Functional Characteristics of Garrison Bay Organisms

Locomotion     Reproduction     Feeding      Protection


Many organisms at Garrison Bay live epifaunally and infaunally. The muddy substrate provides an ideal habitat for burrowing, a common lifestyle for many polycheates and bivalves.  Evidence of their activity can be seen in surface trails on the loose sediment.  Nemerteans, polycheates, bivalves, and gastropods are among those whose locomotion can be seen by such traces, and characteristics of the trail can even be used to deduce the organism that made it.  Various types of trails seen on the mud surface result from the use of parapodia, muscular foot, cilia, undulatory and peristaltic motion, and mucous secretions.

Paranemertes moving along the sediment

Nemerteans use ciliary action in combination with the secretion of mucous to produce a lubricated surface in which the nemertean can easily slither across. This picture shows the mucous trail left behind by the nemertean as it slides along the sediment.

Nereis using parapodia for locomotion

Polycheates use parapodia in combination with muscles and a segmented body plan for locomotion. This Nereis uses its parapodia for crawling and swimming. The parapodia may be modified into paddle-like structures to facilitate swimming or to ventilate tube shafts. Undulatory and peristaltic locomotion is also commonly found among the polycheates.

Battilaria sp. plowing through the surface sediment
Gastropod snails such as Battilaria are often found "trail blazing" on the sediment surface. The gastropod is propelled by ciliary motion and wave action of the foot.

Cancer magister

The crustaceans found at Garrison Bay include some larger crabs, such as Cancer magister, Cancer productus, as well as a pea crab, Pinnixa sp.  Individuals from each of these species crawl about using their articulated limbs; however, Pinnixa sp. are also known to reside as commensals in the mantle cavities of gaper clams, Tresus capax.

Hermissenda crassicornis

Small nudibranchs like Hermissenda maneuver through the sediment using primarily ciliary motion. They can be found in the lower intertidal zone near boulders.

bird tracks in the mud

Other tracks left in the soft sediment are made by a large number species, including terrestrial vertebrates that feed on resident invertebrates.


[Return to themes] - [Garrison Bay Main Page] - [Home]


Evidence of reproduction at Garrison Bay is widespread.  Gastropod egg masses of at least three species are found frequently throughout the mid- to low-intertidal zone.  Free-spawning polychaetes are common both on the mud surface and in burrows, as evidenced by an oweniid polychaete that we observed spawning in the lab on its day of collection. Several species also brood young, including the bivalve Transenella sp. and the brittle star Amphipholis squamata. Organisms that brood may burrow in the mud or inhabit small, protected microhabitats. 

Each strategy may have particular advantages in the muddy habitat.  Because surface drainage is poor, the moist environment and abundance of hard substrate, in the form of shells and rocks, provide suitable conditions for the survival of egg masses.  However, egg masses may be exposed to several threats at low tide, including temperature stress, desiccation, and predation.  Egg masses have a large gel component that can help to protect them from desiccation, but may do little to relieve temperature stress.
External fertilization may have advantages in this environment due to the relatively calm waters and relatively dense aggregations of organisms, such as burrowing clams.  Some free spawners show specialized strategies to increase the chance of fertilization.  For example, polychaetes of the genus Nereis undergo a life-history transformation where the adult body transforms into a pelagic stage devoted entirely to spawning gametes.
Organisms that utilize asexual reproduction can ensure replication even in the absence of nearby mates.  In some sea anemones, asexual fission or budding allows for the production of dense aggregations of genetically identical individuals, an effective strategy for colonizing small patches of hard substrate.

Egg mass of Searlesia dira

Egg mass ribbons of Haminoea

Globose egg mass of Melanochlamys

The most easily observed reproductive strategy in the bay involves egg masses laid by gastropods. We found egg masses from three species: Searlesia dira, Haminoea vesicula , and Melanochlamys diomedea.
The egg masses of S. dira occurred on the inside surfaces of shells near rocks in the lower intertidal zone. These egg masses are relatively small (the one pictured is about 1 cm total diameter).

Egg masses of Haminoea and Melanochlamys , in contrast, are large jelly ribbons and balloons, respectively, found patchily distributed in the lower intertidal of the bay.

The yellow egg ribbons of Haminoea, about 1 cm wide and several centimeters long, are deposited in aggregations by many individuals, which are hermaphrodites.  These ribbons must be deposited on hard substrates, such as shells (pictured to the right).  M. dirodea lay brown to white colored balloon-shaped masses, about 1 cm in diameter. We found solitary egg masses on the mud surface, sometimes partially buried, and attached to the sediment by a thin tether on one end.

Polychaete worms exhibit a variety of reproductive strategies, including broadcast spawning, brooding, and the laying of egg capsules. Most polychaetes are dioecious, with gametes stored in the coelom, the large fluid-filled cavity within the body.  External fertilization occurs by releasing gametes into the water, through ducts or by rupturing the body wall (as shown in the picture of the oweniid polychaete to the left).  Nereids and syllids have developed epitoky, where a sexually reproductive worm is produced that can swim and join in swarming events, increasing the chances of successful fertilization.

Free Spawning of an Owenidae polychaete

Eggs in cirratulid polychaetes

siphon of the gaper clam Tresus capex
Bivalves also display a number of different reproductive methods. Tresus capex (gaper clams) typically spawn in late winter to early spring, and sometimes throughout the year.  In contrast, the tiny burrowing Transenella sp. brood their young, at varying development stages, in their mantle cavity, and release small juveniles in the summertime.

