Survey of the occurrence and morphology of sub-cuticular bacteria in shelf echinoderms from the north-east Atlantic Ocean

Echinoderms from the shelf seas around the British Isles were examined by epifluorescence and transmission electron microscopy for the presence of sub-cuticular bacteria (SCB). Information was obtained on SCB distribution in 63 of the 88 species known from this area, and 40 of these contained SCB. The SCB were present in all five classes. Only 39% of the holothurian species had SCB. In each of the other four classes, 60% of the species were associated with SCB. No correlation was noted between host ecology and SCB distribution or morphology. SCB distribution appeared to be related to host phylogeny. Congeneric echinoderms almost always had or lacked SCB, and this was usually also true for co-familial species. SCB showed limited morphological diversity, with all of them fitting into a classification of three general types. There was no morphological evidence suggesting that they are chemoautotrophs or methylotrophs. Some species had two morphological types of symbiont. SCB load was quantified for four species, three of which had symbiont densities >10⁹ SCB g^-1 ash-free dry wt. SCB are probably heterotrophic and may be co-evolving with their hosts.     

Extremes of metabolic strategy in Antarctic Bryozoa

The physiology of many organisms in many places varies with seasonality of environment characteristics. Polar marine environments are amongst the least and most seasonal. For example, sea temperature varies by <3°C while light climate and primary productivity vary more than elsewhere. Polar nearshore invertebrates generally live life at a slow pace with recorded metabolic rates of 22–454 g O₂/g ash-free dry-mass/h. We measured oxygen uptake by three Antarctic cheilostomatid bryozoan species during summer and winter months. Our data suggest that, in many ways, bryozoans differ from other animals. They have enormous metabolic flexibility; more within the order Cheilostomatida than in all other polar marine animals measured to date. One species alone, Kymella polaris, had both the lowest (16.8) and highest (574.9 g O₂/g ash-free dry-mass/h levels recorded. Furthermore, Camptoplites bicornis showed no significant change in oxygen consumption with season whereas the final species, Isoseculiflustra tenuis, showed the highest winter to summer metabolic rise (ratio =3.45) reported for a polar species. They may not have a resting state analogous to other animals, but exhibit either full activity or dormancy in which most organic tissues are reabsorbed. The concept of resting metabolism in this taxon may be inappropriate.Communicated by J.P. Thorpe, Port Erin     

Bacterial nitrification activity directly associated with isolated benthic marine animals

Potential nitrification rates (PNR) directly associated with isolated marine macrobenthic invertebrates were measured for a range of benthic epifaunal and infaunal species (bivalves, gastropods, polychaetes and crustaceans) collected from the Sacca di Goro, Po River delta, Italy. In the case of the filter-feeding bivalves, Tapes philippinarum and Mytilus galloprovicialis the PNR associated with the shell surfaces and dissected animal tissues (gills, siphons and residual tissue) were determined separately, in order to assess the distribution of the nitrifier populations. Significant PNR was found associated with all the tested macrofaunal species with activities ranging between 12 and 2,250 nmol ind.^–1 day^–1 and specific activities between 150 and 18,400 nmol g^–1 dry weight day^–1. However, no simple relationships were observed between PNR and the animals taxonomic or functional group, or with animal comportment (infaunal or epifaunal) or size class, indicating that more complex interactions may regulate the degree of colonisation of the animals by nitrifier populations. Incubations of shells alone and dissected tissues of the bivalves T. philippinarum and M. galloprovicialis demonstrated that approximately 50% of the total PNR activity was associated with the shell surfaces and 50% with the internal animal tissues, with the highest specific activities of 950 and 1,970 nmol g^–1 dry weight day^–1 determined for the gills of T. philippinarum and M. galloprovicialis, respectively. Thus, specific relationships may exist between the nitrifiers and their animal hosts. Overall, our data indicate that the macrofaunal stimulation of nitrification and/or coupled nitrification–denitrification observed in previous studies may not be solely due to the animals burrow walls serving as sites for nitrification, but also to the fact that the internal and external surfaces of the animals themselves are also colonised by nitrifying bacteria. Tentative calculations based on reported animal densities in the Sacca di Goro and the determined PNRs indicate that animal-associated nitrifier populations could contribute significantly to overall nitrification rates in situ, although further experiments are required to determine to what extent the potential rates measured in this study are realised under in situ conditions.Communicated by R. Cattaneo-Vietti, Genova     

