Five, perhaps six species of Scyphomedusae have been obtained in surface macroplankton catches at the "Texel" lightship. Of these species Pelagia noctiluca has been described in a separate paper (VAN DER BAAN, 1967). The others are discussed in their order of appearance in the course of the season. Cyanea lamarckii had a strobilation period which lasted from early November till early May, with some occasional later appearances of ephyrae. The medusae appeared in two separate periods, clearly separated after cold winters, more or less overlapping after mild winters, each with its own peak period. The medusae of the two periods could be distinguished by their degree of coloration. Arguments for a hypothetical difference in origin and strobilation time to account for the two peaks of appearence of C. lamarckii are given. Aurelia aurita. The usual strobilation period for Aurelia aurita lasted from early March to about May. Sometimes it began as early as in January, though with a cold spring the early start tended to miscarry. Even earlier dates have been found in Zeeland. There were 80 separate periods of occurrence but the numbers rose to a peak in early summer and then declined rather quickly. Chrysaora hysoscella gave very few ephyrae. The big summer catches must come from elsewhere. The influence of the cold spring of 1963 was visible in all Scyphomedusae, but especially in Chrysaora. In 1966 Chrysaora was scarce.Rhizostoma octopus. Young specimens were never collected in the lightship hauls, though the young stages are regularly found in the Wadden Sea. It is plentiful on the Dutch beaches, but only present in the lightship hauls in very low numbers in autumn. Cyanea capillata. Though this species is often recorded from the Dutch beaches, always later and in much lower numbers than C. lamarckii it is suggested that some earlier identifications have been wrong and that these records concerned yellow-coloured varieties of C. lamarckii. They are found during the later peaks of the lamarckii period. |
