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Block Island, Washington County, Rhode Island, USA
July 22, 2015
Very common in tidal flat in Great Salt Pond. I've changed my identification to Sphaeroma sp. based on a comparison of Sphaeroma quadridentatum, my original tentative ID, and Sphaeroma papillae, which has been recorded from Cold Spring Harbor, Long Island, NY, and the north shore of Cape Cod. I initially chose to wait until next year to verify my identification, as I thought the characters differentiating the species were likely impossible to examine in this somewhat out of focus, blurry photo. However, upon further examination, I found that the characters are more obvious than I thought they were, and contradictory. I'm not sure what to make of it and still plan to collect some more specimens and get better photos next summer.

The shape of the telson and of the sixth and seventh abdominal segments fits S. papillae, though there are certain features that are hard to make out. Most obviously, the color points to S. papillae. The Arthrostraca of Connecticut describes the coloration of S. quadridentatum as follows: "Color extremely variable, some individuals of a uniform slatey gray; many marked dorsally with a whitish, creamy, or pinkish patch margined with black." By contrast, Bayliff (1938) describes the color of S. papillae as quite different: "In color the specimens are generally light brown but occasionally light gray. There is scattered black pigment which has a slight tendency to collect in a broad median band on the dorsal surface of thoracic segments."

However, the first abdominal segment is much different than that of S. papillae, which is "two and one half times as long as the head." I'm having a hard time making out the exact extent of the head and first abdominal segment but the ratio is definitely much smaller than in Bayliff's description. Is it an immature, a new subspecies, a new species, a hybrid, or something else? Who knows, which is why I'm getting more pictures.

Some extra thoughts
The main distinguishing factor between S. papillae and S. quadridentatum are the uropod exopod margin (entire in papillae, toothed in quadridentatum) and the telson ornamentation (with two long tubercles vs smooth), both which are a bit hard to see in these photos. The coloration in S. papillae is also a bit more variable than what Bayliff originally described, probably since he had a small sample size to look at. From what I've seen from surveying appropriate habitats along the northwest coast of Long Island and finding a few hundred S. papillae, most appear to be shades of gray or dull rufous but a few can have somewhat dull black-and-white patterns such as the largest one here.

The habitat also seems to be very important too. S. papillae lives in an extremely specific habitat, usually living where freshwater meets the intertidal zone such as freshwater seeps and creek outlet shores. If there wasn't any freshwater nearby (I don't know if the Great Salt Pond is brackish or marine) then it's most likely not S. papillae

Have not looked into this in several years
I will have to do some more marsh/flats/beach/etc. sampling in 2021. I see yours is from Cold Spring Harbor, which is rather appropriate in this case.

The GSP was formerly a variably brackish pond with periodical surface flow to the sea. A permanent cut was made toward the end of the 19th century. (It’s been a while since I’ve read up on the timeline.) It’s now “New Harbor” on the island (vs. Old Harbor on the east coast) and essentially marine. There are some permanent brackish flows into it, to which this isopod may or may not have been near(ish). To better photos and some good diagnostic views next summer!

Moved from Sphaeroma papillae.



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