Nematocysts of stages in the life cycle of Stomolophus meleagris, with keys to scyphistomae and ephyrae of some western Atlantic Scyphozoa

Nematocysts were examined in the planula, scyphistoma, ephyra, and medusa stages of the rhizostome scyphozoan Stomolophus meleagris from South Carolina, U.S.A. Two categories of nematocysts, holotrichous isorhizas and heterotrichous microbasic euryteles, were present in all four stages. Holotrichous isorhizas were of one morphological type (a-isorhizas) in the planula, ephyra, and medusa, and two types (a-isorhizas and α-isorhizas) in the scyphistoma. Several distinct varieties of heterotrichous microbasic euryteles were observed. Those of the planula were all small (5.7–7.0 × 3.9–4.8 μm), those of the scyphistoma and ephyra were either small (5.7–8.4 × 3.8–5.9 μm) or medium (7.5–9.8 × 6.0–7.6 μm), and those of the medusa were small (6.4–10.1 × 4.5–6.5 μm), medium–large (10.5–12.6 × 7.3–9.0 μm), or large (13.2–17.8 × 8.0–10.3 μm). The cnidom is compared with that of other scyphozoans, and identification keys to the scyphistomae and ephyrae of temperate western Atlantic semaeostome and rhizostome Scyphozoa are presented.