Obelia longissima

Abstract: The differentiation of nematocysts and associated structures in the tentacles of the sea anemone Metridium senile fimbriatum and in the tentacles of medusae of the hydroid Obelia longissima was studied by electron microscopy. Stages in the morphogenesis of nematocysts are: 1. transformation of an interstitial cell into a cnidoblast in which the primordium of the nematocyst arises from Golgi vacuoles, 2. development of the basal part of the primordium into a capsular region that is continuous with an external tube with which microtubules, Golgi apparatus, and centrioles are associated distally, 3. growth of the capsular region to include most of the external tube, and 4. differentiation of an internal tube with spines from the matrix of the capsule and of the remaining external tube. The external tube is not invaginated to become the internal tube, but is incorporated into the capsular region by distal growth of the capsular wall. At the same time the matrix, both before and during differentiation into a thread, moves into the capsule. The development of nematocysts in Obelia differs from that in Metridium in several respects. In the former the basitrich has an operculum plus associated cnidocil and dense rods whereas in the latter the nematocysts (amastigophore, b-mastigophore and basitrich) have three apical flaps and an associated flagellum, with the exception of the spirocyst.

Abstract: The effect offlow regime on the ability of the hydroid Obelia longissima to capture plankton was ex amined in a laboratory flume and a wave tank. Feeding effectivenessâ€"the proportion of the gastrozooids with tentacles successfully capturing food in a fixed amount of timeâ€".-is significantly greater in oscillating flow than in unidirectional flow at the same average velocity and particle flux. Thus quantitative feeding studies in uni directional flow may seriously underestimate feeding in the field ifflow in the natural habitat is unsteady. Increas ing colony bushiness (weight/colony length) decreases feeding effectiveness in uni-directional and low fre quency oscillating flow, but not in high frequency oscil batingflow. Longer colonies (0.083 m) show significantly lower feeding effectiveness than short (0.033 m) colonies. This decrease in feeding effectiveness with increasing cob ony length and bushiness is offset by a rapid increase in polyp number with increasing colony length.

Abstract: Medusa blooms are a scarcely known phenomenon. For temperate, southwestern Atlantic wa- ters, reports of such events are almost nonexistent. This research reports the analysis of a total of 1043 plank- ton samples taken from 1993 to 2006 and benthic samples collected by seasonal SCUBA diving since 1997 along the Buenos Aires coast. The medusa Obelia longissima was observed in very low densities and frequen- cies, except in October 2003, when a bloom of this species was detected. Soon afterward, two consecutive mas- sive accumulations of O. longissima and Amphisbetia operculata colonies were detected on the coast by the inhabitants of Mar del Plata (38o00´S-57o32´W) and the coastal Mar Chiquita Lagoon (37o32´S-57o19´W) be- tween March and April 2004. The colonies of both species were collected from the shoreline at low tide at both sites. The environmental factors of that period seemed to be particularly appropriate for promoting optimum growth conditions for these hydroid species. This paper reports and describes the mentioned abnormal phe- nomena, providing baseline information on these little-understood events.

Abstract: This is the first investigation on the steroids of the hydrozoan genus Obelia . Thirty known sterols were identified, 20 of them were 4-desmethyl sterols, four coprostanols, and six 4-methyl sterols. For two other sterols that appear to be new sterols, tentative structures 13d and 14b are proposed on the basis of the mass spectral fragmentation. Contrary to other Hydrozoans, Obelia longissima contains a very complex sterol mixture, cholesterol not being the main sterol. The main sterol appears to be the (22Z)-isomer of brassicasterol 16a (probably a mixture of two C-24 epimers). The only other (22Z)–sterol seems to be 17a . Sitosterol appears to be a 24β isomer (clionasterol). Sterol esters possess a sterol composition different from that of the free sterols.