Disturbance by ice and life-history adaptations of the seagrassZostera marina

The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass     

Parthenogenetic reproduction of Diaphanosoma celebensis (Crustacea: Cladocera): influence of salinity on feeding, survival, growth and neonate production

 Effect of salinity on the feeding rate and parthenogenetic reproduction of asexual females of the cladoceran Diaphanosoma celebensis Stingelin was studied. Short-term (10 h) grazing experiments were conducted using Isochrysis galbana as feed at 5, 17, 25 and 30 psu salinity. Gut pigment concentration showed a significantly higher rate of feeding at lower salinities. Survival, growth, maturity attainment and neonate production of asexual females reared in the above four test salinities indicated preference for lower salinities (5 and 17 psu). The mean size of adult females decreased from 909 to 593 μm, mean life span from 24 to 5 d, mean neonate production from 12 to 2 and mean size of neonates from 434 to 400 μm as the salinity increased from 5 to 30 psu. Salinity variations also affected the size and age of primiparous females. Resting egg formation and sexual reproduction did not occur at the tested salinities. The results indicate that D. celebensis is adapted to low saline, estuarine environments. Received: 14 January 2000 / Accepted: 24 March 2000     

Ecology of two littoral species of caprellid amphipods (Crustacea) from Washington,USA

Ecology of 2 littoral species of caprellid amphipods is compared. Populations of Caprella laeviuscula, a periphyton scraper/filter-feeder, are most dense on eelgrass Zostera marina L. In the absence of C. laeviuscula, periphyton biomass increases 411% in protected Z. marina beds. The light absorption spectra of periphyton and Z. marina are similar, and C. laeviuscula, by its periphyton removal, may allow Z. marina to grow in areas where it would otherwise be excluded. C. laeviuscula is aggressively dominant over sympatric caprellids, and seasonal predators of C. laeviuscula are absent during winter, the period when periphyton would be most limiting to Z. marina. Populations of Deutella californica, a predator, are most dense on the hydroid Obelia dichotoma (L.). In the O. dichotoma epibiotic community, D. californica is the primary predator, but removal did not change the composition of the community structure. In the absence of other macropredators, the structure of the O. dichotoma epibiotic community depends more on the seasonality of O. dichotoma occurrence than on organismal interactions.     

Production dynamics of the eelgrass, <Emphasis Type="Italic">Zostera marina</Emphasis> in two bay systems on the south coast of the Korean peninsula

Production dynamics of eelgrass, Zostera marina was examined in two bay systems (Koje Bay and Kosung Bay) on the south coast of the Korean peninsula, where few seagrass studies have been conducted. Dramatically reduced eelgrass biomass and growth have been observed during summer period on the coast of Korea, and we hypothesized that the summer growth reduction is due to increased water temperature and/or reduced light and nutrient availabilities. Shoot density, biomass, morphological characteristics, leaf productivities, and tissue nutrient content of eelgrass were measured monthly from June 2001 to April 2003. Water column and sediment nutrient concentrations were also measured monthly, and water temperature and underwater irradiance were monitored continuously at seagrass canopy level. Eelgrass shoot density, biomass, and leaf productivities exhibited clear seasonal variations, which were strongly correlated with water temperature. Optimal water temperature for eelgrass growth in the present study sites was about 15–20°C during spring period, and eelgrass growths were inhibited at the water temperature above 20°C during summer. Daily maximum underwater photon flux density in the study sites was usually much higher than the light saturation point of Z. marina previously reported. Densities of each terminal, lateral, and reproductive shoot showed their unique seasonal peak. Seasonal trends of shoot densities suggest that new eelgrass shoots were created through formation of lateral shoots during spring and a part of the vegetative shoots was transformed into flowering shoots from March. Senescent reproductive shoots were detached around June, and contributed to reductions of shoot density and biomass during summer period. Ambient nutrient level appeared to provide an adequate reserve of nutrient for eelgrass growth throughout the experimental period. The relationships between eelgrass growth and water temperature suggested that rapid reductions of eelgrass biomass and growth during summer period on the south coast of the Korean peninsula were caused by high temperature inhibition effects on eelgrass growth during this season.     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Effects of anaerobiosis on root metabolism of Zostera marina (eelgrass): implications for survival in reducing sediments

The temperate seagrass Zostera marina L. typically grows in highly reducing sediments. Photosynthesis-mediated O₂ supplied to below-ground tissues sustains aerobic respiration during photosynthetic periods. Roots, however, experience daily periods of anoxia and/or hypoxia at night and under conditions that reduce photosynthesis. Rhizosphere cores of Z. marina were collected in August 1984 from Great Harbor, Massachusetts, USA. We examined short-term anaerobic metabolism of [U-¹⁴C]sucrose in excised roots and roots of intact plants. Under anaerobic conditions roots showed appreciable labeling of CO₂, ethanol and lactate, and slight labeling of alanine and other metabolites. Over 95% of the ¹⁴C-ethanol was recovered in the root exudate. Release of other metabolites from the roots was minimal. Ethanol was also released from hypoxic/anoxic roots of intact plants and none of this ethanol was transported to the shoot under any experimental conditions. Loss of ethanol from roots prevented tissue levels of this phytotoxin from increasing during anaerobiosis despite increased synthesis of ethanol. Anaerobic metabolism of [U-¹⁴C]glutamate in excised roots led to appreciable labelling of -aminobutyrate, which was known to accumulate in eelgrass roots. Roots recovered to fully aerobic metabolism within 4 h after re-establishment of aerobic conditions. The contributions of these root metabolic responses to the ability of Z. marina to grow in reducing marine sediments are related to light-regulated interactions of shoots and roots that likely dictate depth penetration, distribution and ecological success of eelgrass.     