Sea anemones (Phylum Cnidaria) exhibit different patterns of asexual reproduction.  Some species undergo fission, where one individual will split into two individuals The large frilly anemone, Metridium, observed in abundance in the bay attached at the pedal disk to bits of shell material, display an interesting form of asexual reproduction where new individuals will bud off from the pedal disk..

the sea anemone Metridium


[Return to themes] - [Garrison Bay Main Page] - [Home]


The soft sediment-dwellers of Garrison Bay employ a variety of feeding strategies, including deposit feeding, predation, filter/suspension feeding, and scavenging. Feeding in a muddy habitat presents a unique set of challenges, particularly to predators and filter feeders.  Predators must contend with unstable surfaces that impede rapid movement and suspended particles that can conceal prey.  Sediment in suspension is a significant problem for filter feeders, whose filtering mechanisms can easily become clogged, and for other suspension feeders that may be have greater difficult encountering and distinguishing food particles. On the other hand, mud is an ideal setting for deposit feeders and scavengers because it has a high organic content and, thus, provides an accessible food source.


Subsurface deposit feeders ingest sediment as they burrow, absorbing any nutrients that the surfaces of sediment particles, and then recycling them.  Many polychaete worms, such as those of the families Arenicolidae and Capitellidae, are subsurface deposit feeders. Arenicolids construct U-shaped burrows in which they generate water currents by two mechanisms. First, peristaltic body contractions can be used to actively pump water through the burrow.  Second, buildup of fecal material at one burrow entrance can aid generation of a pressure differential between burrow openings, a la the Bernoulli principle.  Piles of "cleaned sediment" are common near burrow entrances.


A mound of fecal material marks the rear entrance to the U-shaped burrow of an arenicolid polychaete. The front entrance is to the right of the mound.

Click image to see movie of feeding terebellid

Surface deposit feeders collect food particles directly from the substrate surface. This process is more selective than subsurface deposit feeding because individuals can more easily choose which particles to ingest.  The polychaete families Oweniidae and Terebellidae are surface deposit feeders that are largely sessile, inhabiting vertical tubes that they construct out of mucus/sand or mud.  Individuals extend tentacles out of the tube entrance to collect particles from a position of relative protection. When the tentacles are retracted they carry food to the mouth. Watch the movie of a feeding terebellid!

Filter/suspension feeders at Garrison Bay include a number of bivalve species (e.g., Macoma spp. and Tresus capax), brittle stars (Amphipholus squamata ), and a few small crabs (Pinnixa sp.).  Bivalves use extensions of the mantle, called "siphons," to draw water into their body cavities. Then, they collect particles on the ctenidia, specialized gills that are used for both respiration and food collection.  Brittlestars use their arms, or "ambulacrae," as filtering devices. Pea crabs (Pinnixa sp.) live inside the mantle cavities of gaper clams, taking advantage of water currents created by the bivalves to collect diatoms and detritus.

Siphons of the large filter-feeding "gaper" clam
Tresus capax

Shell valve of Protothaca staminea, drilled by predatory gastropod

Predators and scavengers are not especially diverse at Garrison Bay. This is not due to scarcity of prey, but, in all likelihood, to the inability to locate and successfully capture prey in fine sediment. The large crabs Cancer magister and Cancer productus, with their formidable claws, are common at the surface.  Infaunal predators include the nemertean worm Paranemertes sp., which uses its eversible pharynx, armed with a sharp poisoned stylet, to harpoon and stun prey. Some drilling predators such as gastropods are rarely seen, but evidence of their presence is abundant in the form of drillholes in dead shells. Scavengers in the mud include polychaete worms, such as members of the families Nereidae and Phyllodocidae, that swim in search of dead organic material.

A large Cancer crab

a nereid polychaete worm burrowing into the mud


[Return to themes] - [Garrison Bay Main Page] - [Home]


Organisms on the mud flats show different strategies for protection from both physical stresses and predation.  Although rapid flight is a typical response to predators, many invertebrates lack the ability for a rapid flight response.  In such cases, species have adopted a thick shell covering, camouflage, or special defensive mechanisms.

During our observations in the field, we noticed that burrows of the gaper clam Tresus capax decreased in frequency with increasing tidal height (corresponding to increased exposure time during low tide). This may have been in response to dryer conditions at the surface. The deep burrows of Tresus capax, and other bivalves, may be advantageous for protection from desiccation as well as from potential predators.

Metridium sp.

Paranemertes sp.

Hermissenda crassicornis

Special defensive mechanisms include the stinging cells or nematocysts found in anemones. These cells are used as a defensive strategy as well as in prey capture. Hermissenda crassicornis preys upon hydroids and actually incorporates the acquired nematocysts into their own integument as a defensive device. The harpoon proboscis and venomous stylet of nemerteans (e.g. , in addition to being used in prey capture, can also be used as a defensive device against soft-bodied predators.
Shell adaptations such as a thick nacreous layer, a high-spire, and shapes that facilitate suction onto a substrate are found at Garrison Bay.
The thick nacreous layer improves the relative hardness of the shell by aligning the CaCO3 crystals into a cross lamellar pattern that is most resistant to crushing by predators. Increasing the hardness of the shell provides an advantage to predators such as crabs and sea gulls.

Battilaria sp. has a high-spired shell. This shape may allow these animals to retract further into their shell upon encountering predators. On the other hand, limpet shells are dome shaped, which greatly enhances the ability of the animal to clamp the shell to a substrate using the suction of the muscular foot. This prevents predators from lifting the shell and accessing the vulnerable soft tissue.

Battilaria sp.

Lottia pelta

Siphon of Tresus capax
A typical response upon disturbance of the gaper clam Tresus capax, which is abundant at Garrison Bay, is to fully retract its siphon into the sediment. Burrowing deep has multiple advantages, including escape from predators and avoiding the physical stresses of the intertidal at low tide.


[Return to themes] - [Garrison Bay Main Page] - [Home]