UC Davis

Spines of zoeae of the mud crab Rhithropanopeus harrisii apparently have been selected primarily through predation by small fishes rather than by predatory invertebrates, and do not appear to have evolved either to stabilize the organism or to retard its sinking. The settling velocity and swimming and sinking behavior of spined zoeae vs those with their spines removed were similar. Sinking slowed when the antennal spines were flared perpendicularly to their resting position, but zoeae do so only when molested. Only 1 of 10 planktonic and benthic invertebrates preyed on more despined than spined zoeae, but spines were effective against 2 fishes that predominate in upper estuaries of the E coast of the USA where larvae of R. harrisii develop. Spines limited the size at which planktivorous silversides Menidia menidia and opportunistic killifish Fundulus heteroclitus began preying on zoeae, but the effectiveness of spines in preventing predation appeared to diminish with each successive zoeal instar. Gape-limited fishes apparently exert strong selective pressure for crab zoeae to produce spines rather than a large unarmed body. Not only do spines increase the size of the zoeae, but spines are noxious, enhance survival of attached zoeae, regenerate quickly, and are effective when partially regenerated. Rhithropanopeus harrisii zoeae that were attacked by fishes had the same survival and time to metamorphosis as zoeae that were not attacked, whereas the shorter spines and larger bodies of zoeae of the marsh crab Sesarma reticulatum were less effective at preventing fatalities. -from Author