Seagrass and algal beds as nursery habitats for tiger prawns (Penaeus semisulcatus and P. esculentus) in a tropical Australian estuary

We evaluated the importance of seagrass and algae to two species of tiger prawns (Penaeus semisulcatus and P. esculentus) by detailed sampling at four sites (two seagrass, two algae) in the Embley River estuary, and through sampling 26 sites in 7 adjacent estuaries at one time of year. Samples of tiger prawns were collected in the Embley River estuary with a small beam trawl at night every 2 wk from September to May for 2 yr (1990 to 1992). The two seagrass sites, which were 11 and 13 km from the river mouth, showed less seasonal variation in salinity than the two algal sites, which were 15 and 20 km from the river mouth. The algal beds at the two upstream sites almost disappeared during the wet season, but the biomass of seagrass did not change significantly between the wet and dry seasons. The grooved tiger prawn (P. semisulcatus), the main species at all sites, comprised 88% of the total tiger prawn catch over the two years. They were found at all sites during the pre-wet season, but after the onset of the wet season, they disappeared along with the algae, from the upstream sites. The brown tiger prawn (P. esculentus) was found almost exclusively (97% of the total catch) on the seagrass sites downstream. In the study of several estuaries, juvenile P. semisulcatus were caught at all 26 sites, and P. esculentus were caught in much smaller numbers, at 16 sites. Approximately equal numbers of P. semisulcatus were caught in seagrass and algal beds in the pre-wet season. Very few individuals >10 mm carapace length of either species, were caught. The results from this study highlight the importance of algal beds during the pre-wet season as nursery areas for one species of tiger prawn (P. semisulcatus).     

The value of patches of intertidal seagrass to prawns depends on their proximity to mangroves

Penaeid prawns were sampled with a small seine net to test whether catches of postlarvae and juveniles in seagrass were affected by the distance of the seagrass (mainly Zostera capricorni) from mangroves and the density of the seagrass in a subtropical marine embayment. Sampling was replicated on the western and eastern sides of Moreton Bay, Queensland, Australia. Information on catches was combined with broad-scale spatial information on the distribution of habitats to estimate the contribution of four different categories of habitat (proximal dense seagrass, distal dense seagrass, proximal sparse seagrass, distal sparse seagrass) to the overall population of small prawns in these regions of Moreton Bay. The abundance of Penaeus plebejus and Metapenaeus bennettae was significantly and consistently greater in dense seagrass proximal to mangroves than in other types of habitat. Additionally, sparse seagrass close to mangroves supported more of these species than dense seagrass farther away, indicating that the role of spatial arrangement of habitats was more important than the effects of structural complexity alone. In contrast, the abundance of P. esculentus tended to be greatest in sparse seagrass distal from mangroves compared with the other habitats. The scaling up of the results from different seagrass types suggests that proximal seagrass beds on both sides of Moreton Bay provide by far the greatest contribution of juvenile M. bennettae and P. plebejus to the overall populations in the Bay.Communicated by M.S. Johnson, Crawley     

Feeding ecology of the grooved tiger shrimp <Emphasis Type="Italic">Penaeus semisulcatus</Emphasis> De Haan (Decapoda: Penaeidae) in inshore waters of Qatar,Arabian Gulf

