Skeleton shrimp (Caprella sp.)
This so-called shrimp is not a shrimp but a member of the amphipod group of crustacea. However, it differs from other amphipods (think beach hoppers) because of its elongated body. It also looks like a praying mantis with its grasping front limbs.
Adding to this diversity of body form is its habit of moving like an inchworm. With its hind end firmly anchored by its four hooked legs, it reaches forward with its arms, anchors and pulls its hind end forward to meet the front. The inch-long individual pictured here is an adult female with a large brood pouch in the middle of the body where the young develop.
Caprellids can be found by the thousands on branching hydroids under docks or on eel grass blades. There they feed on hydroids, diatoms and detritus (organic particulate matter), or on small animals. They can be easily found by examining eel grass blades or among the attached organisms on a boat bumper that has been left in the water and not cleaned for awhile. Within a minute of exposing the bumper to the air, the capellids will start to wiggle, revealing their locations. They are eaten by a variety of fish.
The various species of caprella occur worldwide, including some called whale lice that live on the skin of whales.
David W. Jamison is a marine biologist and Boston Harbor resident.