Nematocysts comparisons within some Hydrozoans, Scyphozoans and Anthozoans

The largest nematocyst diversity occurs within Hydrozoa and the smallest diversity in Anthozoa. Isorhizas, present in Hydrozoa, Scyphozoa and Anthozoa, are the most diverse nematocyst group. The functions of the isorhizas, their capsule shape and size, and their spine pattern can be similar or different within the three major taxonomic groups. As a rule, the more toxic nematocysts are larger than the less toxic ones. Generally, cnidarians feeding on large prey have larger, more toxic penetrating nematocysts with coarser and more pointed spines than animals feeding on small prey. The potent penetrating isorhizas with spherical capsules and hook-shaped spines along the tubule are larger in Physalia pelagica and Uvaria spp (Siphonophora, Hydrozoa) and in Cyanea capillata (Scyphozoa) compared to the smaller spherical isorhizas in the less toxic Aurelia aurita (Scyphozoa) and Tubularia spp (Hydrozoa). Cothylorhiza tuberculata and Casiopea xamancha (Scyphozoa) have still smaller spherical nematocysts. In addition to feeding on small zooplankton these two jellyfishes also get nutrition from symbiotic zooxanthellae. Spherical penetrating isorhiza are not present in Anthozoa. Penetrating isorhizas, specific for Anthozoa, have broad oval to narrow elongate capsules. Many penetrating isorhizas in Hydrozoa have also oval to elongated capsule but they differ in capsule shape and tubule pattern from those of Anthozoa. The largest isorhizas within C. capillata are not penetrators. These isorhizas entangle the prey with their long distal tubules armed with weak spines. Oval and elongated isorhizas, present in the cold water coral Lophelia pertusa and in the anemone Edwardsiella carnea (Anthozoa), might be used as primary attachment at settling for their larvae. Entangling desmonemes, present in Tubularia spp, Halocordyle disticha and Hydractinia echinata, are specific for Hydrozoa. Other nematocysts, specific only in one of the three major taxonomic groups are the penetrating birophaloids in Siphonophores and stenoteles in Tubularia spp (Hydrozoa), and the penetrating b- and p-mastigophores and p-amastigophores in Anthozoa. The penetrating euryteles in H. echinata (Hydrozoa) are also present in Cyanea spp and A. aurita (Scyphozoa).