Photosynthetic performance of Laminaria solidungula measured in situ in the Alaskan High Arctic

Photosynthetic performance in the kelp Laminaria solidungula J. Agardh was examined from photosynthesis irradiance (P-I) parameters calculated from in situ ¹⁴C uptake experiments, using whole plants in the Stefansson Sound Boulder Patch, Alaskan Beaufort Sea, in August 1986. Rates of carbon fixation were determined from meristematic, basal blade, and second blade tissue in young and adult sporophytes. Differences in saturating irradiance (I _k, measured as photosynthetically active radiation, PAR), photosynthetic capacity (P _max), and relative quantum efficiency () were observed both between young and adult plants and between different tissue types. I _k was lowest in meristematic tissue (20 to 30 E m^–2 s^–1) for both young and adult plants, but consistently 8 to 10 E m^–2 s^–1 higher in young plants compared to adults in all three tissues. Average I _k for non-meristematic tissue in adult plants was 38 E m^–2 s^–1. Under saturating irradiances, young and adult plants exhibited similar rates of carbon fixation on an area basis, but under light limitation, fixation rates were highest in adult plants for all tissues. P _max was generally highest in the basal blade and lowest in meristematic tissue. Photosynthetic efficiency () ranged between 0.016 and 0.027 mol C cm^–2 h^–1/E m^–2 s^–1, and was highest in meristematic tissue. The relatively lower I _k and higher exhibited by L. solidungula in comparison to other kelp species are distinct adaptations to the near absence of light during the eight-month ice-covered period and in summer when water turbidity is high. Continuous measurement of in situ quantum irradiance made in summer showed that maximum PAR can be less than 12 E m^–2 s^–1 for several days when high wind velocities increase water turbulence and decrease water transparency.The Univeristy of Texas Marine Science Institute Contribution No. 695     

Food limitation stimulates metamorphosis of competent larvae and alters postmetamorphic growth rate in the marine prosobranch gastropodCrepidula fornicata

The effects of food limitation on growth rates and survival of marine invertebrate larvae have been studied for many years. Far less is known about how food limitation during the larval stage influences length of larval life or postmetamorphic performance. This paper documents the effects of food limitation during larval development (1) on how long the larvae ofCrepidula fornicata (L.) can delay metamorphosis in the laboratory after they have become competent to metamorphose and (2) on postmetamorphic growth rate. To assess the magnitude of nutritional stress imposed by different food concentrations, we measured growth rates (as changes in shell length and ash-free dry weight) for larvae reared in either 0.45-m filtered seawater or at phytoplankton concentrations (Isoehrysis galbana, clone T-ISO) of 1 × l0³, 1 × 10⁴, or 1.8 × 10⁵ cells ml^–1. Larvae increased both shell length and biomass at 1 × 10⁴ cells ml^–1, although significantly more slowly than at the highest food concentration. Larvae did not significantly increase (p > 0.10) mean shell length in filtered seawater or at a phytoplankton concentration of only 1 × 10³ cells ml^–1, and in fact lost weight under these conditions. To assess the influence of food limitation on the ability of competent individuals to postpone metamorphosis, larvae were first reared to metamorphic competence on a high food concentration ofI. galbana (1.8 × 10⁵ cells ml^–1). When at least 80% of subsampled larvae were competent to metamorphose, as assessed by the numbers of indlviduals metamorphosing in response to elevated K⁺ concentration in seawater, remaining larvae were transferred either to 0.45-m filtered seawater or to suspensions of reduced phytoplankton concentration (1 × 10³, 1 × 10⁴, or 5 × 10⁴ cells ml^–1), or were maintained at 1.8 × 10⁵ cells ml^–1. All larvae were monitored daily for metamorphosis. Individuals that metamorphosed in each food treatment were transferred to high ration conditions (1.8 × 10⁵ tells ml^–1) for four additional days to monitor postmetamorphic growth. Competent larvae responded to all food-limiting conditions by metamorphosing precociously, typically 1 wk or more before larvae metamorphosed when maintained at the highest food ration. Surprisingly, juveniles reared at full ration grew more slowly if they had spent 2 or 3 d under food-limiting conditions as competent larvae. The data show that a rapid decline in phytoplankton concentration during the larval development ofC. fornicata stimulates metamorphosis, foreshortening the larval dispersal period, and may also reduce the ability of postmetamorphic individuals to grow rapidly even when food concentrations increase.     

