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Oyster associates: what are amphipods and isopods?

We take a closer look at the small arthropods who make a regular appearance in our Oyster Restoration Stations!

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Amphipod photo courtesy of Mollie Thurman of Biobus/Biobase.

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Isopod photo courtesy of Mollie Thurman of Biobus/Biobase.

Note for teachers: Amphipods and isopods get a starring role in BOP’s new curriculum, NY Harbor Populations Investigation. This unit starts with NY Harbor food and habitat webs, gets students hands-on designing experiments with their own small tanks of amphipods and isopods, and culminates in students reading middle school-friendly digests of real scientific studies as models to propose their own population study.  This explainer appears as a printable handout in the lesson “Small Tanks for Small Arthropods” (part 5).

What are amphipods and isopods?

You’ve probably noticed dozens of small wriggling creatures, many smaller than a penny, in your Oyster Restoration Station. They might even look a little familiar. But what are they? And how are they related to larger organisms you already know?

Some of these creatures are very likely to be amphipods and isopods.

Take a look at the amphipod pictures below. What organisms do they remind you of?

Drawings of the 4 amphipod families:
A: Family Gammaridae (Gammarus locusta)
B: Family Talitridae (Orchestia gammarellus)
C: Family Corophiidae (Corophium volutator)
D: Family Caprellidae (Caprella linearis)

 

…and here is the side and top view of a typical isopod. What do they remind you of?

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Drawing via crustacea.net

You might find amphipods and isopods familiar because they are both crustaceans, a type of arthropod.

Amphipods and isopods are arthropods
Arthropods are a massive phylum of invertebrate animals that includes insects, spiders, and crustaceans (like shrimp and lobsters). These organisms all have an external skeleton (called an “exoskeleton”- “exo” means “outside”), a segmented body, and jointed appendages (a leg is an example of an appendage). There are over a million species of arthropods, and they make up 80% of all described living animal species. (The word “arthropod” comes from the Greek word “arthron-,” meaning “joint,” and “pod,” meaning “foot” or “leg.”)

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Amphipod photo courtesy of Mollie Thurman of Biobus/Biobase.

Amphipods and isopods are crustaceans
All crustaceans are arthropods, but only some arthropods are crustaceans. Crustaceans are mostly aquatic species, but a few, like roly polies and sow bugs, live on land. Their bodies come in many different forms and they live in almost every major habitat in the world. Some crustaceans you might encounter at your Oyster Restoration Station include: crabs, shrimp, barnacles, and of course, amphipods and isopods.

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Isopod photo courtesy of Mollie Thurman of Biobus/Biobase.

What do amphipods and isopods have in common?
–They are mostly benthic creatures- organisms that live at the bottom of a body of water (but some are planktonic and drift in the water column).
–They can generally swim, although some species are better at it than others.
–The species in New York Harbor are generally an inch or smaller in size.
–They tend to eat things like detritus, algae, tiny invertebrates, and dead animals. They are an important food source for the estuary’s fish, some seabirds, and marine mammals.
–There are a lot of them in New York Harbor. There are some amphipod species with 20,000 individuals per square meter.
–They reproduce sexually.
–They can be useful indicators of different kinds of pollution in the harbor.
–Some species get taken in to the ballasts of ships (compartments that are filled with harbor water to stabilize the ship), and then released outside of their native range. No one can predict what will happen when they show up in a new ecosystem.

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Caprellid amphipod photo courtesy of Mollie Thurman of Biobus/Biobase.

How are amphipods different from isopods?
“Amphipod” comes from the Greek words “amphi-,” meaning “two,” and “pod,” meaning “foot” or “leg.” The name comes from the fact that amphipods have one set of legs designed for walking and one set designed for swimming- hence two different kinds of legs. In contrast, “isopod” comes from the Greek “iso,” meaning “equal,” and “pod.” Many isopods have seven pairs of mostly similar (“equal”) legs.

According to The River Project, “Amphipods are ‘side swimmers’, because they’re propelled sideways by their legs (they’re really propelled backwards, but because they’re laterally [side-to-side] compressed, people usually view them side-on, so it looks like they’re moving sideways). In contrast, isopods usually swim upside-down because their gills are located on the lower abdomen, so swimming upside-down with their legs moving maximizes the amount of oxygenated water flowing over the gills. Another difference is that many local amphipod species burrow in the mud, while isopods generally cling to solid structures like wood pilings or rope using their hook feet.”

Posted 4/3/17 at 5:09 PM