Aglaura Hemistoma Research Articles

Omnivory by the small cosmopolitan hydromedusa Aglaura hemistoma

We investigated the feeding of the small hydromedusa, Aglaura hemistoma (bell diameter < 4 mm), to determine if it occupies a trophic position similar to that of large medusae. Feeding was examined using gut-content analysis of preserved and unpreserved medusae and by analyzing prey-capture events using microvideographic techniques. Analysis of gut contents and prey-capture events revealed that A. hemistoma fed heavily on protistan prey and that it possessed a prey-capture mechanism, specifically a feeding current, that is effective at entraining and capturing protists with low motility. We suggest that many species of small hydromedusae possess prey-capture mechanisms adapted to capture small protistan prey and that many of these small hydromedusae feed omnivorously on microplanktonic prey. The trophic roles of small hydromedusae in different systems are not understood and more studies are needed. However, based on their often high abundances and the cosmopolitan nature, if small hydromedusae are primarily omnivores, they need to be considered when estimating the impact of zooplankton on primary production and, more generally, protistan community dynamics.

Bathymetric distribution of medusae in the open waters of the middle and south Adriatic Sea during spring 2002

Twenty-eight species of hydromedusae and one scyphomedusa were collected from the middle and south Adriatic in Spring 2002. Haliscera bigelowi was recorded for the first time in the Adriatic Sea and Dicodonium adriaticum for the southern Adriatic. The cosmopolitan species Rhopalonema velatum, Aglaura hemistoma and Persa incolorata averaged 88% of total medusan abundance. The Mediterranean species Rhabdoon singulare and Solmissus albescens were numerous at the deeper stations. These species were found either in the southern or in middle Adriatic, and their abundance is greater than in other regions of the Mediterranean Sea. Species diversity and abundance generally were highest in the upper 200 m and decreased with depth. Diel vertical migration patterns included some species that nocturnally sought shallower or deeper layers, and some species migrated in both directions. A few species did not show any vertical migration. Although light likely triggers vertical movements, data suggest that foraging strategies also influence the vertical migration of medusae.

Interannual variation in the composition of the assemblages of medusae and ctenophores in St. Helena Bay, Southern Benguela Ecosystem

The assemblages of medusae and ctenophores were examined from samples collected each winter from St Helena Bay, over the 10-year period 1988-1997. A total of 50 hydromedusae, 1 scyphozoan and 2 ctenophore species were identified from 243 samples. Although the data set is characterised by great interannual variability, two main assemblages could be identified each year. These were characterised by either holoplanktonic medusae (e.g. Liriope tetraphylla, Aglaura hemistoma) or meroplanktonic medusae (e.g. Mitrocomella millardae, Chrysaora hysoscella) and ctenophores (e.g. Pleurobrachia pileus). The holoplanktonic medusae were typical of samples at the southern edge of the Bay, and were positively associated with both depth and temperature. Their abundances tended to increase during warm years (1992, 1993 and 1997) as warm surface water flooded the Bay. The meroplanktonic medusae and ctenophores were typical of samples collected within the Bay, and the density of species tended to be negatively correlated with temperature and depth. In spite of the eurythermal nature of the meroplanktonic species, they were more common during cold years (1990 and 1995). This paper represents the first Bay-wide, interannual study of any zooplankton group, and contributes important base line information on the structure of regional pelagic assemblages.

A Survey of the Reef-Related Medusa (Cnidaria) Community in the Western Caribbean Sea

The species composition, distribution, and abundance of medusae collected during a 4-day plankton survey in a reef system of the Mexican Caribbean were studied. Highest mean medusae abundance was observed over the fore-reef zone and in daytime samples. Lowest abundances occurred in the reef lagoon and at dusk. Seventeen species were identified, with Liriope tetraphylla, Aglaura hemistoma, Cubaia aphrodite, and Sarsia prolifera being the most abundant. They belong to a group of medusae dominant along the world's second largest barrier reef. Cluster analysis revealed primary (fore-reef) and secondary (reef lagoon, channel) oceanic groups, showing the strong oceanic influence along and across the reef system. Day-to-day variation in the reef medusan community seemed relatively unimportant. The community structure of the reef medusa fauna appeared to be quite uniform despite the expected migratory behavior of these predators, tidal exchange across the reef, introduction of oceanic species, and time of day. The species composition was most closely related to that of the Campeche Bank and oceanic Caribbean waters. Dominance of oceanic medusae within the reef lagoon was attributed to the narrowness of the continental shelf and the mesoscale hydrological features of the zone.

