Life in a Tidal Pool; Hermit Crabs and a Sea Worm

March 16, 2017

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Photographer: Mila Zinkova
Summary Author: Mila Zinkova

Shown above is a tidal pool on the California coast, near San Francisco, featuring a hermit crab (Pagarus sp) and a sea worm (Polychaete). Click on the photo to see the interaction between this crab (in the black shell) and two others and the hapless sea worm. I started filming the worm without realizing I was filming the last few seconds of its tenuous life. Hermit crabs are common in tidal pools. They live in shells that were freed by snails. These shells serve as their home and protect them from would-be predators. They're on constant lookout for a new shell and are often seen fighting with other hermit crabs over what they perceive is a good one. Hermit crabs go to a great extent to retrieve a better shell. Although mostly scavengers, they also attack grass shrimps and tube worms when the opportunity presents itself. As I was filming the sea worm, it touched one of the crab's antennas, most likely by accident, but this is how the action began.

According to Dr. Thomas Carefoot, Emeritus in the Department of Zoology, at the University of British Columbia, during the 8.5 seconds of this video, the initial withdrawal of the worm at the 2.5 second mark is mediated by the chemotactic stimulation of sensory appendages (antennae, palps, cirri) on the worm's head. The question arises as to whether the worm's response was innate or was a result of some type of chemical emanation from the crab's antenna. This leads to a corollary question, whether the worm has remembered a similar previous contact and has responded accordingly. For invertebrates such as mollusks and sea stars, such a memory capability is actually quite well known, but whether it exists in a Polychaete worm hasn't yet been determined. Dr. Carefoot took screenshots from the video and made a video montage with detail descriptions of just what is occurring. Click here to see it.