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The Nuggett is a converted fishing vessel that was used to sample shallow-subtidal bottoms in the San Juan Islands of Washington. The vessel belonged to Friday Harbor Laboratories, University of Washington. If you are lucky, you will get to go on a field trip on a similar vessel and get to trawl for benthic invertebrates, fish, or take a plankton tow.

This dredge provides an efficient way to collect from gravelly bottoms, which often bring up a wide variety of epibionts, such as bryozoans, sea squirts, scallops, benthic prawns, and gastropods. The one pictured at left is a simple iron frame with a steel mesh bag, into which bottom organisms are gathered. It differs little from the dredge used by vessels in the early nineteenth century.

Occasionally the dredge brings up a monster. Also pictured is the carnivorous sea star Pycnopodia helianthoides. Many species are eaten by this voracious predator and many have escape responses that can be induced even by exposure of extracts of this species. Nearly no invertebrate can escape them; they even dig for buried clams! Unfortunately, these sea stars have succumbed in recent years to Sea Star Wasting Disease, along with many other species of sea stars in the Pacific northwest of North America,

Photo by Thomas H. Suchanek

This multi-rayed sea star (left) is often common in shallow subtidal hard and soft bottoms of the Pacific coast of North America. This specimen is about 50 cm in diameter. White anemone at right is Metridium senile. When these starfish come by, the large and normally sluggish sea cucumber Parastichopus californicus gallops off in the water.

 

Subtidal hard surfaces are often covered with colonial animals, including hydroids, bryozoans, and tunicates. Here we see a large number of coexisting colonies of the Atlantic colonial hermaphroditic sea squirt Botryllus schlosseri. The limitation of space here raises questions about the degree of intraspecific competition. It also raises questions of competition and relatedness. Does it make evolutionary sense to fight for space with your siblings? These questions have been addressed recently with the experimental study of colonial systems.

For example, Richard Grosberg and colleagues have produced evidence that colony fusion, when it occurs in newly settled B. schlosseri, seems to occur between close relatives. Fusion is to the advantage of the participants, as a larger colony may be more resistant to overgrowth and may become sexually mature earlier.

Larva and Life Cycle of Botryllus schlosseri. Photo by Richard Grosberg

The larva (left) is the classic tunicate tadpole larva, which does not reside very long in the plankton, nor does it travel very far. The latter increases the probability that settling larvae may be closely related. Studies of allozyme genotypes suggests close relationships among settlers, particularly between those that fuse.

The yellow arrows show the direction of overgrowth of the right colony by the left one. But also note that the left colony is a chimera (fused pair of genetically distinct colonies), which we can tell by the different-colored individuals (bright white and light grey). What you might predict about the relative relatedness of the three individual types that you see in this picture (two in the chimera, one in the overgrown colony).

Here we see a large number of colonies of a bryozoan that occurs in shallow waters of northern New England, but here nearly all of the colonies have apparently fused. Sean Craig has shown that fusion is common in this species, which occurs in shallow waters of Maine, and apparently occurs among individuals that are genetically closely related.

Fusion can be seen on the gross scale, but also in the fusion of many anatomically detailed structures. Nutrients, for example, are transferred between fused colonies.

The symmetry of this colony is deceiving, as it arose from the fusion of two founding zooids, or ancestrulae (red arrows). This colony suggests a strong degree of integration between the two colonies, judging for example by the overall colony form.

An even closer view of two fused colonies shows an apparent different trajectory between zooids arising from each ancestrula (red arrows). The zooid designated by the yellow arrow came from both fusing colonies.

This is an electron micrograph of the skeleton of the encrusting bryozoan Fenestrulina sp., collected in shallow waters of Maine. You can see the pores (black) through which the zooids would emerge. Several egg-containing ovicells can also be seen, above the main zooid chambers.

This hermaphroditic bryozoan is a common member of the shallow subtidal fauna of northern New England and is often found on the fronds of brown seaweeds. It is notable for the wide range of proportions of male (blue arrow) and female (red arrow) zooids. Two of the feeding zooids are designated by yellow arrows.