Species Composition, Distribution and Abundance of Hydromedusae in the Exclusive Economic Zone of the East Coast of India

Based on zooplankton collections made by Fisheries Ocean Research Vessel Sagar Sampada Central Marine Fisheries Research Institute, India, during the period 1986-1990, 356 samples from 18 cruises from Sri Lanka to Calcutta coast were analysed. The hydromedusae were encountered in 273 samples and more than 62 species were found. Liriope tetraphylla dominated in the percentage of occurrence and population density. Aglaura hemistoma, Solmundella bitentaculata, Rhopalonema velatum, Cytaeis tetrastyla, Bougainvillia fulva and Phialidium hemisphaericum were the commonly occurring species. The maximum species diversity was noticed away from the nearshore stations, while maximum population density was from nearshore stations. reported by Navas (1971), Vannucci and Navas (1973a, b, c) and Santhakumari (1977a) based on the samples in the International Indian Ocean Expedition (1962-1965). But in this expedition coastal areas were not thoroughly sampled. A comprehensive study from the coast of India, especially in its exclusive economic zone (EEZ), has not been undertaken. The available information on the distribution of hydromedusae is confined to areas off the southwest coast of India (Nair, 1954; Vannucci and Santhakumari, 1969; Santhakumari, 1977a, b, 1991). The only a report about the destribution of hydromedusae from the east coast of India is by Ganapati and Nagabhushanam (1958) who recorded the seasonal distribution of hydromedusae from Visakhapatnam. The present paper deals with the distribution of hydromedusae based on the material collected by FOR V Sagar Sampada from the EEZ of the east coast of India. Materials and Methods Samples were collected in 18 cruises on board FORV Sagar Sampada from the EEZ of the east coast of India, from Sri Lanka to Calcutta, during the period 1986-1990. By an oblique haul from 150m depth to surface using Bongo net (mouth diameter 60 em; mesh aperture 0.33 mm) equipped with a calibrated flow meter, 356 zooplankton samples were taken and preserved in 5% formalin sea water solution, and the hydromedusae were separated and analysed. The sampling details were presented in the cruise reports (Central Marine Fisheries Research Institute, 1986). Hydromedusae counts were transformed to numbers per unit volume using flow meter data.

Influence of the thermocline on the vertical migration of medusae during a 48 h sampling period

The vertical distribution and migratory movements of epiplanktonlc medusae were studied at an oceanic station off northern Namibia (18°00’S /10°30’E). Samples were collected from five separate depth strata (200–100 m, 100–60 m, 60–40 m, 40–20 m, and 20–0 m) using a multiple opening and closing RMT 1 x 6 net over a 48–h period. The area was characterized by mixing of the Angola Current surface waters with the northernmost subsurface waters of the Benguela Current. During the sampling period there was a continuous flow of water from Angola, giving rise to a strong thermocline between 20 and 40 m depth. A total of 17 species of medusae were captured, of which Aglaura hemistoma, Liriope tetraphylla, and Solmundella bitentaculata were the most abundant. Both the number of species and the number of individuals were higher on the second than on the first day of sampling, chiefly in the 20–0 stratum. Differences in abundance appeared to be related to the patch size of each species. The depth diistributlon of the medusae population was characterized by the existence of two assemblages, one associated with each current, and by the presence of the thermocline, which acted as a boundary between the two water masses and did not facilitate migratory movements of the most abundant species. The non-migratory distribution pattern of most of the species was attributable to the high concentration of potential prey items and to the absence of predators in the layer above the thermocline throughout the sampling period.

