Mysterious marine creatures washed up by storm

I was cleaning up storm debris in Nahant the other day with some coworkers and other community members, and we came across these bizarre creatures in a mass of algae that had been washed up onto the sidewalk. None of us had seen anything like them before.


I am a fan of invertebrate zoology, and my initial thought was that these were peanut worms (phylum Sipuncula). These are a group of unsegmented, marine worms whose typical shape resembles that of the mystery creature. Peanut worms live buried in sediment or under rocks. Here is typical peanut worm:

Themiste petricola, everted

Upon further reflection though, something wasn’t adding up, and I began to doubt my first identification. A feature of peanut worm anatomy is that they have a mouth at the narrow end, and an inconspicuous butt hole located halfway down the body. But our animals clearly had holes at both ends. Also, I noticed our animals had 5 faint lines down their length, and this 5-part symmetry is found almost exclusively in the phylum Echinodermata (the sea stars, sea urchins, sea cucumbers, and their relatives).

After scouring some field guides and the internet, I concluded that these creatures are actually unusual sea cucumbers. Specifically, they are Caudina arenata, the “rat-tailed cucumber” of which there are scant few pictures on the internet, but here are illustrations from some very old, digitized guidebooks.

Seaside studies in natural history (Page 97) BHL11583285
A manual of the common invertebrate animals, exclusive of insects (Page 650) BHL7260166

The species is known from Massachusetts and other places in the Northeast. According to a book on tubefoot-less sea cucumber identification, The Apodous Holothurians, published in 1907, they live in shallow waters below the tide line, buried in firm sediment, with only the tip of the skinny end (which is the posterior, and the end that sea cucumbers breathe out of) at the surface. Like most sea cucumbers (and also peanut worms) they are deposit feeders, eating up organic material in and on the sediment. They are rarely captured in dredges – the only time they’re ever really observed is washed up on beaches after severe storms. The Apodous Holothurians reports how a large number of specimens washed up on Revere Beach after a storm.

I had brought the 2 specimens we found back to the Marine Science Center and placed them in our touch tanks, on the off chance they were still alive after being tossed around by the storm and sitting the algal wrack, out of water, for a few days. Believe it or not, they were actually still alive! You can go check them out yourself.


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