Bryozoan Overgrowth. Photos by Sean Craig

Bryozoans are often found densely packed on seaweed fronds in shallow waters of Maine. Here we see two cases of overgrowth.

Left: A Celloporella hyalina colony is just beginning to overgrow a Fenstrulina sp. colony (upper right).

Right: A Fenestrulina sp. colony has overgrown a Celleporella colony.

When the bryozoan Membranipora membranacea encounters a predator such as the sea slug Doridella steinbergae it grows protective spines near the periphery of the colony. This shot is a closeup of the edge of a colony. Drew Harvell has found that although the spines defend against the predator, they encumber a cost and overall colony growth is reduced. Thus it is valuable to have this defense “programmed” only when predators are encountered. Otherwise the colony will grow faster and presumably also reach a reproductive size sooner.

On the lower right, you can see the orange cup coral Balanophyllia elegans. Note the bare space between this solitary coral and the surrounding didemnid colonial ascidian. The standoff may be determined by the coral’s stinging tentacles. B. elegans also occurs in the intertidal zone. This shot was taken near the Hopkins Marine Station in Pacific Grove, CA.

The hydrocoral Allopora californica (?) is found commonly on shallow subtidal and lower intertidal rocky surfaces. It can often be found growing in a sheet-like colony, but here it is growing vertically, which is perhaps a response to taking advantage of zooplankton food that can be taken off of the bottom in this relatively quiet-water habitat.

Here in the very shallow subtidal of the outer coast of Washington we see one of the many complex competitive situations, where plants, colonial and solitary animals compete for the same space. Here, an anemone, Epiactis prolifera, competing with a red coralline alga (pink) and a sponge colony (orange).

Yellow arrow points to concentration of defensive stolons with nematocysts

Many species of the colonial hydroid Hydractinia live on the shells occupied by hermit crabs; the relationships are known in some cases to be species-specific. Colonies of Hydractinia are presumably initiated as a hermit crab drags its shell along the bottom, picking up larvae. Space on the shells is limiting and Hydractinia colonies often come into competition for space. At left, we see a typical reaction: stolons armed with dense concentrations of nematocysts (yellow arrow) are produced by the right colony to defend against overgrowth by the one on the left.

This anemone is common on subtidal bottoms on the Atlantic and Pacific coasts of the United States. Note the feathery and delicate tentacles.It feeds upon smaller zooplankton. Individuals often exceed 15 cm in length and may be over a meter in length. Under strong current conditions the feathery tentacles are retracted.

This solitary tunicate (10-15 cm long) is usually far cleaner than this, and internal organs can be seen clearly through the whitish translucent tunic. It is found on lower intertidal and subtidal hard surfaces throughout the world, owing to its transport on ships. It figures importantly in the food of benthic-feeding fishes, such as plaice. It has been a favorite in studies of physiology. This individual was dredged from the sea bed in the San Juan Islands, Washington.

This snail, with its egg case. Fusitriton oregonensis is carnivorous and a common hard substratum resident of the Pacific coast. It feeds principally upon sea squirts and sea urchins. The eggs are laid in a spiral pattern.

The subtidal snail Calliostoma annulatum feeds on smaller colonial invertebrates, such as bryozoans and hydroids. This picture comes from the Channel Islands of California. Corynactis californica, the anemone-like colony to the left, is a corallimorph, belonging to a group of cnidarians that are related to corals.

Nudibranchs are often a common and very lovely feature of subtidal rocky surfaces, sea grasses, and seaweed habitats. They usually feed on colonial invertebrates and Phidiana pugnax feeds commonly on hydroids. It is very aggressive and usually attacks and tears apart other eolid nudibranchs. It is usually 3-4 cm long.

Here the nudibranch Eubranchus rustyus is feeding on the hydroid Plumularia lagenifera. The nudibranch typically crawls along the central axis of each stalk, feeding on the gonophores, which are arrayed along the axis; it also reaches out onto the lateral branches to pick off the feeding polyps. The nudibranchs lay their eggs on the main axis of the hydroid.

This shot, taken in shallow water in the San Juan Islands of Washington, shows several colonies of the hydroid Plumularia lagenifera. They often are exposed to strong flow.