Small-scale distribution of a cnidarian population in the western Mediterranean

The neritic area of the coast of Catalonia (north-west Mediterranean) has a continental shelf narrower in the northern half and broader in the southern half. This area is crossed from north to south by a coastal current whose intensity decreases to the south. Both conditions, together with other local geographical and hydrographical features, are thought to be the key factors in determining the distribution of planktonic cnidarian assemblages, along two main variational axes: a north-south axis and a coast-open sea axis. Aiming to find support to this hypothesis, the planktonic cnidarians collected in two seasons of the year 1983 have been studied. The first season (April-May) corresponds to the highest abundance and heterogeneity, and the second (September-October), to the epoch when the cnidarian population is lower, but constant, along the coastal area. In a global principal component analysis the factor that best explains the group distribution in the area is the coastal character, as given by the bottom depth of the sampling station. The number of individuals is greater near the coast (due to the dominance of a euryhaline species, Muggiaea atlantica), but the species number increases towards the open sea (due to the addition of more oceanic species, such as Narcomedusae and Trachymedusae and the effect of a permanent hydrographic front at the shelf-slope boundary). The second factor is the latitudinal axis. The northern half of the coast is under the influence of waters favouring a greater number of species and individuals. On the contrary, the southern half shows a less marked influence of the north-south current, their cnidarian assemblages being more constant over time. Although the global cnidarian population follows this general pattern, the most abundant and frequent species (M.atlantica and Aglaura hemistoma) show a broad distribution over all the studied area because they are rather insensitive to hydrographica l variations.

Considerações em tôrno das hydromedusae da região de Fernando de Noronha

During a seven days trip to the oceanic island of Fernando de Noronha, plankton was hauled from depths varying between 50 m and the surface. All the nets used were open nets, one was 1 m wide at the mouth, the other 25 cm wide. Due to lack of suitable equipment only the surface temperature and salinity were taken. Five holoplanktonic species wore collected, being Aglaura hemistoma the most abundant (145 specimens) and present at almost every station. Next in frequency of specimens and number of stations are: Liriope tetraphylla (27 specimens), Rhopalonema velatum (25 specimens), Solmundella bitentaculata (2 specimens) and finally Geryonia proboscidalis (1 specimen). Most of the specimens were immature. Of the meroplanktonic species, the most abundant was Eutima mira (7 specimens), next Obelia (perhaps Obelia geniculata). 4 specimens), Phialidium sp. (the medusa of the hydroid Clytia. 1 specimen). a fragment of Clytia cylindrica (hydroid). Sarsia sp. (the medusa of the hydroid Syncoryne, 1 specimen) and a few hydroids of Syncoryne sp. Numerous fragments of thecate hydroids, some taken alive, some not, were also present in the samples. On the whole, the present collection is characterized firstly by a great majority of oceanic forms both in number of species and number of individuals and secondly by relative abundance of benthonic specimens in the plankton. The author believes that the first fact may be explained by the geographical position of the island, placed 175 miles away from the edge of the continental shelf or north-eartern Brazil. The second fact might be explained by the strong impact of the South Atlantic current on the insular shelf that may be strong enough to tear fragments from the benthonic hydroid colonies. Due to the impact upon the island's submerged portion, upwelling or strong turbulence might insue, responsible for the presence of benthonic animals in the plankton. Benthonic nematodes were also present.

