Animal Form and Function 7: Echinoderms and Chordates

As my dissection course draws to a close, it naturally begins moving toward the clade of animals most familiar to my students: the chordates.  On its way there, it visits the chordates’ closet living relatives, the echinoderms.  Students usually need some convincing that echinoderms and chordates and echinoderms have anything in common, because the adult forms do indeed have no real similarities.  The evidence of the kinship between these groups, and between the vertebrates and the other chordates, is mostly genetic and embryonic.  These highly divergent animals have a number of highly improbable similarities in the way their embryos form and the anatomy of their larvae, revealing that their adult forms are highly specialized rather than ancestrally distinct.

I usually start with the primitive chordates, because they’re somewhat less spectacular in shape than the echinoderms.  The students get to see whole lancelets as well as numerous sections through them, so this sister group to the vertebrates is well known to them.  The other non-vertebrate chordate group is the highly underrated Urochordata.  These animals have all of the classic chordate characters as larvae but lose almost all of them as adults.  All urochordates are filter feeders, but they have highly dissimilar means of realizing that lifestyle.

One subset, the larvaceans, are pelagic.  These animals construct a mucous net around their bodies with a well-defined set of openings at both ends.  They consume it as it becomes clogged with plankton, and their mucus is another important component of the organic detritus that feeds the deep oceans.

A larvacean inside its speckled mucous net

Another rather imposing urochordate is the salp.  These animals are colonial, building tube-shaped pelagic colonies that can be many meters in length and comprise hundreds or thousands of individual animals.  Each animal is about the size of a mandarin orange.

The best-known urochordates are the group affectionately dubbed “sea squirts” for their habit of spitting water at beachgoers who poke them.  This is not a nice thing to do, as sea squirts on land are generally waiting for the next high tide and rely on that water to breathe until then, but it does tend to distract and confuse creatures that might otherwise try to pry them from their rocks.

Small, dull temperate sea squirt adhered to rock.

Other sea squirts are not quite as bland as this typical example, and instead show the radiant colors that typify tropical reef biota.

Colony of radiant blue sea squirts on rreef.

Opposite to the chordates, echinoderms began with other lifestyles and developed filter-feeding habits late in their evolutionary trajectories, echinoderms got their start as filter feeders.  The well-known modern groups only later developed the predatory and substrate-feeding strategies for which they are known.  The quintessential filter-feeding echinoderm is the crinoid, or sea lily, whose kin dominated entire reefs in the Paleozoic era.  This creature consists of a forest of feathery arms on a long calcareous stalk that serves as a holdfast.  Modern sea lilies live only in deep oceans, rarely seen by unaided human eyes.

Unlike the sea anemones it superficially resembles, the sea lily has very limited movement modes.  Its stalk is made of its skin and body wall, and has no locomotor functions.  If a sea lily is tipped over or desires to find a new home, it must drag itself along the seafloor using its arms, a somewhat pathetic sight.

Not all crinoids are so limited.  Tropical reefs have their own crinoids, known as the feather stars and lacking the sea lilies’ distinctive stalks.  These brightly colored animals are, like their deep-sea kin, filter-feeders.

Unlike their deep-sea kin, they are powerful swimmers, flapping their dozens of arms in waves to paddle about and using their long, central tube feet to hold themselves in place when they arrive.

Both more familiar and more alien are the basket stars.  Kin to the brittle stars that populate many marine aquaria, the basket stars have arms subdivided many times over into a huge net of tentacles that they use to seize small crustaceans and bring them to their central mouths.  The one in this video has been fed brine shrimp and sped up to show the movements more clearly.

It is an absolute travesty that these three animals aren’t better known. Such weirdness must be broadcast across the world.

The animation of this creature is raised dramatically in the brittle stars.  So named because of the ease with which their arms break when the animal is grasped roughly, these creatures are much more active feeders than other echinoderms.

With their lashing arms, brittle stars can move about at surprising speed, changing directions instantly and wrapping their arms around prey to bring them inward.  Around large food items, groups of them can form tangled feeding frenzies, each trying to drag the entire item away to feed at its leisure.

And then there are the classic representatives.  Sea urchins are dealt with in detail in class, so I don’t generally feature them in my supplemental presentation.  Sea stars, though, are inescapable.  Though the main dissection in this session is a sea star, the course materials are conspicuously silent on the sea star’s most famous feature, its highly distensible stomach.  This striking video not only shows the sea star’s external anatomy in great detail, highlighting the dermal branchia, spines, and sensory tube feet…

…but allows us to watch the animal insert its stomach into its prey mussel and digest it inside its own shell.

Another, less intense look at the stomach is here, on a scavenging sea star that dissolves the mulm off of its aquarium glass.

Sea stars get a lot of attention even outside of biodiversity surveys like this one because they are often highly important to their ecosystems.  In tide pools, they are often top predators whose pressure on mussel populations keeps the mollusks from completely crowding out everything else.  Their low metabolic rates also mean that they are quite capable of adapting to very low-energy environments, such as the Antarctic shore.  Everything moves slowly when the temperature is consistently near the freezing point of freshwater, or colder.  Here, though, sea stars face an altogether more dangerous hazard.

When sea ice forms, it excludes from its expanding crystals the salt in the water.  This creates a plume of hypersaline water with a lower freezing point immediately adjacent to the new sea ice.  This water then sinks to the bottom and cools the adjoining ordinary seawater to freezing (and creates more hypersaline water…) all the way down.  The end result is a snaking tendril of underwater sea ice that eventually reaches the bottom and spreads across the seafloor until its cold is completely dissipated.  Eventually, the ice is broken up by waves or melts on its own, but that generally takes too long for the sea stars and other scavengers trapped by the speeding cold.

That would be a satisfying ending note, but it wouldn’t be the echinoderm lab without a nice dose of gross and weird.  The sea cucumber fits that bill nicely.  The sea cucumbers are a surprisingly diverse lot, featuring pelagic, substrate-feeding, and roughly sessile filter-feeding members.  Some of them look like animate feces, others like obese flowers, but one thing that almost all of them have in common is a truly bizarre defense mechanism.

Sea cucumbers are far too slow to run from predators and usually lack long, easily shed limbs like other echinoderms, so they defend themselves by dumping organs at their predators through their anuses.  In the most primitive sea cucumbers, these are usually the intestines, and the predators are, with any luck, satisfied with eating those and leave the sea cucumber alone to regenerate.  More advanced sea cucumbers instead excrete specialized sticky tubules that clog up a fish’s gills and otherwise put the predator in danger, or fill those discarded intestines with poison.  The key to this strategy is the echinoderms’ famous regeneration, which enables the sea cucumber to regrow the lost material in a few days or weeks the same way a sea star regrows its lost arms.

Few vertebrates can match that healing factor, but vertebrates have more than enough quirks of their own.  Those, we’ll see next time.

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