Influence of temperature and salinity on the uptake,distribution and depuration of mercury,cadmium and lead by the black-lip oyster Saccostrea echinata

Saccostrea echinata (Quoy and Gaimard) were exposed to 10 g 1^-1 of either mercury, cadmium or lead at 30 °C, 36S; 30 °C, 20S; 20°C, 36S and 20°C, 20S for 30 d and were then transferred to clean seawater for a further 30 d to depurate. Specimens were removed at regular intervals during the exposure and depuration periods, dissected into gills, mantle, visceral mass and adductor, and analysed for the appropriate metal by atomic absorption spectroscopy. Mercury was concentrated more than the other metals in all tissues under all conditions. Cadmium uptake was greater than lead in all tissue in the high-temperature experiments, whereas both metals were concentrated to similar extents at low temperature. The gill tissue generally accumulated the greatest amount of all 3 metals, whilst the adductor concentrated the least amount. At both salinities, mercury and cadmium accumulation by all tissues was significantly greater at the higher temperature whereas lead uptake was only marginally increased. The accumulation rates of mercury at high temperature were significantly greater in all tissues at low compared with high salinity, whereas at low temperature, differences were not significant. Accumulation rates of cadmium and lead in the majority of tissues examined were significantly greater in lowsalinity water at both temperatures. In general, lead was lost the most rapidly from oyster tissues, followed by mercury and then cadmium. The residence times for mercury and cadmium differed significantly between tissues, with the gills showing the highest turnover rate. In contrast, residence times for lead were similar between tissues. Losses of all 3 metals from oyster tissues were not obviously influenced by temperature and only mercury losses differed significantly between salinities.     

Ecology of a corallivorous gastropod,<Emphasis Type="Italic"> Coralliophila abbreviata</Emphasis>, on two scleractinian hosts. I: Population structure of snails and corals

Despite their potential importance in structuring reef communities, invertebrate corallivores and their population structures are poorly understood. We found distinct differences in the population structures (length-frequency distribution and sex ratio) of the corallivorous gastropod Coralliophila abbreviata residing on two coral-host taxa, Montastraea spp. and Acropora palmata, in the Florida Keys. In each of two survey years, around 50% of the Montastraea spp. colonies were infested, with a mean snail density of eight snails per infested colony (range 1–45), while around 20% of A. palmata colonies harbored three snails per infested colony (range 1–23). Variation in patterns of snail occurrence was also observed within a host taxon. A. palmata occurred in low- and high-density stands (0.4 and 1 colony m^–2, respectively, at the initial survey) at different sites. Hurricane Georges struck the area in September 1998. When resurveyed in 1999, density of colonies in low-density stands had decreased by 75% to 0.1 colonies m^–2. This decrease was accompanied by a doubling in the proportion of colonies infested with snails (from 19% to 46%) and an increase in snail density per infested colony (from 3.7±3.3 SD to 5.4±4.6 SD) as snails apparently concentrated on surviving A. palmata. In contrast, sites with high density A. palmata stands (thickets) retained colony densities of about ~1 colony m^–2 among years, while snail infestation increased only from 9% to 14% of colonies surveyed and snail density essentially remained unchanged (from 2.7±1.8 to 2.9±1.9 snails per infested colony). Snails collected from Montastraea spp. were shorter than those from A. palmata in low-density stands and were longest on A. palmata in thickets. On both host taxa, female snails were longer than males. The sex ratio of snails on Montastraea spp. hosts was even (1:1), while that of snails on A. palmata was skewed (70% males). Factors that could explain observed differences in size structure and sex ratio between Coralliophila populations on the two coral host taxa include: differential susceptibility to predators, influence of host tissue nutritional quality and/or secondary metabolite content, and genetic differences (cryptic species). The host-specific characteristics of C. abbreviata populations imply that the impact of gastropods on reef communities will vary with the coral species composition.     