Hydrozoa e Scyphozoa existentes no Instituto Oceanográfico II

In this paper are given distributional records of species already known to frequent brazilian waters. Full description and figures are given of speoies not hitherto known from this coast. Of the mentioned 2O speoies of hydroids, 5 are new for brazilian waters (Eudendrium carneun, E.capillare. Campanularia laevis, Amphisbet ia pulchella, Aglaura hemistoma); 3 are known only from the ooast south of Cabo Frio (Eudendrium carneun, Campanular ia laevis, Ob*lis commit sura 1 it. O.griffini, Thaumant ias raridentata, Orthopyxia crenata = O. lennoxent it, Geminella tubtilit, Amphisbetia pulchella); 7 are found both to the north and south of Cabo Frio (Pennaria fragil is, Clytia cylindrica = C.attenuate, Hebella scandens, Dynamena crisiodes f. typica, Sertularia marginata f. typica, Schisotricha billardi, Aglaophenia rigide); 4 species have a wide distribution (Eudendrium capillare, Obelia bicuspidata = Gonothyrea bicuspidata, Macrorhynchia philippina, Aglaura hemistoma) and finally of one species the known area of distribution was extended further south than Cabo Frio: Hebel la scandens. Cabo Frio has been tentatively considered as a natural zoogeographical frontier. The name Pennaria is used instead of Halocordyle, beoause the first described species certainly belonging to this genus was Sertolara pennara of Cavolini, later redescribed by Goldfuss in 1820 as Pennaria disticha. Stechow's opinion that considers oorreot the name Halocordyle is abandoned and the author now follows Bedot's view according to which the correct name for this genus is Pennaria. The speoies described for the Brazilian coast must therefore be called Pennaria fragilis (Van.). Eudendrium carneum was kept alive in sea-water aquaria. The by dran the have a well developed pseudohydrotheoa, they feed voraciously on small animals, mainly copepods and require thoroughly oxygenated water. They are rather difficult to raise in aquaria, and frequently the heads fall off or are resorbed. In these instances, the gonophores development proceeds at the expenses of the remainder portions of the coenosaro. The gonophores are produced around hydrants which become totally resorbed during their development, quiokly so in the male colonies, muoh more slowly in the female ones. Only one planula is produced in each female gonophore. The larvae emerge by rotating slowly using the orifice as fulcrum and so unscrewing themselves, so to speak, out of the gonangia. The pianolas locomotion is partly due to their oiliary covering and to a greater extent to gliding and active contraction and distension of their body. They are bright orange red and have a strong positive phototaxis. They become attached and from 60 to 66 hours after eclosion (water temperature from 23 to 84o C), the first polyp is completely formed. Eudendrium capillare was found in an aquarium and is here recorded for the first time for this coast. Campanularia laevis previously known only from cold sub antartic waters, was now found in the region of Cabo Frio at 67m depth. The hypothesis has been presented earlier of the existence of upwelling in this region (Vannucci 1951). A short description and figures are given of this species, which seems to be a cold water species. The gonothecae of Obelia bicuspidsta of these waters are here recorded for the first time. An extensive sinonimy of this species is presented, based principally on the work of Hummelinck. The finding of Orthopyxis billardi nom. nov., described by Billard as Eucopella crenata has made it possible to determine the sinonimy of Orthopyxis crenata and O.lennoxensis and clear the confusion until now existing for what concerns this species. Orthopyxis billard i has an interesting type of articulation by means of soft peridermic portions connecting successive hard segments of the peduncle. This articulation must confer to the peduncle a high flexibility associated with high resistance. Amphisbetia pulchella new for Brazil, was collected at Rio de Janeiro, a large, dead colony. Aglaura hemistoma, as was to be expected since it is a circumtrepical planctonic Trachynemid, was found to be abundant at 5-10m depth about 20o lat. S - 36o long. W, Jaseur Bank. Four species of Scyphoxoa are recorded, none of them is new. The first was described primarily by Fritz Muller from Sta.Catarina; it is the common Chiropsalmus quadrumanus, a voracious carnivore it is a very common species and is usually entangled in the fishermens nets, specially during summer. Chrysaora hysoscella was already known from Brazil, and was now collected in the low salinity waters of the region of Cananeia (southern part of the coast of the State of S.Paulo). Stomolophus meleagris and Lychnorhisa lucerna are two very common, even if not very abundant, species.