No abstract

SEM investigations of laboratory-reared larvae of Briarosaccus tenellus Boschma, 1970, revealed five naupliar instars, one more than previously described for the Rhizocephala . The external features of these and the cypris larva are described in detail . The youngest instar (NO) is characterised by a better developed furca than in subsequent stages and one additional antennulary seta . Differences in outline, shape of antennulae, and hind-body denticulation, offer the potential of individual discrimination . All the nauplii possess a transparent, hollow cuticular ring around their body. This `flotation collar' represents the bulged margin of the larval head shield and is attached to the body along a delicate ridge . Three pairs of tiny pores in contact with the ridge possibly regulate inflation of the ring, but details of this mechanisms remain unknown . Due to total lecithotrophy, the nauplii of B . tenellus have limbs setation reduced to that needed for swimming only, and other feeding structures such as the labrum are also highly reduced . In the antennulae, the morphological changes in form and setation were followed from nauplius to cypris and shown to largely resemble events in ontogeny of the thoracican barnacle Semibalanus balanoides . On this basis we establish a homology scheme between antennulary setae in these two species . In both B. tenellus and S. balanoides, a naupliar seta, apically on the fourth antennulary segment develops into a conspicuous aesthetasc while one (B . tenellus) or two (S. balanoides) subapical and postaxially sited setae on the same segment develops into into four short setae in the cypris . Our study reveals that the terminology used in describing cirripede nauplius and cypris larvae is often misleading or even erroneous and improvements are suggested . Notably replacing 'cypris carapace' with the ontogenetically and phylogenetically more informative term `head shield' .

Aurelia labiata medusae occurred in aggregations with hundreds to millions of jellyfish. The aggregations were widely distributed in inlets of Prince William Sound (PWS), Alaska. Aerial surveys of PWS in May to August in 1995August in , 1996August in , and 1997 showed marked interannual variation in the numbers of aggregations observed, from a minimum of 38 in 1997 to a maximum of 557 in 1996 Acoustic surveys showed that the aggregations extended from near-surface (0 to 5 m) to 15 m depth. Schools of young-of the-year walleye pollock Theragra chalcogramma were associated with A. labiata, both within and below the aggregations. All seine catches that contained juvenile pollock also contained jellyfish. Medusa swimming was analyzed from underwater videotapes in order to elucidate how aggregations might be formed and maintained. Medusae did not swim in circles in the aggregat i o n~. Medusae all swam vertically in the same direction, either up or down, in crowded parts of the aggregations, suggesting that they had become concentrated in flow features, like convergences, in the water column. Reduced swimming, due pnmarily to frequent collisions among medusae in the aggregation~, also may have caused the medusae to become concentrated. The potential advantages of aggregation include increased fertilization success, retention near shore where planula settlement sites and zooplankton prey may be more abundant, retention in convergences where zooplankton are concentrated, and reduced predation.

Higgins. R. P . , Storch, V . & Shirley, T. C. 1993. Scanning and transmission electron microscopical observations on the larvae of Priupulus cuudatus (Priapulida).-Acta Zoologica (Stockholm) 74: 301-319.Scanning electron microscopical studies revealed four distinct morphological larva-types (not instars) of Priapulus cuudatus whose lorica-lcngth measured 82-860 pm. The smallest of the larvae are round to oval. have 20 longitudinal ridges, a series of transverse ridges, and have two pairs of laterally situated tuhuli near the posterior limits. The second larva-type is dorsovcntr;illy compressed. has a single dorsal and bingle unsculptured ventral plate each with a prominent midridge near the anterior limits. three infolded lateral plates with a pair of tubuli near the posterior limits. The third and fourth larva-types remain dorsoventrally flattened; the third larva-type has less pronounced sculpturing than the fourth larva-type and is smaller; two pairs of tubuli are situated as in the previous stage. The number and arrangement of scalids on the introvert and pharyngeal teeth diffcr according to each of the four larva-types and are described. The ultrastructure of all organ systems is described. All scalids are sensilla equipped with ciliated receptor cells. The cuticle is similar to the cuticle in the adult and differs fundamentally from the cuticle of larval Halicryptus. The trunk is richly supplied with sensory structures. Trunk tubuli contain secretory cells. Posteriorly. a gland complex was found which presumably is the precursor of the equivalent gland in postlarvae and adults. The implications of the first larva-type's structural similarity to the larva of Tuhiluchus are discussed.