Cnidarians and Mollusks

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ON THIS PAGE:

Cnidarians:

  • Red Eye Medusa

Mollusks:

  • Heath’s Dorid and Monterey Sea Lemon
  • Hooded Nudibranch
  • Frosted Nudibranch
  • Opalescent Nudibranch

Brachiopods:

  • Transverse Lamp shell

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Red eye medusa (Polyorchis pencillatus)

The red eye medusa gives a pretty good description of what it is through its name. Jellies have two general life stages: the attached phase called a polyp, which looks like a tiny anemone, and the planktonic stage of medusa (the French word for jellyfish is méduse, just to save time). The red eye medusa’s polyp form is pretty much unknown, but the medusa we found, with a tall, jellybean shaped bell and long spaghetti-like tentacles, is fairly common. This species likes to live in eelgrass meadows or shallow sandy areas, where it can stir up the bottom to catch food. The red eye medusa is easy to identify: it has a ring of bright red dots where the tentacles and bell meet. The markings aren’t the eyes themselves, but actually circles around them that help reflect light, giving them a sense of direction. Starngely these jellies may be on decline, unlike other species, although it could also be a seasonal thing, as red eyes have been known to virtually disappear for a year then reappear in enormous numbers later.

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Heath’s Dorid (Geitodoris heathi) and Sea Lemon (Doris montereyensis)

During one of our days on Glass beach, we came across a tiny little nudibranch, pebbly and pale yellow on a sponge-covered rock. According to our ID sheet, it was a Sea Lemon, a citrus-smelling, breadcrumb sponge eating animal that grows up to 15cm. That fit, almost. Our specimen was about 2cm long, with very few, very pale spots, but the right shape and habitat. With more research, I came across Heath’s Dorid (whose other scientific name is Discodoris heathi!) and although it looks extremely similar, is smaller in size, around 4.5cm.Either way, they’re are adorable and interesting! Nudibranchs are picky eaters, many species feed almost exclusively on one species of sponge, anemone or other animal, which makes them more difficult to keep, but also easier to find. Both species are from a group of nudibranchs called dorids, meaning they have a tuft of gills on their backs for breathing. The other obvious appendages are the rhinophores, a pair of quill-shaped horns found on the head, which nudibranchs use to detect chemicals and water currents… All pretty impressive for the marine cousin to the slug!

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Hooded nudibranch (Melibe leonina)

Hiding amongst the algae, opening and closing its gaping venus-fly trap mouth, the hooded nudibranch is a fierce predator, gobbling up tiny crustaceans, mollusks and cnidarians. Its latching “foot” helps it cling to surfaces, but when it wants to, the nudibranch can swim by thrashing back and forth almost gracefully. On its back, lines of cerata filled with nerves allow it to breath and give it a strange lobed look. But beware, predators, although it may look soft and vulnerable, the hooded nudibranch has evolved certain defences to protect itself with. To distract predators while it escapes, the hooded nudibranch will let go of its cerata as a decoy, leaving the attacker with a handful of jello-like protrusions. Even more impressive, it will excrete a somewhat unappetizing watermelon smell, letting potential predators know that it will taste awful. Those more interested in studying the hooded nudibranch than eating it will take note of the spiral of sponge-like yellowish-white eggs that they lay, and that are often guarded by the parent. But make no assumptions about its sex – nudibranchs are hermaphrodites, all the better when you’re a small creature in a big ocean.

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Frosted nudibranch (Dirona albolineata)

It looks a bit like something from the nineties, or a weird new veggie, but it is a nudibranch, and although it might be behind the times, it’s still very cool. The frosted nudibranch is an aeolid, a group of nudibranchs that have cerata for breathing and not gills like dorids. They also have two front feeding tentacles, unique to this group. The cerata with their white ends and clear sliver shapes give this species its scientific name, which means white lined. The rhinophores also have white lines, making them harder to tell apart from the cerata. Some, like the one we found, has a small black area on its dorsal area, above its muscular foot. We found our nudibranch clinging to a dock, surrounded by its favourite food, snails. With powerful jaws, frosteds can break open the shells to get at the animal’s insides. They will also eat sea anemones, small crustaceans, ascidians (colonial tunicates) and bryozoans (moss animals).  Just like the hooded nudibranch, the frosted nudibranch will release its cerata when its stressed.

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Opalescent nudibranch (Hermissenda crasicornis)

This nudibranch we often find when it’s still small- one or two centimetres long- but its colouration makes it stand out quite clearly! The orange-tipped cerata set off by bright blue, all packed tightly together give a jewel-like sheen.  The colours are for more than just show. Many scientists believe they are used to warn off predators, the way many tropical species do. At the end of each cerata is a chamber called a cnidosacs, where stinging cells (stolen from cnidarians) can be stored. By taking the cells from other organisms and keeping them on their back, the opalescent nudibranch creates a defence system without the effort of producing the poison-injectors themselves. This ability, along with the large hammerhead-like tentacles make it easy to identify as an aeolid. For such tiny, delicate-looking creatures, opalescent nudibranchs are surprisingly ferocious. They have been known to fight each other to the death upon meeting. If nudibranchs are ambitious, this species is cut-throat: opalescents live in all sorts of habitats including pilings, mud flats and rocky shores and eat all sorts of other animals including hydoids, anemones and sea pens.

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Transverse Lamp shell (Terebratalia transversa)

It looks a bit like a mollusc. It acts a bit like a mollusc. But is it a mollusc? NO! It’s one of the few members of the very ancient Phylum Brachipoda and should not be confused with any upstart bivalve.  In the Paleozoic era, brachiopods were some of the most numerous and successful species, and only starte3d to decline in numbers at the beginning the Triassic period, when a mass extinction event occurred. The majority of species alive today are found in shallow, cold and calm water, like the underside of the dock where we found our lamp shell. It lives attached to the substrate (or floating objects like the washed up foam where we found ours) using a short stalk called a pedicle to hold on.  The pedicle, which is part of the lamp shells body, enters through a hole in one shell and loops around the inside, where it is called a lophophore. The two valves, which protect the filter feeding body or lophophore within, called the upper and lower shells, are opened and closed by simple muscles. Lamp shells aren’t super common, so it was nice to stumble on to one!

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