This post is part of an occasional series on local marine species compiled by New England Aquarium aquarist Peter Gawne. Unless otherwise noted, all photos are Peter's. When he's not diving locally for these posts or researching coral reefs in Belize, Peter takes care of the touch tank's sharks and rays and the jellies exhibit. Today's post is about skeleton shrimp (family Caprellidae).

According to evolutionary theory, half a billion years ago each of the major arthropodan groups, the Crustaceans (shrimp, crab, amphipods), Uniramians (insects, millipedes, centipedes), and Chelicerates (spiders, mites, horseshoe crabs) shared a common ancestor, probably something resembling a trilobite. Skeleton shrimp are classified as Crustaceans, alongside shrimp, crabs, and lobsters.

Skeleton shrimp are classified as amphipods of the family Caprellidae.  They are sometimes known as caprellid amphipods.  While they might resemble a science-fiction monster, skeleton shrimp rarely achieve 2 inches in length.

While outwardly skeleton shrimp look quite different from crabs and lobsters, they are actually closely related. They have branched appendages, two pairs of antenna, the same number of legs, and a similar general body form. They differ, however, in that they lack a carapace and females possess a brood pouch. Skeleton shrimp also do not have a free-living larval stage. This means that the population is dispersed not by currents, but rather the actual migration of the adults.

The body of an amphipod is divided into three sections: head, thorax, and abdomen.  In skeleton shrimp, and all amphipods, the legs of the thorax differ from the legs of the abdomen.  The thorax of the skeleton shrimp also contains the respiratory structures: two pairs of paddle-like gills.

Skeleton shrimp bodies are long and cylindrical. They possess two pairs of legs at the front of their bodies, and three pairs at the back. The front legs are praying mantis-like claws, used for defense, grooming, and food capture. The rear legs are used to hold onto algae or other surfaces.

Skeleton shrimp are found throughout the marine waters of New England, usually clinging to algae, bryzoans, or hydroids with their posterior legs. Skeleton shrimp are especially abundant in fouling communities, which means they are frequently found on docks, pilings, and ropes. Their slender body resembles the very algae that they cling to, allowing them to virtually disappear as they wait to ambush prey.

Microscopy reveals the brood pouch of this female skeleton shrimp, an amphipod of the family Caprellidae.  Female skeleton shrimp brood their larvae until they are ready to emerge as miniature-sized adults.

As a group, skeleton shrimp are opportunistic omnivores. They are able to feed upon diatoms, detritus, various larvae, and even other skeleton shrimp. Some species use fine hairs on their antennae to filter particles from the water column. Most skeleton shrimp are predators, using their front claws, called gnathopods, to capture smaller invertebrates.

An array of feeding and sensory structures allows skeleton shrimp to capture and consume a wide variety of food sources.

Most skeleton shrimp possess a venomous tooth on their gnathopods. Males may use this tooth as a weapon during competition with other males. This tooth is capable of inflicting serious damage, and combat can result in death. This is an extremely rare case in the animal kingdom–the only other animals known to apply venom in intraspecific completion are male platypuses and slow lorises. Females sometimes use this tooth to kill a male after mating. But rest assured, the venomous tooth of skeleton shrimp is tiny, and poses no threat to humans.