Some strange Hemigrapsus specimens have been recently found in La Rochelle Harbour. I'm not sure it is Hemigrapsus sanguineus.
Hemigrapsus sanguineus seems to be the only Hemigrapsus species with fleshy vesicles on chelipeds. But in the same family (Varunidae) Brachynotus sexdentatus (Risso, 1827), B. gemmellaroi (Rizza, 1839) and B. foresti Zariquiey Alvarez ,1968 (mediterranean species) also show this particular anatomical feature.
Try asking this question on crust-l, the listserver for Crustacea. I've seen deposits on H. oregonensis and H. nudus. I thought they were associated with the molt cycle.
Many thanks,
Actually, concerning the genus I was wrong, I finally found the identity of these crabs: Brachynotus (probably B. gemellaroi).
Anyway I'm still interesting to get the answer to my question, so I will follow your suggestion.
The structures on Hemigrapsus oregonensis and H. nudus are dense patches of plumose setae, most prominent in males and not associated with molting. It's been claimed they hold water like a sponge and are used to wet the eyes when the crab is out of water, but I don't find it very convincing.
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