THE FUNCTIONS OF NEMATOCYSTS IN PREY CAPTURE BY EPIPELAGIC SIPHONOPHORES (COELENTERATA, HYDROZOA)

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The nematocysts of 24 siphonophore species were examined by light and scanning electron microscopy (SEM) for differences that could relate to differences in the sizes and types of prey captured. The siphonophore species in the suborder Calycophorae had 4-30 microbasic mastigophores (0.7-18.0 µl volume), and 50-2000 smaller homotrichous anisorhizas in uncoiled nematocyst batteries. The physonect siphonophore species had 4-120 stenoteles or microbasic mastigophores (1.8-40.7 µl volume), and 150-20,500 smaller homotrichous anisorhizas in coiled nematocyst batteries. The sizes of crustacean prey (primarily copepods) captured by species in both suborders increased with increasing nematocyst size and numbers. Examination by SEM of captured, but uningested prey showed that the heavily-spined threads of these nematocysts adhered to the prey surface, and primarily entangled the prey. In contrast, the tentacles of siphonophores in the suborder Cystonectae, which includes Physalia physalis, have only isorhizas of 1.0-18.0 µl volume with and without small spines on the threads. These nematocysts penetrate the soft-bodied prey (mostly fish larvae) of these siphonophores, but apparently do not penetrate or entangle hard-bodied prey. Thus prey capture by siphonophores differs with the sizes, numbers, and types of nematocysts present in each species. The possible functions of nematocyst batteries in tentacle spreading, and luring of large zooplankton prey are discussed.