great number and the arrangement of tentacles
and the behavior of large Metridium are adaptive toward
feeding on numerous microfood particles while the small
anemones can manage pieces larger in relation to their
body size. The deciding factor seems to be the distance
between the mouth and tentacles and the corresponding
difficulty in transferring particles to the mouth.
These conclusions are in agreement with the observations
made in previous studies.
Natural Foods of Metridium
In order to determine foods consumed by Metridium
under natural conditions, animals were collected in the
field, returned to the lab, and settled in clean aquaria
provided with filtered running seawater. Batham and
Pantin (1950) state that Metridium produce fecal pellets
within 48 hours after feeding. In the present study,
Feeding Differences in Metridium
animals freshly brought from the field defecated in be-
tween 24 and 48 hours. Pellets from 50 small, 15 inter-
mediate sized, and 18 large Metridium were examined under
a dissecting microscope. The pellets contained detritus
and food animals in various stages of digestion. Re¬
cognizable remains of organisms were visually separated
from the unidentifiable detritus. When necessary for
identification, the remains were mounted in glycerine
and examined under a compound microscope. Relative
abundance of the different species or taxa was deter-
mined from counts and from qualitative estimates of their
percentage of the total biomass. To determine foods
available, material was scraped from the pilings adjacent
to the site where anemones were collected, and plankton
tows were taken at the collecting site. These collections
were examined, and relative abundance of organisms est-
imated as above. Phytoplankton, unrecognizable detritus,
and sand, abundant in the water, were not included as
food sources.
The small anemones each excreted several tiny pel-
lets and a dark reddish-brown mucus. The total volume
eliminated by each animal was approximately 0.1 ml, con-
sisting of 65% mucus, 30% detritus and 5% animal matter.
Large Metridium produced one or a few sizeable pellets,
total volume for each animal averaging 1.4 ml. The
Feeding Differences in Metridium
percentage of detritus ranged from 40 to 60%. One day
later the large anemones regurgitated large amounts of
mucus containing bubbles which floated to the surface.
Microscopic examination showed that mucus from both sizes
contained high populations of bacteria. Occasional
undigested phytoplankters and foraminifera occurred.
The relative abundance of the organisms most fre-
quently encountered in the environment and in the fecal
pellets of small and large Metridium are compared in
Fig. 2. The diet of small Metridium consists mainly
of the three types of worms from the pilings. The re-
mainder also consists largely of immediately adjacent
benthos. In sharp contrast, the food of large Metridium
represents mostly planktonic forms. However, the diet
here showed a greater variety; additional food organisms
not indicated as utilized by large Metridium in Fig. 2
due to their rare occurrance, include crab chelae, shrimp
appendages, gastropod larvae, tunicate tadpole larvae,
and barnacle nauplii. Fecal pellets from both large
and small anemones included sponge fragments and spicules,
crustacean fecal pellets (possibly from the guts of di¬
gested prey), and the exoskeletons of barnacle appendages.
Most of the barnacle exoskeletons were probably molts
captured by the anemone, however fresh mouth parts and a
barnacle ovigerous lamella were also found on occasion.
Feeding Differences in Metridium
10
A partially digested barnacle was seen being extruded by
one small Metridium in the field. Also included in the
pellets of large and small anemones were variable frac¬
tions of sand, barnacle and mussel shell pieces, sea
urchin spines, anemone mesentarial filaments, and in
the large individuals, several fish scales. One large
anemone extruded intact chiton plates in the lab.
Clearly there is a marked difference in the sources
of the organisms eaten by small and large Metridium;
benthic and pelagic diets, respectively, are indicated.
Even most of the scattering of benthos including bryozoan
and hydroid fragments taken by the large anemones could
have been captured waterborne. Pieces of mussel and bar-
nacle may have been broken from higher on the pilings
and cascaded down. The food of the small Metridium is
found on or in close proximity to the surface of the
pilings. Certainly part of the dietary difference be-
tween large and small anemones relates to the distance
of the tentacles from the substrate in each case. In
a typical feeding posture, the animals are bent upword
from horizontal on a vertical face (Fig. 1d-e). Here
the tentacles of small anemones are close to the pilings,
less than 2 cm away and often brushing the surface.
The tentacles of large Metridium are usually at least
15 cm distant from the pilings. The same general argu-
ment holds for anemones on horizontal surfaces.