Field survey of the occurrence and significance of regeneration in Amphiura chiajei (Echinodermata: Ophiuroidea) from Killary Habrour,west coast of Ireland

The boreo-Mediterranean amphiurid Amphiura chiajei Forbes occurs in high numbers (700 individuals/m²) in Killary Harbour, a fjordic inlet on the west coast of Ireland. 99.1% of the adult individuals show signs of arm regeneration, while 0.5% show regeneration of the disc. A study of the phenomenon commenced in November 1987 and continued on a seasonal basis until July 1989. Specimens were collected by SCUBA. The overall mean length of regenerated arm tissue over the 21 mo sampling period was 33.5±6.5 mm. Regeneration points were most common in the proximal and in the distal one-third of each arm. An overall mean of 4.21±0.3 arms per individual showed evidence of regeneration, while biomass assays revealed that regenerated tissue accounted for up to 57.9% of the total body weight. Anthropogenic activity (e.g. bottom trawling) may contribute to regeneration, although sub-lethal predation appears to be the main causitive factor of regeneration. Gut analyses of potential predators at the sampling station revealed that fish species (especially the Pleuronectidae) are particularly implicated. This predation seems to vary seasonally, being greatest in the summer months. Since the larger individuals showed the greatest evidence of arm regeneration, it is assumed that such sub-lethal damage is a persistent experience. Whatever the causality, the population of A. chiajei in Killary Harbour is deemed to be highly resilient.     

The role of chemosensory behavior of Symbiodinium microadriaticum,intermediate hosts,and host behavior in the infection of coelenterates and molluscs with zooxanthellae

Symbiotic dinoflagellates, Symbiodinium microadriaticum (=zooxanthellae), may gain access to aposymbiotic hosts (i.e., those lacking zooxanthellae) by chemosensory attraction of the motile algae by the potential host or via an intermediate host. Laboratory experiments showed that motile zooxanthellae were attracted to intact aposymbiotic host animals, but not to starved symbiotic hosts. Fed symbiotic hosts and brine shrimp (Artemia sp.) nauplii also attracted motile zooxanthellae. The attraction of these zooxanthellae was directly correlated with nitrogen levels in the seawater surrounding the hosts; thus ammonia and possibly nitrate could be atractants. Brine shrimp nauplii, acting as intermediate hosts actively ingested both motile and non-motile zooxanthellae. the ingested zooxanthellae tended to remain morphologically unaltered during and after passage through the gut of the brine shrimp. Capture and ingestion of brine shrimp containing zooxanthellae by aposymbiotic scyphistomae of the jellyfish Cassiopeia xamachana led to infection of the scyphistomae with zooxanthellae. Zooxanthellae isolated from 17 different species of coelenterates and molluscs could be transferred via brine shrimp to the endodermal cells of the scyphistomae. However only 10 of these isolates persisted to establish a permanent association with C. xamachana. Scyphistomae in suspensions of motile zooxanthellae responded by a classical coelenterate feeding response, which may facilitate ingestion of the potential symbionts and establishment of a symbiosis.     

Dinoflagellate infections of Favella panamensis from two North American estuaries

Favella panamensis Kofoid and Campbell, 1929 is seasonally abundant in meso- to polyhaline waters of Chesapeake Bay and Indian River, Florida, USA, where it reaches densities of 10³ cells l^-1. During the summers of 1986–1992. F. panamensis populations of the two estuaries were commonly infected by the parasitic dinoflagellate Duboscquella aspida Cachon, 1964. The intracellular phase of the parasite reached maturity in 21 h (30 °C) and consumed 35% of the host's biomass. Infections were not typically lethal to F. panamensis, but sometimes forced the host from its lorica. Several D. aspida were found in the cytoplasm of many hosts, and the number of parasites infection^-1 was directly related to infection level. Parasite prevalence averaged 24.0 and 11.5% with mean number of parasites infection^-1 being 1.5 and 1.3 for Chesapeake Bay and Indian River samples, respectively. D. aspida was estimated to remove up to 68% of host standing stock d^-1 with a mean of 10% for all samples. The average impact of parasitism on F. panamensis populations was somewhat less than would be expected from copepod grazing.     