The feeding ecology of the green tiger shrimp Penaeus semisulcatus was studied in inshore fishing grounds off Doha, Qatar, using a combination of stable isotope (δ¹³C and δ¹⁵N) analysis and gut contents examination. Samples of post-larvae, juvenile and adult shrimp and other organisms were collected from intertidal and subtidal zones during the spawning season (January–June). Shrimp collected from shallow water seagrass beds were mostly post-larvae and juveniles and were significantly smaller than the older juveniles and adults caught in deeper macroalgal beds. Gut content examination indicated that post-larvae and juvenile shrimp in seagrass beds fed mainly on benthos such as Foraminifera, polychaetes, benthic diatoms and small benthic crustaceans (amphipods, isopods and ostracoda), whereas larger shrimp in the macroalgal beds fed mainly on bivalve molluscs and to a lesser extent polychaetes. In shrimp from both seagrass and algal beds, unidentifiable detritus was also present in the gut (18, 32%). δ¹³C values for shrimp muscle tissue ranged from −9.5 ± 0.26 to −12.7 ± 0.05‰, and δ¹⁵N values increased with increasing shrimp size, ranging from 4.1 ± 0.03 to 7.7 ± 0.11‰. Both δ¹⁵N values and δ¹³C values for shrimp tissue were consistent with the dietary sources indicated by gut contents and the δ¹³C and δ¹⁵N values for primary producers and prey species. The combination of gut content and stable isotope data demonstrates that seagrass beds are important habitats for post-larvae and juvenile P. semisulcatus, while the transition to deeper water habitats in older shrimp involves a change in diet and source of carbon and nitrogen that is reflected in shrimp tissue stable isotope ratios. The results of the study confirm the linkage between sensitive shallow water habitats and the key life stages of an important commercially-exploited species and indicate the need for suitable assessment of the potential indirect impacts of coastal developments involving dredging and land reclamation.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Diets of piscivorous fishes in a tropical Australian estuary,with special reference to predation on penaeid prawns

The diets of fish from the tropical Embley Estuary in the eastern Gulf of Carpentaria, Australia, were analysed with particular reference to piscivory and predation on juveniles of commercially important penaeid prawns from October 1986 until July 1988. Of the 77 species caught, 52 were piscivorous, and of these 37 ate penaeid prawns. The most numerous piscivores wereScomberoides commersonianus, Arius proximus, Lates calcarifer, Polydactylus sheridani andRhizoprionodon acutus, the first four of which accounted for over 90% of all prawns eaten. Twenty species are commercially important species of prawns. The proportions of penaeids in the diets varied seasonally, according to the density of penaeids in the estuary. The proportion was highest during the pre-wet period (November) and lowest during the dry period (July–August). Predation on prawns was highest in the lower and middle reaches of the river. Estimates of the rates of predation on the two most common juvenile commercial prawns,Penaeus merguiensis andP. semisulcatus, are presented and discussed.     

Influence of salinity and temperature on the oxygen consumption in young juveniles of the Indian prawn Penaeus indicus

Routine oxygen consumption of very young juveniles (0.1 g) of Penaeus indicus H. Milne Edwards was significantly influenced by ambient temperature and weight of the animal, but not by ambient salinity, when tested at salinities (7, 21, and 35) to which they had been long-term (over 10 days) acclimated. Standard oxygen consumption of young juvenile prawns (1 to 3 g), subjected to step-wise changes in ambient salinity, from sea water to low salinity waters (2 to 6), and measured after short-term (24 h) salinity acclimation at each step, was lowest at salinities where prawns such as those tested occur naturally (10 to 15). The metabolic rates do not appear to have a direct relation to the osmotic gradient, even when the influence of interfering activity is eliminated. It appears that factors other than osmotic gradient will have to be sought in order to explain the metabolic patterns of P. indicus in relation to salinity.     

Effects of dusk and dawn on locomotor activity rhythms in the norway lobster Nephrops norvegicus

The endogenous locomotor activity rhythm of Nephrops norvegicus (L.) shows peaks during darkness, around 3 h after dusk. At the peaks, activity occurs mainly within the burrow, although the prawns repeatedly vacate the burrows during the same periods. The phase of the rhythm shifts with a change of light-dark regime and in response to changed timing of dusk when dawn is fixed. Activity is suppressed immediately at dawn, but recommences before complete darkness during gradually reduced light at simulated dusk. This difference in response to light at dusk and dawn indicates phase responsiveness which is characteristic of endogenous rhythms entrained by light. The light-entrained endogenous rhythm is complementary to the rhythm of emergence which, reflected in commercial catches, appears to be controlled by exogenous factors.     

