Animal Form and Function 2: Mollusks

The next big chapter in the course I teach is on the mollusks.  The most visible component of any seashore’s biota, mollusks are an incredibly diverse group of animals.  One could be forgiven for not knowing in advance that snails, clams, and the colossal squid are about as close together on the tree of life as spiders and lobsters, or humans and pipefish.  Mollusks are, in this way, a classic example of adaptive radiation, in which an ancestral body plan somewhat like a very primitive snail was reshaped into widely dissimilar beasts in response to very different selection pressures.

At the core of those differences is movement.  The various mollusk classes all have very different movement modes available to them, in addition to the more specific differences between lower taxonomic levels.  Cephalopods all have a jet-propulsion mechanism, for example, while most snails pull themselves with rhythmic contractions of their muscular feet.  Many bivalves, such as mussels, are stationary animals, while others are very effective burrowers.  One unexpected bivalve bores into wood and plays a major role in humanity’s maritime history.  Perhaps the most surprising locomotor mode among the mollusks is this one.

Scallops, unlike other bivalves, can clap their hinged shells together rapidly to push water through them and swim.  This whimsical display means that adult scallops can escape one of their primary predators–sea stars–much more effectively than clams, mussels, and other bivalves that have to either burrow away or outlast a sea star’s punishing embrace.

Spectacular for a different reason are the giant African land snails.  Often kept as pets due to their striking appearance, these snails are important agricultural pests in their native lands.  What tends to get my students’ attention is their imposing size.  Unfortunately, my preferred video of one of these behemoths peacefully eating lettuce while resting in its owner’s hand won’t upload properly.

For those who follow lower-tier American football, in particular teams from Washington state, the banana slug might be a more familiar creature:

Yet even these creatures and the tree snails that the Everglades burned over don’t hold a candle to the spectacle of the nudibranchs.

“Nudibranch” means “naked gills,” and these sea slugs have lost the characteristic mantle cavity in which ordinary snails and slugs house their gills.  They instead breathe through the large tuft of tentacles sticking out of their backs–the titular naked gills.  Combined with their typical bold colors, this might make nudibranchs exceptionally vulnerable creatures, except that many of them prey on toxic sponges and/or stinging corals and take those defenses into their own bodies, rendering their anemone-like decorative tufts into venomous turrets.  Others take in algal cells and camouflage among seaweed, mostly giving up on eating thereafter.

The one at the end is a sea hare, a different sort of shell-less gastropod that is often used as a model organism for neurology and animal behavior because of its highly stereotyped, well-understood escape response.

To help finish off any notions my students might have that snails are slow and boring, I show them Glaucus atlanticus, one of several contenders for the title of “real animal that is most likely to secretly be a pokémon.”

This sea slug is pelagic and swims at the water’s surface using its limb-like foot extensions.  Its primary prey is a hydrozoan sea jelly known as Velella, the by-the-wind-sailor, so populations of Glaucus atlanticus accumulate in ocean eddies where all manner of floating organisms often become trapped.  A whole ecology forms in these places, featuring crabs, anglerfish, and seaweeds found in abundance nowhere else.

And then I try to surprise my students with these creatures, the pteropods or “sea butterflies” and “sea angels.”

I challenge them to tell me what mollusk class (cephalopods, gastropods, bivalves, something more obscure…) holds these animals, and they rarely guess correctly.  Pteropods are another highly derived snail, whose shell is reduced to a flexible support structure or missing entirely.  They swim using flaps of the snail’s muscular foot and form a large component of marine snow, along with microscopic radiolarian shells and other organic detritus.  Creatures like these, who have very thin shells, are among the most threatened by ocean acidification, which renders calcareous shells thinner and more difficult to form.

If you’re wondering why I spend so much time on the gastropods, it’s because the lab portion of the course features dissections from the Cephalopoda and Bivalvia, making those groups more effectively covered by the existing material.

With that in mind, I segue into the cephalopods here, in particular the chambered nautilus.  Few students in my course have had the chance to see one of these strange creatures alive, though they all get to handle a sectioned shell and see its internal gas-bladder system for regulating buoyancy.

There are six extant species of chambered nautilus, which represent a much older lineage of cephalopods than the one that gave rise to modern squid and octopodes.  Unlike their more derived kin, nautiluses have dozens of tentacles, none of which bear suckers.  They also have pinhole-camera eyes rather than the elaborate focusing eyes that otherwise characterize cephalopods.  Most Canadian students aren’t personally familiar with these strange and wonderful creatures because they are native to the tropical South Pacific.  They are usually nocturnal predators and maintain a complex migration cycle between deeper and shallower waters.

On the subject of octopodes (never get tired of the Greek plural), this one is particularly impressive.  The giant Pacific octopus also provides a chance to discuss the physiological strengths and limitations of different body plans.

The giant Pacific octopus–wider across at its largest than a person is tall–is a massive knot of muscle with razor-edged suckers, a venomous beak, and a full-body suit of chromatophores and iridophores to keep it out of sight.  The spiny dogfish has spines on its dorsal and anal fins, a mouth made of razors, skin made of sandpaper, and the ability to detect electric fields.  Many sharks preferentially feed on cephalopods, relying on the cephalopods’ extremely limited capacity for endurance exercise to run them down and their soft bodies to make tearing them to pieces relatively easy.  This time, though, the octopus gets the drop on the shark, and takes advantage of the shark’s key weakness: its inability to breathe when it isn’t swimming.  The dispatch is swift, and swifter once the venom hits.

Octopodes are just as formidable at the other end of their size range.  The blue-ringed octopus of Australian seas is venomous enough that bathers who foolishly handle them rarely make it to shore.  Nevertheless, it’s passably common in the marine pet trade, as one of the smallest of its kind.

That vaunted venom does it no good against the speed and strength of the peacock mantis shrimp…but more on that when we get to crustaceans.

Animal Form and Function 2: Mollusks