Magazine article Natural History

Getting to the Point: Self-Defense in Crab Larvae

Magazine article Natural History

Getting to the Point: Self-Defense in Crab Larvae

Article excerpt

Of the many predators that crab larvae face, plankton-- eating fishes (such as anchovies and silversides) pose the greatest threat. Female crabs living in the marshes, mangroves, and sea grass beds of estuaries and bays-the productive but perilous habitats where such fishes are especially abundant-brood their embryos beneath their bodies until the embryos have developed into larvae and are ready to be released. The release is timed carefully: it occurs under cover of darkness, when fish are least active, and during the strongest ebb tides of the month, when the vulnerable crabs-to-be have the best chance of being swept out to the relatively safer waters of the open continental shelf.

Their conspicuousness already reduced by partial transparency, the larvae of many species further boost their chances of survival by descending into dimly lit waters during the daytime and ascending only at night to feed in the more productive surface waters. If approached by a fish, a crab larva generally does not attempt to avoid or escape attack. Instead it relies on a heavily armored exoskeleton and spines that effectively increase its size many times over. In addition, a pair of antennal spines flare upon attack, transforning the larva into a prickly ball, difficult for small-- mouthed planktivorous fishes to swallow. Indeed, young fish sometimes die when larvae catch in their throats.

To avoid such a fate, a fish may attack a larva repeatedly before swallowing it, spitting it out each time in an attempt to break down the spines. …

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