Sources of primary production supporting food webs in an arid coastal embayment

Penaeid shrimp can be useful ecological indicators of linkages between shallow tropical coastal habitats, acting as integrators of carbon and nitrogen sources due to their generalist feeding habits and their mobility between habitats and with tidal cycles. In the current study, the contribution of mangrove, seagrass and microbial mat to the nutrition of two penaeid shrimp species, Penaeus semisulcatus and Metapeneus ensis, in a shallow arid embayment in the Arabian Gulf was assessed through a combination of analysis of stomach contents and dual carbon and nitrogen stable isotope signatures. Shrimp tissue stable isotope signatures identified seagrass as a major source of carbon and nitrogen for both species, contributing 21–38?% (1–99?‰). Microbial mat was also detected as a significant nutritional source for early-stage Metapenaeus affinis postlarvae (1–27?%). However, mangroves were not identified as a significant source, with the range of results including the possibility of a zero contribution. Moreover, the greatest possible contribution of mangroves as source of carbon was less than for the other primary producers. This may be due the high salinity and wide temperature range limiting mangrove productivity as well as the low export of dissolved and particulate organic material out of the mangroves due to low rainfall.     

Re-appraisal of age composition,growth and survivorship of the deep-sea brittle star Ophiura ljungmani from size structure in a sample time series from the Rockall Trough

Samples were taken from a 2 900 m deep permanent station (centred on 54°40N; 12°16W) in the southern Rockall Trough, from November 1975 to September 1980, inclusive. Revision of juvenile size frequencies was necessary after removal of postlarvae of an annual non-viable settlement of Ophiocten gracilis (G. O. Sars) that were mistakenly included in a previous analysis. The present study of the revised data shows that settlement of Ophiura ljungmani (Lyman) occurs in summer when disk-size frequencies are dominated by the newly metamorphosed postlarvae. The likely age composition in larger sizes was suggested from the progression of modes along the size axis in juvenile size classes in consecutive were developed from Gaussian mixtures fitted by constrained computer search using, as starting values, component parameter values suggested from the frequency histograms. Choice between competing solutions was made on the basis both of goodness-of-fit and biological plausibility. However, identification of year-class structure amongst sizes >3 to 4 mm disk diameter remained conjectural owing to the low frequencies of adults in the samples. Comparison of the fitted component-means in the time series suggested a marked seasonal growth pattern, with maximal growth in spring. Probable age composition amongst adult sizes was extrapolated from von Bertalanffy growth parameters estimated from the means of juvenile year classes.Communicated by J. Mauchline, Oban     

Metabolism of proline by the tiger prawnPenaeus esculentus

The distribution and fate of¹⁴C-proline were investigated in immature tiger prawns,Penaeus esculentus Haswell, collected in Moreton Bay, Cleveland, Australia, by trawling during 1986/1987. Initially the prawns were fed¹⁴C-proline in food pellets to follow the pathway of proline absorption and distribution in the body.¹⁴C-proline was also injected directly into the prawn to provide sufficient tracer to follow the incorporation of¹⁴C into other amino acids and into proteins. A comparison was made of the metabolism of injected¹⁴C-proline over 48 h in prawns that had been fed and those that had been starved for 10 d. Free amino acids (FAA) in the muscle and protein-bound amino acids were analysed separately. Labelled proline was completely absorbed and distributed within the body 3 h after ingestion, about 80% being in the tissues, mostly in muscle. There was no significant difference between the total CO₂ output in fed and starved prawns, but the latter metabolised about twice the amount of labelled proline over 48 h. At this time, in abdominal muscle of fed prawns, about 95% of the total muscle label was in the FAA; of the label in the FAA, 78% was proline and 18% glutamic acid, with the remainder in hydroxyproline, aspartic acid, glutamine, alanine and Kreb's cycle intermediates. In the starved prawns, proline was 58%, glutamic acid 24%, with correspondingly higher amounts in the other compounds. In the muscle protein, the distribution of label was similar in fed and starved prawns, with 72 to 74% as proline. The experiments showed that proline is not very labile in the tiger prawn and its rate of synthesis is slow. It does not appear to be an important source of energy as in some insects and cephalopods, but during starvation is only slowly oxidised for energy.     

