THE ECOLOGICAL SIGNIFICANCE OF SEASONAL OCCURRENCE AND GROWTH RATE OF VELELLA (HYDROZOA)

Abstract

The sudden unpredictable occurrence of oceanic neuston on beaches around the world has stimulated yet greatly hindered the study of neuston biology. Part of this apparently haphazard occurrence is due to the complexity of sorting processes in coastal areas (Woodcock, 1956). The difficulty of studying left and right handed-ness in stranded Physalia lead to Woodcock's laudable plea for more studies at sea. However the scientist on board ship is often as frustrated at the scattered and sporadic occurrence of neuston as his colleagues ashore. Savilov (1958) had the good fortune to cruise through Velella, Janthina and other animals of the sea surface community for weeks. Bigelow (1911) on the other hand reported that during the Albatross Expedition of 1904-05 only one large Velella was sighted. Thus with everything ready the neustologist may cruise for days or weeks and find not a trace of the larger species. Then unexpectedly in the middle of another research program huge swarms may appear (see for example Doe, 1955), but problems of time, schedule, personnel and equipment may prevent even some random collecting. These vagaries are not completely whimisical much as they often seem. Bingham and Albertson (1974) have doccumented the importance of wind velocity, duration and direction in beach strandings. Much of the scattered occurrence is also due to "patchiness", a characteristic of plankton in general. It results from the turbulent nature of mixing, from the behavior of the organisms, from predation, food availability and other factors and some of it is due to the scientist's techniques of collection and observation. For example, thrice weekly examination of the beach at Shirahama for eight months amply confirmed the statement of Dakin, Bennett and Pope (1952) that a careful search of newly stranded flotsam reveals a great variety of plants and animals that are there for at most a few hours before being washed again into the sea for final destruction. The data presented below show that Velella has a seasonal pattern of growth and reproduction that accounts in large measure for the vagaries of its appearance. Because it is the food base for many other sea-surface animals (Bieri, 1966; Savilov, 1968) one would expect to find the seasonal occurrence reflected in its cohorts.