My First Loves, The Amphipoda

I must start with the amphipods.  For a moment, I tried to avoid it so that I would not feel so predictable, but I could not let them go.  It was a member from this group, an animal no greater than two centimeters in length, that dragged me into the fields of taxonomy and marine science.

What are these?  Amphipods are crustaceans, close relatives to the ones you might find on a dinner plate such as crabs, shrimp, and lobsters.  The are about 6,000 species in this group.  That means there are more described species of these mysterious things called amphipods than there are mammals.  Amphipods are ubiquitous in the marine realm.  I'll show you some that you can find within 60 seconds of stepping onto the beach and some you will likely never see, unless you have plans to visit the ocean depths.

The beach hopper is one of thousands of different species of amphipods.

Remember walking along the beach, perhaps a little too close to that rotting pile of kelp, when you saw some tiny organisms jump about in front of you? Beach hoppers are amphipods.  I confess that I am no big fan of these particular "pods," but they are among the more conspicuous members of the group.  Of course, they also play a role in making those piles of algae disappear so that you can better enjoy your experience at the beach.  And can you really look at the picture and tell me it's not cute?

Alicella gigantea from deep waters.

If we look at a couple of close relatives from the deep sea, you'll see that not all amphipods are created equal. Alicella gigantea is a touch bigger, reaching lengths of more than a foot.  Aren't you glad that the beach hoppers are the small ones?  Despite their impressive size (for an amphipod), these look pretty bland to me.

Epimeria rubrieques, a poorly known deep-sea species.

There are some other forms that have more eye-catching ornamentation.  Check out the spines on Epimeria, for instance.  I don't know why it needs such ornamentation or what it is doing way down there in the ocean depths, but it has one cool suit of armor (one that presumably remains unseen by its neighbors, as there is no light where it dwells).

If we do head upward, toward the sea surface, we can take a look at another kind of amphipod.  Some species are associated with other floating organisms, like jellies and salps.  "Hyperiid amphipods" actually live inside these larger, floating animals.

Phronima emerging from a salp

Members of the genus Phronima are great examples. You will have to trust me that it looks a touch scarier when you peer at it through a microscope.   Evidently, this animal was the inspiration for the queen in the movie Alien.

Whale lice aggregating on fluke of dead gray whale.

Ventral surface of a whale louse from humpback whale.

I could share many more examples, from amphipods that burrow into kelp to terrestrial forms you might find in soil with potted plants.  However, I will finish this post with mention of some epibiotic forms that drew my attention as a college student and that continue to interest me today.  These are the skeleton shrimp and the whale-lice.  Yes, that's right: whales have lice.  Of course, they're crustaceans, not the insects that normally come to mind when you hear the word lice.  They earned that name because of their unusual life history.  Whale lice are found exclusively on the surface of whales, dolphins or porpoises.  They do not swim at any stage of their life cycle; they spend their entire life clinging to the skin of their blubbery mammalian hosts.  In this lineage, natural selection then has channeled the mutations occurring over millennia into some pretty interesting anatomy.  Whale lice are dorso-ventrally flattened and possess recurved claws on their limbs, for it pays to be able to stay on the whale.  The sometimes occur in large numbers.  Interestingly, a given species of whale louse is specific to a given host species.  For instance, the gray whales that migrate along the West Coast of North America every year are associated with only three species of whale lice, and these three species are not found anywhere else.

A close relative of the whale lice is the skeleton shrimp, so called because of their narrow, elongate bodies.  They also have been referred to as the "praying mantis of the sea" because of their large forelimbs (relative to body size) and characteristic posture.

Rendering of new skeleton shrimp from California.

 There are about 350 species of skeleton shrimp known to science and many more waiting to be discovered.  I have found them living on everything from seaweed to sea grasses to sea stars to stone fish.  They cling to other organisms with their rear limbs and then scavenge for food with their front two pairs of limbs and rasping mouthparts.  To the naked eye, they could pass for pocket lint.  Under the microscope, some look more like Godzilla.  The skeleton shrimp often occur in abundance.  The groom the organisms to which they are attached but serve as food for small fishes.  The exhibit a variety of behaviors that make them interesting to watch, from their quick, inchworm-like movements to male-to-male combat.

Caprella californica.

I just shared seven of the 6000+ species in this group.  We'll call it quits at that. These animals play enormously important roles in marine ecosystems, admittedly most often by serving as links in marine food webs.  Still, with the diversity of their form and the wide array of habitats in which they are found, they make my short list of invertebrate animals that deserve mention.