Relationships between the movements,growth, age structures,and reproductive biology of the teleosts Sillago burrus and S. vittata in temperate marine waters

Sillago burrus and S. vittata both use sheltered, nearshore shallow waters ( 1.5 m) as nursery areas. However, the juveniles of the former species remain there for only a few months, before migrating into deeper waters (5 to 15 m) as they increase in size, whereas some juvenile S. vittata do not undergo a similar migration until considerably later. S. burrus rarely exceeded 2 yr of age and was never found beyond 4 yr of age. Although only a small number of S. vittata exceeded 2 yr of age, a few individuals of this species were caught between 4 and 7 yr old. The maximum and asymptotic lengths of S. burrus (251 and 180 mm, respectively) were far lower than those of S. vittata (325 and 320 mm, respectively), whereas the growth coefficients (K) were much higher for the former species, i.e. 2.4 vs 0.4. Virtually all S. burrus, and also those S. vittata that moved into deeper waters early in life, spawned at the end of their first year of life. Since relatively few S. burrus reached 2 yr of age, the attainment of almost full size by the end of their first year of life enables a relatively large number of eggs to be produced by fish at the end of their first year-for many, their only spawning period. Those individuals of S. vittata that remained in their shallow nursery areas until the end of their first year of life, did not reach maturity until the end of their second year of life. The proportions of mature gonads and the numbers of yolk-vesicle and yolk-granule oocytes and post-ovulatory follicles in ovaries were far higher in both S. burrus and S. vittata during December to February than in any other month, demonstrating that these two species spawn largely in these summer months. During this period, the ovaries of individual S. burrus and S. vittata often contained post-ovulatory follicles, as well as yolkvesicle and yolk-granule oocytes that ranged widely in size, strongly suggesting that both species are multiple spawners. During the spawning period of S. burrus, the ovaries possessed large numbers of hydrated oocytes and no post-ovulatory follicles or the reverse situation, and the oocytes tended to form several relatively discrete size groups. This indicates that S. burrus produces eggs in batches and that the spawning of the members of this species is synchronised. The presence of large numbers of yolk-granule oocytes with migrating nuclei in the ovaries of many S. vittata at certain times suggests that this species is also a batch- and synchronised spawner. Comparisons between the results of the present study and past work emphasise that the relationships between the timing of offshore movements and the sizes and ages at first maturity vary considerably amongst whiting species.     

Fish assemblages in seagrass beds are influenced by the proximity of mangrove forests

Mangrove forests and seagrass beds frequently occur as adjacent habitats in the temperate waters of southeastern Australia. At low tide when fish cannot occupy mangroves they might utilise adjacent habitats, including seagrass. We first sampled small fish from seagrass beds close to and far from mangroves in the Pittwater estuary, NSW, Australia. Seagrass beds close to mangroves had a greater density of fish species than beds far from mangroves (close: mean 16.0 species net⁻¹, SE 1.0; far: 13.2, 1.3; P < 0.05). In particular, juvenile fish were in greater densities near to than far from mangroves (close: 5.3, 0.4; far: 3.1, 0.4; P < 0.05). We then sampled the mangrove forests during the high tide and seagrass beds during the low tide, in beds along a continuum of distances from mangroves. Multivariate analysis showed that fish assemblages differed with distance from mangroves, and the differences were attributed to the composition of the fish assemblage (i.e. presence/absence of fish species), not the abundances of individual species. In particular, fish that utilise mangrove forests at high tide were found in greater species densities and species richness in seagrass nearer to mangroves. A negative relationship was found between the density of mangrove-utilising fish species and the distance of the bed from mangroves (R ² = 0.37, P < 0.05). This confirms the important connectivity between mangroves and seagrass for fish in temperate Australian waters.     

