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.