Nematoscelis tenella vs. N. gracilis

[Mary Rapien]

Hello everyone.

I am wondering if a more experienced euphausiid taxonomist can help me
with a little dilemma.  I have been using a key by Baker, Boden, and
Brinton, which has gotten me down to a choice between two species --
Nematoscelis tenella and Nematoscelis gracilis.  My samples are from the
Arabian Sea, an area in which both of these species reside.  A couple of
the characteristics which Baker et. al. use to distinguish the two are the
presence of a single long seta near the proximal end of the dactylus of
the first thoracic leg (for N. tenella) and the arrangement of setae on
the propodus of the first thoracic leg.  My problem is that several of my
specimens do not appear to have the single long seta characteristic of N.
tenella, but the arrangement of setae on the propodus most closely
resembles that of N. tenella (although it matches neither species
exactly), and the eyes seem to be shaped more like the eyes of N.
gracilis.  Which of these traits is the most reliable?  Are there any
other traits not mentioned here that I might be able to use to distinguish
the two?

Thank you very much for your help.

Regards,
Mary
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Mary Rapien
University of Rhode Island
Graduate School of Oceanography
Box 200
South Ferry Rd.
Narragansett, RI 02882
(401) 874-6689
maryr at gsosun1.gso.uri.edu