Carotenoids in the tiger prawn Penaeus esculentus during ovarian maturation

Female Penaeus esculentus Haswell were collected by 15 to 20 min duration trawls during 1990. Carotenoids were analysed in the digestive gland, abdominal muscle, the remainder of the body (hereafter called integument) and ovary of prawns in Stage 2 through Stage 4 (fully mature) of maturation. The only oxycarotenoids (xanthophylls) identified were astaxanthins or astaxanthin esters; occasionally low levels of -carotene were detected in the digestive gland. The concentrations of astaxanthin monoesters (AM) and diesters (AD) were highest, with only minor amounts of free astaxanthins (Ast), except in the maturing ovaries, where free astaxanthins predominated (up to 80% of the total carotenoid). Of the total carotenoid, 82 to 94% was in the integument, but at maturity the digestive gland contained 10.7±3.4% and the ovary 5.6±0.9% of the total carotenoid. Only the ovary increased in mass during maturation, reaching up to 5.2% of total prawn mass. During this period, digestive gland concentrations of AM, AD and Ast all increased (tota 20 to 120 g g^-1); levels in the muscle and integument varied little throughout maturation (total 0.4 and 100 g g^-1, respectively); ovary AM levels remained low throughout (1.5 to 1.2 g g^-1), AD increased from only 2 to 5 g g^-1, but Ast increased from 2 to 34 g g^-1. Apart from the ovary, AM concentrations were the most variable. In common with other decapod Crustacea, the maturing ovary of P. esculentus contained high levels of carotenoids, indicating that these may have an important role in early development. The natural diet of P. esculentus includes a variety of carotenoids, but except for a little -carotene, the digestive gland, where absorption occurs, contained astaxanthins, with only an occasional trace of -carotene. This suggests that the conversion of dietary carotenoids to astaxanthin occurs soon after ingestion.     

Influence of pycnocline topography and water-column structure on marine distributions of alcids (Aves: Alcidae) in Anadyr Strait,Northern Bering Sea,Alaska

Systematic ship-board surveys were used to simultaneously record seabird abundances and resolve coarse-scale (3 to 10 km) horizontal and fine-scale (1 to 10 m) vertical variability in water-column structure and bathymetry for portions of the coastal zone in Anadyr Strait near western St. Lawrence Island, northern Bering Sea, Alaska, during August and September 1987. Three plankton-feeding alcids, parakeet (Cyclorrhynchus psittacula), crested (Aethia cristatella) and least (A. pusilla) auklets, each exhibited distinct associations for different pycnocline characteristics. Least auklets were more abundant in mixed water, but they also occurred within stratified water where the pycnocline and upper-mixed layer were shallow (8 m) and thin (10 m), respectively. Low body mass (85 g), high buoyancy, and relatively poor diving ability may have restricted this auklet to areas where water-column strata nearly intersected the surface, or to areas from which strata were absent altogether due to strong vertical mixing. Parakeet and crested auklets, which are larger-bodied (ca. 260 g) planktivores with presumably greater diving ability, were more abundant in stratified water, and both species exhibited less specific affinities for water-column characteristic at intermediate and shallow levels. All three auklets avoided locations with strong pycnocline gradients (0.22 _t m^–1), a crude index of the strong, subsurface shear in water velocities characteristic of this region. Auklet distributions in Anadyr Strait were consistent with: (1) strata accessibility, as estimated from relationships between body mass and relative diving ability, (2) possible avoidance of strong subsurface water motions, and (3) habits and distributions of plankton prey. In contrast, largebodied (>450 g) alcids [i.e., common (Uria aalge) and thick-billed (U. lomvia) murres, pigeon guillemots (Cephus columba), tufted (Fratercula cirrhata), and horned (F. corniculata) puffins feeding on fish or benthic invertebrates] showed no consistent relationships with either the pycnocline or upper-mixed layers. All large alcids were more common in stratified than in vertically-mixed waters, but differences in abundance between mixing regimes were small or equivocal. The only measured variable with which all large alcids were associated was total water-column depth: murres, guillemots, and puffins each used areas with shallow sea floors and avoided areas with deeper sea floors. Failure of large alcids to discriminate among foraging areas in Anadyr Strait as a function of pycnocline topography and strength may be attributable to: (1) greater reliance on large pelagic and benthic prey not associated with the pycnocline; (2) higher body mass, lower buoyancy, and greater diving ability; (3) foraging over a uniquely shallow continental shelf where all vertical strata, including the sea floor, are potentially accessible from the ocean surface.     