Glycerol translocation in Condylactis gigantea

Sodium cyanide (NaCN) was used to partially uncouple respiration and photosynthesis in the symbiotic sea anemone Condylactis gigantea. NaCN significantly increased the ratio of gross photosynthesis to respiration in both intact tentacles and isolated zooxanthellae (Symbiodinium microadriaticum), increased carbon translocation from 17.7±3.5% of total fixed in controls to 43.5±5.8%, and doubled the amount of photosynthetically fixed carbon accumulating in the coelenterate host over that in controls. Only 2% of the non-particulate radioactivity recovered in the host tissue was ¹⁴C-glycerol when uninhibited symbiotic tentacles were incubated in ¹⁴C-bicarbonate for 1 h. At 10^-5 M NaCN, approximately 25% of the host nonparticulate radioactivity was recovered as ¹⁴C-glycerol, the absolute concentration of glycerol in the host tissue was three times higher than in controls, and ¹⁴C-glycerol was found in the medium. While glycerol has been proposed to play a major role in the translocation of photosynthetically fixed carbon from zooxanthellae to their coelenterate hosts, its concentration has never been measured in the animal and algal components of the symbiosis. The isolated zooxanthellae contained 3.62±0.33 mM glycerol, 26x the 0.141±0.02 mM found in the anemone. Aposymbiotic anemone tissue contained 0.169±0.06 mM glycerol. The rate of glycerol mineralization was not saturated even when exogenous glycerol levels were 70x internal concentrations. These data show that respiration and photosynthesis in symbiotic associations may be partially uncoupled by NaCN, and that this uncoupling allows the verification of the translocation and rapid catabolism of glycerol within the host.     

Diet and trophic position of Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) inferred from stable carbon and nitrogen isotope analysis

Stable ¹³C and ¹⁵N isotope analyses of scale, bone, and muscle tissues were used to investigate diet and trophic position of North Atlantic bluefin tuna (Thunnus thynnus Linnaeus) during residency in the northwest Atlantic Ocean off the northeast coast of the United States. Adult bluefin tuna scales collected from fish between June and October 2001 were significantly enriched in ¹³C compared to both muscle and bone across all months, while muscle was significantly enriched in ¹⁵N compared to either bone or scale throughout the same period. In muscle tissue, there was evidence of a shift over the summer from prey with ¹³C values (–17 to –18) that were characteristic of silver hake (Merluccius bilinearis) to species with ¹³C values of –20 to –21 that were similar to Atlantic herring (Clupea harengus) and sandlance (Ammodytes americanus). Depletion of ¹⁵N values in adult scales and bone compared to muscle tissue may be explained by bone and scale samples representing juvenile or life-long feeding habits, isotopic routing, or isotopic differences in amino acid composition of the three tissue types. Adult bluefin tuna were estimated to be feeding at a trophic position similar to pelagic sharks in the northwest Atlantic Ocean, while the trophic positions of yellowfin tuna (Thunnus albacares), albacore tuna (Thunnus alalunga), and juvenile bluefin tuna were indicative of a diet of up to a full trophic position below adult bluefin tuna. The close relationship between the juvenile bluefin ¹⁵N values and those of suspension feeders suggests that nektonic crustaceans or zooplankton may contribute significantly to the diet of bluefin tuna, a food source previously overlooked for this species in the northwest Atlantic Ocean.Communicated by J.P. Grassle, New Brunswick     

Biology,population dynamics and secondary production of<Emphasis Type="Italic"> Tylos europaeus</Emphasis> (Isopoda,Tylidae) on the western coast of Portugal

