Digging through a grimy lake bottom for a diving job in New York's Adirondack Park is one of my favorite underwater memories. Let me picture how it went . . .
About ten feet below the surface, I peered out into the brown and blue-green expanse of lake water. The view was occasionally interrupted by a small fish, turtle, or duck, and vegetation covered the lake's bottom as far as I could see (which was only about ten to twenty feet). I turned my attention back to the invasive milfoil plants we were hired to remove, and pushed my gloved hand into the sediment so I could coax out the nearest milfoil plant. If I didn't take it out by the roots, it would just grow back, and if I pulled it roughly, fragments would break off from any part of the plant and produce new plants--a big factor as to why the plant is so invasive along the east coast and elsewhere.
When I began to pull, something caught my attention. Out of the corner of my eye, a little white dot had raced by. Wait--it was swimming! But by the time I looked in its direction, it was gone. I went back to work, and as soon as I'd dug my hand further through the sediment, a few more white objects flew out from the dense vegetation. As I continued, more still came from the surrounding plants that had previously concealed them from view. They came out in multitudes, arcing like miniature fireworks whose paths sparkled from launch to landing as they went out of sight in a different refuge of plant matter. I couldn't get a good look at them to discern what they were exactly, but they were definitely some kind of live, swimming animal!
Public Domain photo by NOAA Office of Ocean Exploration and Research, 2015 Hohonu Moana
Above is a photo of what one of these creatures looked like underwater.
A while after I was out of the water, another diver from our boat crew surfaced and boarded the boat for the end of his shift. I was busied with on-deck tasks when I heard him yelp behind me. I came over to help out, and he was trying to pick off live and wriggling, white somethings off of his wetsuit. They were the same creatures I'd seen in the water! I spent an hour or two researching what the little white wonders could be, and found matching Google images with names that matched descriptions of the macroorganisms (creatures visible to the naked eye) in a registry of species native to Adirondack Park. In the registry was a list of different species of amphipods, which I've assumed since to be the little white creatures I saw on multiple dives there.
Amphipods belong to Phylum Arthropoda, the same group as crabs, lobsters, insects, and spiders. Like all arthropods, they have jointed appendages and a relatively tough exoskeleton for their size. They belong more specifically to order Amphipoda under Class Crustacea--yes, they're crustaceans! While their exoskeleton is made of overlapping plates like other crustaceans, they lack the carapace, or hard shell, that other crustaceans such as crabs and lobsters possess. There are about 6,000 known species of these little critters.
Amphipods can be found virtually anywhere in the ocean, as well as in lakes, rivers, caves, and even in sandy beaches and the warm, damp substrate (plant matter, soils etc.) that makes up the topsoil of many islands. One square foot in any of these environments often contains over 1,000 amphipods! They are also known as "scuds", or "sand hoppers" and "sand fleas" particularly when they inhabit sandy beaches. They will bite people, though rarely at that, and even then their bites very typically only itch or hurt for a short period of time. One species, Arcitalitrus sylvaticus, can be a nuisance to property owners in tropical regions when damp soils give them an opportunity to wriggle onto streets and inside of homes in large numbers, where they die and make a mess to clean up.
Because amphipods are soft and translucent, they are commonly mistaken for tiny shrimp; hence they are sometimes also referred to as "lawn shrimp". Many species are found in bright colors including red, pink, yellow, green, and blue. Their long and tall, but thin body structure allows them to slide quickly through plant matter underwater. They use the three pairs of appendages on their abdomen to actively swim and in some cases jump--the latter ability is a known specialty of the sand flea! Some marine amphipod species are sedentary, or nonmoving, and have specialized mouthparts to latch onto other marine organisms: these species live their lives as parasites. All amphipods have compound eyes like many other crustaceans do, but their eyes are sessile (without a stalk).
Amphipods are an important food source for many fish, invertebrates, birds, small cetaceans (dolphins, whales, etc.) and pinnipeds (seals, sea lions, and walruses). They are valuable nutrient recyclers, too, as they frequently scavenge for carrion (the bodies of deceased organisms). Their diets, otherwise, can be very diverse: some eat plants, while others, in the case of the amphipod family Cheluridae, dine on wood!
Amphipods molt, meaning shed their exterior, and after having done so from six to nine times over anywhere from one to four months, they are full-fledged adults. Adult amphipods fertilize eggs externally, and fertilized eggs can vary from only one to a clutch of over 250. Females carry fertilized eggs in their gills, likely to provide the eggs an environment with a reliable flow of oxygen. Eggs hatch in anywhere from 2 to 59 days. Most species complete their life cycle in one year or less. Also, most species produce only one brood of eggs in their lifetime--though one species, Hyalella azteca, averages at 15 broods over five months.
Michal Maňas, CC BY 2.5 https://creativecommons.org/licenses/by/2.5 via Wikimedia Commons
An up-close view of an amphipod, specifically Gammarus roeselii. It is native to Europe; this specimen was photographed in the Czech Republic.
Sources:
Short video about amphipods of the Lysianassid family that (in a very unusual case) bit a young man's legs to the point of meriting a hospital visit.
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