Variation in assemblages of fish associated with deep and shallow margins of the seagrass Posidonia australis

Little is known about the fish fauna of deep Posidonia australis seagrass beds in Australia. We investigated associated with the deep and shallow margins of P. australis during two surveys in Jervis Bay, New South Wales. Fish associated with deep and shallow seagrass at each of two areas within a large bed of P. australis were compared over a period of 3 mo (November 1986 to January 1987). This was followed by a larger scale survey in which we compared fish found at the two depths within three large beds of P. australis in Jervis Bay over 2 yr (September 1988 to June 1990). In the smaller scale survey there were marked dissimilarities in relative abhundance and composition of species between samples from deep and shallow P. australis. There were also significant differences in abundance between deep and shallow seagrass on most occasions for the majority of the dominant fish species. As a result of the small-scale survey, we concluded that differences in species composition of fish associated with deep and shallow P. australis may have been due to the presence of species from nearby bare substrate among the sparser leaves of the deep seagrass. The results of the smaller scale survey were not confirmed by the larger survey. The relative abundances of species in samples from deep and shallow P. australis were not dissimilar for all beds. In addition, it was rare for any of the dominant species to have significantly different abundances in deep and shallow seagrass. There was little evidence that the species composition of fish associated with deep P. australis was strongly influenced by the fauna of deep bare substrate. Rather, samples from deep P. australis were far more similar to those from shallow P. australis than to a comparable set of samples collected from deep bare substrata. This study demonstrates how the results and conclusions of small-scale survey work, a feature of many environmental impact asseesments, can be misleading when applied to larger spatial and temporal scales, even within the same bay.     

Effect of salinity on oxygen consumption in postlarvae of the penaeid shrimps Penaeus monodon and P. stylirostris without and with acclimation

The effects of salinity on oxygen consumption was investigated in cultured Penaeus monodon Fabricius and P. stylirostris Stimpson postlarvae (32 and 35 d after metamorphosis, respectively). In both species, no difference appeared between non-acclimated and fully-acclimated individuals. The metabolic rate was unaffected by salinity variations, but P. stylirostris displayed a tendency (non-significant at P<0.05) to increase respiration at lower salinities. These observations are discussed in reference to the ecology of the young prawns.     

Kinetics of zinc uptake from solution,accumulation and excretion by the decapod crustacean<Emphasis Type="Italic"> Penaeus indicus</Emphasis>