The biology, population dynamics, and production of Tylos europaeus were studied in two sandy beaches of the western coast of Portugal. At both sites, reproduction occurred seasonally, from April to July, with only one new cohort produced per year. Regarding population dynamics, cohort-splitting events were detected in males at the beginning of the reproduction period (April/May), resulting in two groups with distinct growth rates (fast-growing vs slow-growing males). Different biological characteristics were consequently detected in these two groups, namely regarding body size, lifespan, and contribution to the reproductive effort. Lifespan was estimated as approximately 3 years, for females and fast-growing males, and 4 years for slow-growing males. Cohort-splitting among males appeared as a possible strategy to cope with the highly male-biased sex ratios observed, which could lead to a strong male-male competition for mating. T. europaeus appeared as an annual species, with a univoltine life-cycle (one generation per year), and iteroparous females reproducing twice during their lifespan. Average growth production (P) was estimated at 0.082 g.m^–2.yr^–1 AFDW (ash-free dry weight) and the average annual biomass () (standing stock) at 0.052 g.m^–2, resulting in a P/ ratio of 1.58. These results produced baseline information for the construction of a population-dynamics model and highlighted the potential of this species as an environmental quality-assessment bioindicator on sandy shores.Communicated by S.A. Poulet, Roscoff     

Lipids and stable carbon isotopes in two species of Hawaiian corals,<Emphasis Type="Italic"> Porites compressa</Emphasis> and<Emphasis Type="Italic"> Montipora verrucosa</Emphasis>, following a bleaching event

Mass coral bleaching events have occurred on a global scale throughout the worlds tropical oceans and can result in large-scale coral mortality and degradation of coral reef communities. Coral bleaching has often been attributed to periods of above normal seawater temperatures and/or calm conditions with high levels of ultraviolet radiation. Unusually high shallow-water temperature (>29°C) in Kaneohe Bay, Hawaii, USA, in late summer (20 August–9 September) and fall (1–7 October) of 1996 produced visible bleaching of two dominant corals, Porites compressa Dana, 1864 and Montipora verrucosa Dana, 1864. The present study examined chlorophyll a (chl a), total lipid concentrations, and lipid class composition in corals of both species in which the entire colony was non-bleached, moderately bleached, or bleached. Skeletal, host tissue, and algal symbiont ¹³C values were also measured in non-bleached and bleached colonies. In additional unevenly bleached colonies, paired samples were collected from bleached upper surfaces and non-bleached sides. Samples were collected on 20 November 1996 during the coral recovery phase, a time when seawater temperatures had been back to normal for over a month. Chl a levels were significantly lower in bleached colonies of both species compared with non-bleached specimens, and in bleached areas of unevenly bleached single colonies. Total lipid concentrations were significantly lower in bleached P. compressa compared with non-bleached colonies, whereas total lipid concentrations were the same in bleached and non-bleached M. verrucosa colonies. The proportion of triacylglycerols and wax esters was lower in bleached colonies of both species. Both bleached and non-bleached M. verrucosa had from ~17% to 35% of their lipids in the form of diacylglycerol, while this class was absent in P. compressa. ¹³C was not significantly different in the host tissue and algal symbiont fractions in non-bleached and bleached samples of either species. This suggests that the ratio of carbon acquired heterotrophically versus photosynthetically was the same regardless of condition. Skeletal ¹³C was significantly lower in bleached than in non-bleached corals. This is consistent with previous findings that lower rates of photosynthesis during bleaching results in lower skeletal ¹³C values. The two species in this study displayed different lipid class compositions and total lipid depletions following bleaching, suggesting that there is a difference in their metabolism of lipid reserves and/or in their temporal responses to bleaching and recovery.Communicated by J.P. Grassle, New Brunswick     

Population dynamics and secondary production in an estuarine population of Nephtys hombergii (Polychaeta: Nephtyidae)

From July 1978 to March 1980, a study was made on the distribution, population dynamics and secondary production of Nephtys hombergii Audouin et Edw. occurring in the sublittoral industrialised region of Southampton Water in south England. The distribution of the worm was related to the silt content and copper level of the sediment, the greatest densities of N. hombergii being found in sediment containing 60 to 100% silt. Breeding occurred at a low level throughout the year, with a maximum in July to September and November to January in the second year of growth. Spawning occurred when the oocytes measured 200m in diameter, and unshed gametes were resorbed. Annual production varied between 0.092 and 4.32 g C m^-2 yr^-1 (ash-free dry weight) and amounted to 1.9–39.4% of the total macrofaunal production at the sampling stations. The production:biomass (P:B) ratio of the species varied between 1.6 and 2.9.     

