Well, for a couple of journal entries now I have mentioned my ongoing process of dissecting small amphipods to determine their gut contents. I am sure many readers have thought, “Wow, that is so cool. I wonder what he finds in those guts?” Alright, probably not, more likely readers are thinking “Wow, dissecting the stomach contents out of small bug-things….his mom must be so proud.” Ultimately though I have seen some pretty interesting things in gut material and have been amazed at the diverse range of amphipod feeding habits, especially across the various species and depths we collect them.
Most of the amphipods I dissect are between 0.5 – 1 cm, about the size of a jelly bean (and just as colorful), but some species get much smaller. How does one retrieve the guts from something so small? Well, a steady hand (ever play the game ‘operation’ as a kid) and a whole lot of patience. I must admit that several of my initial attempts to degut an amphipod had all the surgical precision of Conan the Barbarian.
My less-than-gentle, yet seemingly random, prods and incisions resulted in a conglomeration of internal organs, chitin, and seawater reminiscent of a Jackson Pollock painting. However, with a little practice I managed to fine-tune my surgical prowess and am now able to dissect dozens of amphipods in a fraction of the time and nearly half the frustration.
What do I find in the guts? Glad you asked. First, the most common occupant of the digestion tract is diatoms. Diatoms are small unicellular algae that surround themselves in glass houses (they are not known to throw stones) called frustules. They can range in shape and size quite dramatically. To the right you will see pictures of just a couple of the different morphologies that diatoms possess.
It is rare to find gut contents without diatoms in them (even predators as opposed to herbivores), although this may be because diatoms are so abundant in seawater that it is hard to not have them in the gut. Ever been swimming in the ocean? Than I guarantee you have swallowed a diatom. Some amphipods, like the cracker-jack amphipods (aptly named from a previous post) seem to feed exclusively on diatoms.
It seems that amphipods do not just dine on the unicellular algae, but larger macroalgae as well. There are many traces of algal filaments and thalli (a macroalgae’s "body") in the guts as well, though these are found in particular species and not as prevalent as the diatoms. Many of the filaments range in size and cellular structure, indicative of different algal divisions.
These physiological differences help us narrow down the algal species that are being eaten. I would love to be able to tell you what exact species those filaments are, but that is still a question we are researching. Later posts will focus on feeding experiments and our techniques for determining the palatable species of algae.
Of course, in the vast world of amphipods you also have your predators. Amphipods, like Bovalia gigantia, guts’ contain mainly other crustacean parts. Using an epi-fluorescent microscope I can readily enhance the detail of these crustacean parts so they are easier to see. Some, believed to be non-predatory, species of amphipods also have crustacean parts in their gut but that is likely because many species ingest their own molts.
Amphipods, like snakes, shed their skin as they grow too big for their exoskeletons. Unlike snakes, many amphipods do not leave the excess material behind but rather consume it to meet nutritional demands.
These are just some of the many things I find in amphipod guts. I have also seen an array of different sponge spicules, bryozoans, invertebrate larvae, and other unicellular algae. Using these data, our group hopes to determine what role the abundant amphipods serve in shaping the benthic Antarctic community and mediating mesograzer-macroalgae interactions.
Working toward that goal, I will continue to painstakingly dissect the smallest of amphipods to determine its diet…even if I never was very good at ‘operation.’