Juveniles of the dendrobranchiate decapod Penaeus indicus take up radiolabelled zinc from solution at all exposure concentrations studied from 5.6 to 100 g l^–1, with an uptake rate constant of 0.045 l g^–1 day^–1 at 15 salinity and 25°C. Over the first 10 days of zinc exposure, the new zinc taken up is added to the existing zinc content of the prawn with no significant excretion; over this period the rate of accumulation of radiolabelled zinc is a measure of the absolute zinc uptake rate from solution. Over the next 10 days of zinc exposure to 10 g Zn l^–1, however, zinc is excreted at about half the rate of uptake resulting in a raised body concentration of zinc. Moulting had no significant effect on the accumulation of zinc. Newly accumulated zinc is distributed to all organs with the highest proportions of body content being found in the exoskeleton, followed by the muscle, the hepatopancreas and the antennal organs. Radiolabelled zinc is subsequently lost from all organs. Unlike caridean shrimps or prawns (pleocyemate decapods), therefore, penaeids (dendrobranchiate decapods) do not show regulation of zinc body concentrations to a constant level over a range of dissolved zinc bioavailabilities by matching zinc excretion to zinc uptake. Nevertheless, unlike amphipod crustaceans, P. indicus does excrete some of the zinc newly accumulated from solution after a time delay. Unlike their caridean counterparts, penaeid prawns inhabiting anthropogenically contaminated coastal waters with raised zinc bioavailabilities can be expected to contain raised body concentrations of zinc.Communicated by J.P. Thorpe, Port Erin     

Migration and stock studies on the Australian school prawn Metapenaeus macleayi

The results of tagging studies conducted to determine the relationship between estuarine (juvenile) populations and adults at sea suggest that maturing Metapenaeus macleayi leave the estuaries and move along the coast in a northerly direction; the longest migration recorded was 120 km, but most prawns appear to disperse in shallow water (<40 m) within about 70 km of their juvenile habitat. There are approximately 60 estuarine populations of M. macleayi in south-east Australia; the geographical range of prawns emigrating from most estuarine populations overlap (at sea) to some degree with that of adjacent or nearby populations, but there is little or no overlap between any two of the 6 major populations. The mixing of individuals from a major population and from nearby smaller populations is negligible for the purposes of fisheries management, hence the prawns in each of the major estuarine populations and the adults in the coastal area north for about 70 km may be regarded as a unit stock.     

Contrasting effects of sulfide and thiosulfate on symbiotic CO2-assimilation of Phallodrilus leukodermatus (Annelida)

Experiments were conducted in order to specify the reductants responsible for the carbon dioxide fixation of the symbiotic sulfur bacteria in the gutless marine oligochaete Phallodrilus leukodermatus (Annelida) from shallow calcareous sediments in Bermuda. Carbon dioxide-uptake rates were suppressed by S⁼ and stimulated by S₂O₃ ⁼. Individuals which hosted bacteria containing reserve energy substances maintained a high short-term CO₂-uptake activity, while bacteria in worm homogenates and in worms treated with an antibiotic (Baypen) did not show any significant metabolic activity. Absolute uptake rates in P. leukodermatus were usually considerably higher than those reported for other animals harbouring prokaryotic sulfuroxidizing symbionts. Utilization of thiosulfate rather than sulfide is compatible with the preferred occurrence of the worms around the redox discontinuity layer and has been confirmed in other thiobiotic animals. Sulfur stored in the symbiotic bacteria appears to be oxidized to sulfate and be excreted when the worms are held under energy-limited conditions. The data emphasize the complexity of the possible metabolic pathways involved in the oxidation of reduced-sulfur compounds by bacterial symbionts in marine invertebrates.     

The effect of temperature-salinity combinations on survival and growth of juvenile Patiriella pseudoexigua (Echinodermata: Asteroidea)

Adult Patiriella pseudoexigua were collected in October 1989 from Wanlitung, Taiwan and then induced to spawn in the laboratory. Post-metamorphosed juvenile P. pseudoexigua were reared on a diet of benthic algae Navicula sp. at 25°C and salinity (34). Six weeks after metamorphosis, juvenile P. pseudoexigua at ca. 400 m in radius were reared on a diet of benthic algae Navicula sp. at different combinations of temperatures (20, 25, 30°C) and salinities (26, 30, 34) for 40 d. Both temperature and salinity had a significant effect on juvenile survival and growth. Juveniles survived best (>90%) at 25°C and 34 and grew best (to ca. 750 m in radius) at 30°C and 34. Variation in juvenile size was small immediately after metamorphosis and increased with time.