Sistrum arachnoides (Lamarck, 1816)
24.1mm
Sistrum
arachnoides is generally considered a synonym or subspecies of Sistrum
ricinus. One difference between the two, at least among Kwajalein specimens,
is that S. arachnoides has orange dashes around the perimeter of the
glossy white aperture, which are lacking in S. ricinus, except perhaps
for light orange patches near the anterior siphon. Also, S. arachnoides
tends to have longer spines, but that may be due to the third, and to me
the most important difference. S. arachnoides is found subtidally,
under rocks but also in seaward caves and ledges at depths of up to 20m at least.
Shells that look more like S. ricinus live intertidally or on very
shallow subtidal reefs. From what I have seen, they do not appear to overlap
in habitat. While they may well be the same species with different forms depending
on where they settle out, I consider it best to leave them separate until someone
has the inclination to compare their DNA.
With an egg mass.
Created 5 October 2017
Updated 16 March 2024
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