#intertidal invertebrates

[uncharismatic-fauna]

Slip on the Common Slipper Limpet

The common slipper limpet, also known as the boat shell or the fornicating slipper snail (Crepidula fornicata) is a species of sea snail native to the North American coast of the Atlantic Ocean. In addition, it has been introduced to the eastern coasts of Europe and parts of the Pacific Northwest and Japan. They can reside in a variety of habitats including bays, estuaries, island shores, and rocky intertidal zones; their maximum depth tolerance is 70m (229 ft).

Fornicating slipper snails are noted for their unique mating methods. Adults typically live stacked on top of each other, with up to 12 to 14 individuals in a group. The largest, and oldest adults are at the bottom of the stack, while the younger, smaller adults are at the top. C. fornicata is a sequential hermaphrodite; new adults are all male, and will change into females as they get older or if they become the oldest in a stack of all males.

Breeding can occur between Februrary and October, although the peak season is in May or June. Unlike other marine mollusks, which are broadcast spawners, the common slipper limpet utilizes internal fertilization. The male closest to the female at the bottom extends his penis under her shell and fertilize up to 11000 eggs. These eggs hatch after about 3-4 weeks, and the planktonic larvae are released into the water. These larvae take 4-5 weeks to develop into juveniles, at which point they settle either on bare rock or on top of an established limpet chain. If it settles in isolation, the young adult immediately changes into a female; if it settles on a chain, it remains a male. Adults can live on these chains for up to 6 years.

Adult boat shells are rather small, ranging in length from 20–50 mm (0.7-1.9 in). The shell is distinctly arched, with a flat underside that gives it a slipper-like appearance. The shell can be white, pink, or yellow with red or brown streaks; older adults are often covered in algal growth.

Conservation status: The common slipper limpet has not been evaluated by the IUCN. Although they are commonly harvested for food, populations are considered stable. Outside its native range, this species is considered invasive and harmful to other limpet snails.

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Photos

Dr Keith Hiscock

Sytske Dijksen

[uncharismatic-fauna]

Another Day, Another Pacific Sand Dollar

The eccentric sand dollar, aka the sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar (Dendraster excentricus), are found in the intertidal zone and near-shore sandy bottoms from Alaska, US to Baja California, Mexico.They are the only sand dollars endemic to the Pacific Northwest, though they share the rest of their range with other species. Live individuals are seen either partially buried upright or lying flat on the ocean floor, depending on the strength of the current. To prevent themselves from being swept away, juveniles will also ingest sand to weigh themselves down. Although they are not social, they can form large colonies with as many as 6 sand dollars in a square m (1 sq yd).

Pacific sand dollars are named for their resemblance to silver dollars, especially the bleached exoskeletons that commonly wash up on beaches. Most adults average about 8 cm (3 in) across, though individuals as big as 10 cm (4 in) have been found. The body is a flat disc coated in small, purple tube-like feet and sensory organelles called cilia. The feet are used both for moving across the ocean floor and for pulling oxygen from the water. The mouth and anus-- a single opening-- are located on the sand dollar’s underside. Inside the mouth are five teeth and jaw plates known as doves; together they form a structure known as Aristotle’s lantern, which is unique to echinoderms like sand dollars and sea stars.

D. excentricus is a suspension feeder, using its feet and cilia to pull food from the water or direct it along special groves on the body’s underside. Their main prey are microscopic larvae, copepods, diatoms, algae, plankton, and detritus. The sea-cake is predated upon by a number of sea stars and fish, as well as crabs and sea gulls. To avoid being eaten, adults bury themselves in the sand and larvae will duplicate themselves via a process known as budding and fission, which creates smaller individuals that can distract potential predators.

Although western sand dollars have seperate sexes, they are broadcast spawners. In late spring or early summer, males and females congregate and release gametes into the water where they become fertilized. Larvae, also known as prisms, hatch just a day later. This larvae floats freely through the water, growing arms and metamorphosing into a echinopluteus larva. Once they reach 8 arms, the larva begins to develop an exoskeleton or echinus, and resembles a small adult. The final stage of growth is triggered by chemical cues released by other adults; after this, individuals become sexually mature and settle on the ocean with other sand dollars. In the wild, adults can live up to 13 years.

Conservation status: Although the IUCN has not evaluated the Pacific sand dollar, they are regularly threatened by ocean acidification, warming, and bottom trawling.

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Photos

Chan Siuman

Brian Starzomski

Alison J. Gong

[uncharismatic-fauna]

Superb Black Turban Snails

For beach-goers along the North American Pacific coast, the black turban snail (Tegula funebralis) is a common sight. They can be found from the northern reaches of British Columbia, Canada to southern California in the United States, almost exclusively in the intertidal zone. Although they are small, weighing only 20g and about 5 cm long, their clusters are hard to miss. Groups of hundreds of black turban snails congregate in seaweed beds, under rocks, and in the tight crevices of the rocky shoreline, and disperse only when they are covered by the sea.

    The most distinctive feature of T. funebralis is the shell. It is typically black or purple, but over time the outer layer of chiten can wear away to reveal a pearly inside. These shells are sought after by people for decoration, and by hermit crabs for shelter, although in both cases the shell is typically scavenged after it has already been emptied by predators. The rest of the snail’s body is black, soft, and can curl easily inside its shell to protect itself. As a member of the order Trochida, these snails also have an ‘operculum’; a hard trap door- like structure that the snail can close the aperture of the shell with to further protect itself. Interestingly, turban snails also have a unique organ called a bursicle that can sense chemicals released by their predators; primarily sea stars and crabs. When a threat is detected, the snail may either curl up in its shell or attempt to flee at a racing 8cm per minute. 

    Like many gastropods, black turban snails are herbivorous. Their primary food is algae, especially macroscopic algae like seaweeds and kelp. They compete with other grazers, mainly hermit crabs and urchins, for this food, although it’s not uncommon to see all three in the same area. In turn, black turban snail shells are sometimes colonized by limpets and slipper snails, which feed on the red algae that grows there. Periwinkles or other predatory snails will also sometimes attach themselves to turban snail shells and predate upon them by boring holes through the shell itself, but this is less common. 

    The main driver behind the black turban snail’s abundance is its high reproductive rates. It is a broadcast spawner, meaning that when it is covered by water, both sexes of the snail will release hundreds of thousands of gametes into the water column . Within a day or two, many of these gametes will combine and become microscopic larvae, which then disperse, metamorphose into juvenile snails, and settle in less than three weeks. Juvenile turban snails are a favorite food of sea stars, crabs, and intertidal fish, but the sheer number of young ensures that most will survive. 

    Conservation status: Although black turban snails were and still are a source of food for humans, they are extremely common and are considered of least concern. Their primary threat is ocean acidification and warming temperatures, but they are in turn a much larger threat to intertidal zone community stability when their primary predator, the sea star, is removed and their population grows unchecked.