The term “pods” to most people more than likely conjures up images of those popular outdoor storage units. Savvy readers of this UAB in A website know that it is our short-speak for amphipods. But to other biologists I know, it refers to another type of crustacean and close cousin of amphipod called isopod.
Odds are really good that you are familiar with isopods. You know those roly polys found in and under decaying vegetation out in your backyard? Those are terrestrial isopods. You may also know them as pill bugs or wood lice.
The name isopod dissects to mean “same footed”. All seven pairs of isopod legs are of the same size and shape. (Sorry, there are many exceptions to this rule…) This is in contrast to cousin amphipod whose name you may recall means “other footed” and in my previous pod entry I introduced you to some of the different types of amphipod legs (gnathopods, for instance).
Amphipods as you know are flattened side to side. Isopods are flattened top to bottom or in bio-speak dorsal-ventrally. This body form results in a more benthic, bottom dwelling lifestyle than that of the amphipods which are more capable swimmers due to their more streamlined body shape.
At the right are images of two types isopods that are often bycatch, accidentally gathered in our seaweed collections. One is the diameter of a pencil eraser, a serolid isopod. This tiny guy is so cute! Look at those eyes peering out at you! Don’t be deceived, those eyes take in a huge field of vision such that the serolid does not need to move its head. I like watching these guys walk across the bottom of the dish because every now and then, the antennae (now hidden) will sweep across the body like windshield wipers. This apparently in nature, and not under a microscope, keeps the dorsal surface of the isopod clear of sediment.
The serolid body is not completely flattened to the bottom but raised up in the center in part to keep the gills out of the sediment. Opposite the eyes is a clear area, sort of a vent out which the water exits the body after crossing the underside gills. Serolids are sort of like roly polys in that they are scavengers.
The raised off the bottom body plan of serolid isopods not only keeps gills from getting clogged with sand, it also helps mom keep her incubating eggs free of sediment. Yup, the serolid at right is a mommy-to-be incubating half a dozen teeny tiny eggs I did not want to disturb by trying to get her upside down to photograph.
At the extreme size to our small serolid requiring a microscope to photograph is the palm-sized spiny Glyptonotus antarcticus. The image at the right includes a quarter-size limpet (type of snail) for a scale bar. This trilobite or cockroach-looking critter is the largest shallow water isopod in Antarctica reaching 4-6 inches long! It is a predator and may fill the ecological role of crabs in warmer ecosystems. During the day it hides under rocks or at the base of algae, coming out at night to feed on whatever it wants, it is an omnivore, chomping and crushing its prey with hefty mouthparts. I am glad we are not night diving!
I have to admit, isopods do not fuel my pod passion with equal interest as amphipods. So in keeping with good, parallel construction writing style, I will introduce you to a pair of bycatch amphipods of extreme sizes. Wandelia crassipes, has a sweet, puppy dog of a face – albeit small and not so clear in the image at right. My apologies. Kate and I found vast numbers of these critters, less than an eighth of an inch long, merrily tunneling along the edges of one of the red algal species she used in her substrate experiment. Wandelia is a lignivorous pod and is specially equipped with its ‘other feet’ to burrow into tough algal (not quite lignin or woody as the term suggests) material.
I have yet to see the largest shallow water polar amphipod, Paracerodocus miersii in the wild. It is hard to believe that we have not sighted this 4 inch chestnut brown amphipod with paired white racing stripes down it back while diving. According to the limited scientific literature available on such a handsome species, it is a detritovore which means it feeds on detritus. P. miersii burrows itself amongst stones and makes a home in the surrounding sediment. I will really have to fine tune and narrow my underwater focus if I want to see this one while I am swimming about.
In the meantime, I am an equal opportunity podist - welcoming pods of any flavor into collections and subsequently Palmer Station’s sorting seawater table. Stay tuned for my next installment of Passion for Pods...