The Garibaldi, Hypsypops rubicundus, is a large (nearly 40 cm) damselfish common on rocky subtidal reefs of southern California and Baja Califonia. During the spawning season, males defend an algal nest within permanent territories on which females deposit eggs. Females prefer males that are defending nests, which may be a key to the fitness of the preferred males, as compared to others that have no eggs.

The scallop Chlamys hastata is common in subtidal gravel bottoms of the San Juan Islands in Washington. Most are covered on the left (upper) valve by a sponge, Myxilla incrustans. While the shell gives the sponge an excellent hard substratum to which it can attach, one might expect that the scallop suffers a disadvantage, since the sponge coating would weigh the scallop down and alter the scallop’s airfoil shape, all of which would reduce the escape swimming velocity of the scallop. Normally, swimming is a major means of avoiding starfish predators. Experimental research by Steve Bloom demonstrated that the sponge coating allows the sea star Evasterias to grip the shell, but it soon lets go. Either a noxious secretion or perhaps the sponge’s spicules is a deterrent to the starfish, so the sponge confers a direct anti-predatory benefit to the scallop. If the scallop is wired shut and the sponge is scraped away the starfish quickly grips the scallop successfully and eats it. The swimming reponse of the scallop may allow escape of the sponge from its own nudibranch predators.

Parastichopus californicus is a large sea cucumber living off the Pacific coast of North America, often exceeding 30 cm in length. It feeds on sediment by means of an anterior crown of feeding tentacles and absorbs sugars and amino acids across its gut wall. In the winter, when food is scarce, it does not feed at all and actually resorbs the entire digestive system. Although usually sluggish it responds to sea star predators by vigorously contracting the longitudinal muscle bands, which allows it to “gallop” away in the water.

Cnemidocarpa finmarkiensis is a common subtidal solitary tunicate only a few cm high. It is often found in crevices.

The Wolf Eel, Anarrhichthys ocellatus, is common in crevices of eastern Pacific rocky reefs and on sandy bottoms. They feed on crabs and starfish, which means that the Patiria in the picture is in mortal danger! Wolf eels are strongly territorial and probably do not move far from “home” to hunt prey. The starfish often includes seaweed and smaller invertebrates in its diet.

Photo by Ken Lohman, Courtesy of Friday Harbor Laboratories

Tritonia diomedea is occasionally abundant in shallow subtidal bottoms of the eastern North Pacific. It preys upon sea pens and other benthic cnidarians.

Tritonia diomedea under flow. Photos by Jim Murray

Subtidal sea slugs often face strong bottom currents. While they are elegantly adapted to moving upstream to locate prey, an especially strong current can lift up a slug and carry it to an unsuitable habitat. The top diagram shows the sea slug Tritonia diomedea with a fully expanded structure known as a veil. In the bottom photograph, the slug has been placed in a flume. As currents increase, the veil is withdrawn, because it would otherwise generate intolerable drag. The animal also hunches up, which improves its static stability in the flow. Tritonia can readily orient and approach its prey, which includes sea pens.

The blue crab, Callinectes sapidus, is a voracious predator, common in estuaries on the east and Gulf coasts of the United States. Even though it is much faster than a sea slug, it has the same problem of locating prey in an environment with sometimes complex flow. These crabs can prey on shallow-burrowed crabs by following the odor plume that leaves the clam’s siphon. But is the odor plume a good signal?

What’s the funny device on the crab’s back? It’s a radio transmitter to allow an investigator to locate the crab.

The two photos at left are dye-stained simulated odor plumes. The top plume emanates from a simulated bivalve siphon hole in a low flow environment. If the current is relatively weak, the flow is laminar and the odor plume maintains its integrity. If the current is stronger, or if the bottom is rough (as in the photo below) then the flow is turbulent, which presents a more complex signal that might be difficult for the crab to follow. Work by Richard Zimmer-Faust, Dean Pentcheff and David Wethey shows that odor flow in the field, at least in estuaries within which blue crabs live, is often in a rather quiet and laminar regime.

