Adaptive Significance of Spination in Estuarine Crab Zoeae

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 invertebrates preyed on more despined than spined zoeae, but spines were effective against two fishes that predominate in upper estuaries of the east coast of the United States where larvae of R. harrisii develop. Spines limited the size at which planktivorous silversides, Menidia menidia, and opportunistic killfish, 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 attacked zoeae, regenerate quickly, and are effective when partially regenerated. Spined first—instar zoeae were preyed upon by silversides less often than despined second—instar zoeae, in much the same numbers as despined third—instar zoeae, and more often than despined fourth—instar zoeae. The sharp multiple spines of zoeae prevented fishes from manipulating their prey for easy ingestion, so that zoeae often lodged in the pharynxes of small fishes and sometimes killed them. 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. Dorsal spines were broken most often during attacks. The partially regenerated spines of third—, but not fourth—, instar zoeae were as effective as unbroken spines in preventing predation by silversides. The effectiveness of spines as an antipredatory adaptation is enhanced, because fishes apparently detected spines and quickly learned to prefer despined to spined zoeae. Zoeae did not attempt to evade or escape attacks by fishes, but instead relied on armor and postcontact antipredatory behaviors for their survival. The differential effectiveness of spines of marine and freshwater zooplankters at deterring predation by fishes and invertebrates is discussed.