Feeding Differences in Metridium
11
Previous investigators have assumed that large and
small Metridium may select foods of different size.
This may indeed be true to a degree, but present results
indicate the two size classes also geed on the organisms
most readily available for capture. Crab zoea larvae
bulk large in the diet of large Metridium compared to
their abundance in the plankton at the time of sampling.
This may not reflect selectivity, but merely a greater
number in the plankton when the anemone fed than at the
time the plankton tow was taken.
Summary
1. Studies were made of the feeding mechanisms and
sources of food in natural populations of small and
large Metridium senile on wharf pilings in Monterey,
california during April and May, 1975.
2. Artemia larvae and adults, the copepod Tigriopus,
and squid and Mytilus pieces produced different responses
in the small and large anemones when dispensed to tentacles
at various distances from the mouth.
Small Metridium (3-15 cm in column height) captured
food larger than 1 mm, the number of tentacles and por-
tion of the oral disc involved increasing with increased
size and activity of the prey.
Feeding Differences in Metridium
Large Metridium (20-35 cm in column height) cap-
tured Artemia larvae and Tigriopus and held them in the
tentacles, closing completely later. Larger prey re¬
quired more time and tentacular movement to be trans-
ferred to the mouth.
The large Metridium seem ideally adapted for cap-
ture of numerous planktonic organisms and the small
anemones seem better suited to larger prey in relation
to their body size.
3. Microscopic examination of the facal pellets pro-
duced in the lab revealed their contents to be mucus,
bacteria, animal remains, and unrecognizable detritus.
Small Metridium utilized mainly benthic organisms
from the wharf pilings as food. Nematode, polychaete,
and nemertean worms constituted the bulk of their diet.
Food organisms of the large Metridium consisted
largely of zooplankton. Copepods and crab zoea larvae
were the most abundant organisms in the plankton and in
the diet of the large anemones.
These results show a marked difference in the food
sources of the two sizes of Metridium. Feeding in each
case appears to be on those organisms most readily avail-
able for capture. The tentacles of small Metridium are
in close proximity to the surface of the pilings and its
characteristic benthos. The large anemones, with
12
Feeding Differences in Metridium
tentacular crowns distant from the pilings, are well
adapted to capture of planktonic organisms.
Acknowledgements
My warmest thanks go to Dr. D.P. Abbott and Chuck
Baxter for numerous suggestions, identifications and
a great deal of time patiently and cheerfully given.
13
Feeding Differences in Metridium
Literature Cited
Batham, E.J. and C.F.A. Pantin. 1950. Phases of Activity
in the sea anemone Metridium senile and their re-
lation to external stimuli. J. Exp. Biol. 27:
77-99.
1951. The organization of the muscular
system of Metridium senile. Quart. J. Microscop.
Sci. 92:
27-54.
1925. The feeding habits of the sea anem-
Elmhirst, R.
one, Actinoloba. Scot.Naturalist 12, No. 3: 149-
152.
1955. The sea anemones of central Cal¬
Cadet.
Hand,
Wasmann J. Biol. Part 3(13: 189-251,
ifornia.
1955).
The reactions of Metridium to food
Parker, G.H.
1896.
and other substances. Bull. Mus. Comp. zool.
Harvard 29: 107-119.
1905. The reversal of the effective stroke
of the labial cilia of sea-anemones by organic sub¬
stances. Am.J. Physiol. 14: 1-5.
1917. Actinian behavior. J. Exp. Zool.
221 193-229.
Pax, F. 1928. Anthozoa. Die Tierwelt Deutschlands,
Jena, Tiel 4: 189-240.
Stephenson, T.A. 1935. British sea anemones. Ray
Soc. London 429p.
Feeding Differences in Metridium
Figure Captions
Figure 1 Diagrams of Metridium senile. a-c Oral views
showing outline of disc and arrangement of tentacles
(Circles indicate approximate positions of tentacles
not shown). a) small individual (less than 3 cm in
column height). b) intermediate size individual (10-
15 cm). c) Large individual (20-35 cm). d-e Typical
feeding postures of small (d) and large (e) individuals.
Stippling represents capitular tissue.
Figure 2 Relative abundance of organisms in the environ-
ment and in fecal pellets of small and large individuals
of Metridium senile. p - present; quantity small, or
difficult to measure.
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