These plumes were created in a plume, using dye-stained water emanating from a model clam siphon hole. Upper panel: low flow; Lower panel: higher flow on a rough bottom.

Sea pens live in soft subtidal sediments and have a muscular peduncle that is inserted in the sediment. When bottom currents are strong, or when a predator approaches, the crown of polyps can be withdrawn below the surface. The fan shape array allows the polyps to intercept water laden with smaller zooplankton prey.

The work of Charles Birkeland on subtidal soft bottoms of Puget Sound, Washington, demonstrated that the starfish Hippasteria specializes on the sea pen Ptilosarcus gurneyi, which may be limiting when the sea pen population is at low density. Other sea pen predators, such as Mediaster and Dermasterias are more generalized and consume other prey species when Ptilosarcus is rare. The top predator in this subtidal food web, Solaster, consumes all sea stars but Hippasteria, and may be repelled by its large sessile pedicillariae.

See Birkeland, C., 1974, Ecological Monographs, v. 44, pp. 211-232

This is a common shallow subtidal Pacific sea star that has close relatives in the deep sea. It has a dorsal membrane that helps enclose a dorsal chamber within which embryos develop into lecithotrophic larvae and then disperse in the plankton. Its ancestors probably used the dorsal pouch to brood young sea stars.

Pteraster tessellatus is a sluggish sea star, but it is well defended against predators, such as other sea stars. When disturbed, it rapidly produces mucus, which contains saponins that chemically deter predators. This starfish was removed from a container of water and secreted all the mucus seen in a matter of seconds.

We tend to expect that species and even larger taxonomic groups have strongly restricted morphologies, such as the 5 rays typical of many sea stars. But many individuals violate the rules, in too many instances to believe that some sort of strong developmental program prevents the “wrong” number of rays. Starfish as seen at left are the exception that proves the rule that the usual number of 5 is probably maintained by Darwinian natural selection.

The sea star Solaster dawsoni (on top) is a voracious predator, and commonly attacks its congener Solaster stimpsoni. It takes several hours to a day for one to subdue another. Solaster dawsoni is so voracious that one literally cannot keep it with most other invertebrates in an aquarium.

Here we see the spiral egg strips of a nudibranch (center) that have been laid on a red sponge. It is common for nudibranchs to consume a sponge and to sequester a pigment that gives it and its egg strip the exact same color as the sponge. It is interesting that a prey organism such as a sponge is also a substratum for egg laying. Why do you think that this should be?

This species is found commonly in shallow subtidal bottoms and intertidal pools of the Pacific coast of North America. Like other aeolid nudibranchs, this slug has prominent dorsal cerata, which are brightly colored in this species (this specimen is about 3 cm long). The cerata include several organs including branches of the digestive gland, but they are often used for defense against predators. This species feeds on a wide variety of smaller sessile prey, but when they feed on hydroids, they can deploy the hydroid’s nematocysts in the tips of the cerata, and these will fire when a predator bites them.

This littorinid snail (less than 1 cm long) is found commonly on various substrata in the shallow subtidal of the eastern Pacific. When on eel grass, it scrapes microalgae from the surface, but when it is on kelp or sea lettuce, it punctures the seaweed with its radular teeth and scrapes material into its mouth.

Lacuna vincta – Different Teeth for Different Foods? Photos by Dianna Padilla

Work by Dianna Padilla and her students demonstrates that exposure to different food types results in different types of radular teeth. Blunter teeth (left scanning electron micrograph) are found when the snail is on eel grass and pointier teeth (right) are found when the snails are on seaweeds. The need to puncture seaweeds selects for the growth of pointier teeth. This response has a strong non-genetic component, since a snail can change its radular teeth when switched from one plant substratum to another. Such switching is possible because the radula is regenerated every few days.

Wood-Boring Bivalve Bankia. Photos by Rudolph Scheltema

Very few marine animals are capable of boring into wood. Species of Bankia and Teredo both use a combination of mechanical rasping and secretion of cellulytic enzymes to work away at the wood. These bivalves get carbon by means of their cellulase enzymes, but they have symbiotic nitrogen-fixing bacteria that help to gather nitrogen.

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