Temporal and spatial variation in bacterial production in the water column over a coral reef

Production and doubling times of the bacterial populations in the water around and over the reefs at Lizard Island, Great Barrier Reef were measured during summer and winter, 1982 and 1983. Bacterial productivity, determined from the rate of tritiated thymidine incorporation into DNA, was high over the reef flats and a Thalassia hemprichii sand flat (28 to 58 g Cl^-1 d^-1). Bacterial growth rates increased during the day and fell at night over the reef flats and seagrass bed. Growth rates were slower over the reef front and in open water. Doubling times ranged from about 2 d in the open water to about 3 h over the reef flat in summer. As numbers did not increase, grazing was probably intense on the reef flats. Growth rates were much slower in winter. The main source of organic nutrient used by the bacteria was probably mucus released following photosynthesis in the corals. The cyanobacterium Synechococcus sp. was sometimes very numerous, especially in summer when 2×10⁸ cells l^-1 were recorded in one water mass. The number of bacteria was also very high in summer, with values ranging from 1×10⁹ to 2.5×10⁹l^-1.     

Carbon metabolism and strobilation in Cassiopea andromedea (Cnidaria: Scyphozoa): Significance of endosymbiotic dinoflagellates

Scyphopolyps and scyphomedusae of Cassiopea andromeda Forskål (Cnidaria, Scyphozoa) containing dinoflagellate endosymbionts (zooxanthellae) were investigated for rates and pathways of carbon fixation. Photosynthesis by the algae, accounting for 80 and 15 mol C h^-1 on a dry weight basis in medusae and polyps, respectively, by far exceeds dark incorporation of inorganic carbon by the intact association. Photosynthetic carbon fixation is operated via C₃ pathway of carbon reduction. DCMU-treatment (1×10^-6 M and 1×10^-5 M) completely inhibits light-dependent carbon assimilation. Major photosynthates presumably involved in a metabolite flow from algal symbionts to animal tissue are glycerol and glucose. A total of 5–10% net algal photosynthate appears to be seleased in vivo to the host. This is probably less than the energy supply ultimately required for the nutrition of the polyps and medusae. The presence of zooxanthellae proved to be indispensable for strobilation in the scyphopolyps. However, photosynthesis by algal symbionts as well as photosynthate release is obviously not essential for the initiation of ephyrae as is shown by DCMU-treatment, culture in continous darkness, and aposymbiotic controls. It is therefore concluded that strobilation is supported, but not triggered by algal photosynthetic activity. The induction of strobilation thus seems to depend on a more complex system of regulation.     

Responses of Metaphycussp. nr. flavus to semiochemicals released from a scale host, Coccus hesperidum

Summary. Metaphycus sp. nr. flavus (Encyrtidae: Hymenoptera) is a parasitoid species collected from the Mediterranean region which lays its eggs in the immature stages of several economically important soft scale insects (Hemiptera: Coccidae), including brown soft scale, Coccus hesperidum L. (= host insect). Preliminary tests suggested that the parasitoid is most successful in producing offspring when it oviposits in the younger stages of brown soft scale. In Y-olfactometer bioassays measuring wasp choices and residence times, naïve parasitoids were significantly more attracted to yucca leaves infested with 26, 27, or 28 d-old scale than to uninfested leaves, whereas leaves with older (29-30 d-old) scale were no more attractive than uninfested leaves. Parasitoids also spent significantly more time in the arm with yucca leaves infested with 26 d-old scale than in the arm with uninfested leaves. These results are consistent with observations of the parasitoids reproductive success on scale of different ages, whereby older scale are more likely to encapsulate the developing eggs of M. sp. nr. flavusfemales than are younger scale. Further bioassays determined that yucca leaves that had been infested with 26 d-old scale but from which the scale had been removed were as attractive as infested leaves. In contrast, infested yucca leaves from which scale had been removed and the leaves subsequently washed with distilled water were less attractive than infested leaves. Furthermore, the wash water containing scale residues was attractive to female wasps. In total, these results suggest that Metaphycussp. nr. flavus females utilize volatile, water soluble compounds produced by brown soft scale as cues to locate suitable hosts.     

Nemertean,copepod, and amphipod symbionts of the dimorphic ascidian Pyura stolonifera near Melbourne,Australia: specificities to host morphs,and factors affecting prevalences

On the central coast of Victoria. Australia, the dimorphic ascidian Pyura stolonifera (Heller, 1878) harbors three endosymbionts: the nemertean Gononemertes australiensis Gibson, 1974, the copepod Doropygus pulex (Thorell, 1859), and the amphipod Paraleucothoe novaehollandiae (Haswell, 1880). The specificities of these symbionts to two host colour morphs were studied during 1989 to 1991 as part of a multidisciplinary investigation aimed at determining whether the two morphs are genetically distinct. Distributional surveys revealed that nemerteans and copepods occur only in yellow and brown ascidians, respectively, and that amphipods live in both forms. These specificities held true not only when the two morphs were in allopatry, but also in sympatry. These observations, especially the sympatric data, suggest that the two host morphs might be genetically distinct. For example, the two morphs might have different genetically encoded internal milieus that favour the survival of nemerteans in yellow ascidians, and copepods in brown hosts. In transplant experiments, which involved moving ascidian morphs within and between habitats, the wrong symbionts never colonised the wrong hosts. These results, although consistent with the hypothesis of genetic maintenance of specificity, were deemed inconclusive because of the difficulty of establishing reliable controls (i.e. vacant hosts). The relationships between symbiont prevalences and several factors (season, year, site within host, host individual, host habitat, host size/age, host breeding condition, and co-occurrence of other symbiont species) were also analysed. Both simple (e.g. greater prevalences for large hosts) and complex (e.g. prevalence x season x gonad state of host) interactions were detected for all three symbiont species. These are among the very few quantitative analyses of factors affecting prevalences of ascidicolous nemerteans and amphipods. The present report identifies one of very few definite nemertean-ascidian symbioses. Since no differences in gross condition were ever noticed between occupied and vacant hosts, it is suggested that all three symbionts are commensals rather than parasites or mutuals.     

Growth,mortality and shell morphology of cultivated mussel (Mytilus edulis) stocks cross-planted between two Scottish sea lochs

One-year-old rope-grown blue mussels (Mytilus edulis L.) were held in experimental lantern nets and cross-transplanted between two sea lochs (Lochs Etive and Leven) on the west coast of Scotland. Growth and mortality were monitored from May 1991 to May 1992, as well as shell morphology in native and transplanted mussels. There were highly significant differences (P0.001) in growth rates between lochs; both native and transplanted mussels performed better in all shell and tissue growth-parameters in Loch Etive than in Loch Leven. Stock also had a significant (P0.05) influence on shell length and live weight, but its contribution to total variance was considerable only in the latter case. Wet and ash-free dry meat weights were governed mainly by site and to some extent by site x stock interaction. Mortality rates were quite low (6 to 7% yr^-1) and did not show any significant variation either between sites or stocks. There were significant morphological differences (ratios of shell length, height and width) between the Loch Etive and Loch Leven populations (P0.001) which persisted for one year after transfer. Stock appeared to be the main factor affecting shell morphology, as opposed to site. Linear regressions of shell height and width on length were significantly different between the native stocks, but became indistinguishable from those of the recipient populations one year after reciprocal transfer. This did not, however, conceal the effect of stock origin on dimensional ratios. It is concluded that site differences related to environmental factors, notably chlorophyll a levels and currents, and also possible stress caused by high levels of Zn and Cu, are major determinants of growth, which are of main interest to aquaculture, while morphological differences most probably result from genotypic variation.