The effects of temperature and salinity on egg production and hatching success of Baltic Acartia tonsa (Copepoda: Calanoida): a laboratory investigation

The functional response of the aspects of reproductive success of a southwestern Baltic population of Acartia tonsa (Copepoda: Calanoida) was quantified in the laboratory using wide ranges in temperatures and salinities. Specifically, daily egg production (EP, # female⁻¹ day⁻¹) was determined for 4 or 5 days at 18 different temperatures between 5 and 34°C and the time course and success of hatching were evaluated at 10 different temperatures between 5 and 23°C. The effect of salinity (0 to 34 psu) on egg hatching success was also examined. The highest mean rates of EP were observed between 22 and 23°C (46.8–50.9 eggs female⁻¹ day⁻¹). When studied at 18 psu, hatching success of eggs increased with increasing temperature and was highest (92.2%) at 23°C. No hatching was observed for eggs incubated at low temperatures (≤12°C) that were produced by females acclimated to temperatures ≤10°C indicating a possible thermal threshold between 10.0 and 13.0°C below which only the production of diapause (or low quality) eggs exists in this population. When tested at 18°C, the hatching success of eggs incubated at 15 different salinities increased asymptotically with increasing salinity and was maximal (81.4–84.5%) between 17 and 25 psu. The high reproductive success observed over wide ranges in temperatures and salinities in this Baltic population demonstrates one of the mechanisms responsible for the cosmopolitan distribution of this species within productive, estuarine and marine habitats.     

Development of epiphytic communities on eelgrass (Zostera marina) along a nutrient gradient in a Danish estuary

The effect of nutrient enrichment on epiphyte development was examined by following the seasonal development of epiphyte biomass on eelgrass (Zostera marina L.) at four localities along a nutrient gradient in Roskilde Fjord, Denmark between March and December 1982. In the most nutrient-poor area, epiphyte biomass followed a distinct bimodal seasonal pattern with maxima in spring and early fall. Low nutrient availability and a high rate of eelgrass leaf renewal kept epiphyte biomass at a low level throughout the summer period. Unlike phytoplankton, the epiphytic community was not stimulated by nutrient enrichment during spring, however, from May through August, the biomass of both components increased exponentially with increasing concentrations of total N in the water. Along the nutrient gradient, phytoplankton biomass increased 5- to 10-fold, while epiphyte biomass increased 50- to 100-fold. Thus differences in nutrient conditions among study sites were more clearly reflected by epiphytes than phytoplankton.Contribution No. 419 from the Freshwater-Biological Laboratory, University of Copenhagen     

Labyrinthula sp., a marine slime mold producing the symptoms of wasting disease in eelgrass,Zostera marina

Coastal ecosystems along the eastern United States are presently threatened by a recurrence of the wasting disease of eelgrass, Zostera marina L. Using Koch's postulates, a species of the marine slime mold, Labyrinthula, is identified as the causal microorganism of this disease. Our disease tests for pathogenicity performed on eelgrass, using four Labyrinthula spp., indicate only one species produces the disease symptoms identical to those found associated with the wasting disease. The pathogenic Labyrinthula sp. has morphological characteristics that distinguish it from the other three species. Identification of Labyrinthula spp. is difficult because species described in the literature are not clearly characterized or identifiable. Tests at various salinities demonstrate that disease symptoms appear infrequently at salinities of 10%. or less.     

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 g g^-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.     

Photosynthesis in Codium fragile (Chlorophyta) from a Nova Scotia estuary: responses to desiccation and hyposalinity

Codium fragile ssp. tomentosoides from Caribou Harbour, an estuarine site in the southern Gulf of St. Lawrence, was extremely tolerant to stresses from desiccation and reduced salinity. Photosynthetic responses of both rhizomatous and erect growth forms were measured using pulse amplitude modulation (PAM) fluorometry of chlorophyll a fluorescence to determine effective quantum yield (Φ_PSII) and relative electron transport rate (rETR). After 5 h of desiccation, thalli lost 20% of their mass, but still showed high levels of Φ_PSII. Thalli survived for at least 6 h in freshwater, and showed virtually complete recovery of photosynthetic capacity within a few hours of return to full seawater. Immersion in 8 psu showed virtually complete recovery until the 24 h treatment period. Combining desiccation and salinity stresses produced a synergistic effect, but plants still showed strong recovery even after 86% dehydration and reimmersion in 16 psu. These results suggest that the photosynthetic physiology of Codium fragile is highly adapted to growth in estuarine conditions.     

The effects of temperature and salinity on reproductive success of <Emphasis Type="Italic">Temora longicornis</Emphasis> in the Baltic Sea: a copepod coping with a tough situation

At specific locations within the Baltic Sea, thermoclines and haloclines can create rapid spatial and temporal changes in temperature (T) and salinity (S) exceeding 10°C and 9 psu with seasonal ranges in temperature exceeding 20°C. These wide ranges in abiotic factors affect the distribution and abundance of Baltic Sea copepods via species-specific, physiological-based impacts on vital rates. In this laboratory study, we characterized the influence of T and S on aspects of reproductive success and naupliar survival of a southwestern Baltic population of Temora longicornis (Copepoda: Calanoida). First, using ad libitum feeding conditions, we measured egg production (EP, no. of eggs female⁻¹ day⁻¹) at 12 different temperatures between 2.5 and 24°C, observing the highest mean EP at 16.9°C (12 eggs female⁻¹ day⁻¹). Next, the effect of S on EP and hatching success (HS, %) was quantified at 12°C for cohorts that had been acclimated to either 8, 14, 20 or 26 psu and tested at each of five salinities (8, 14, 20, 26 and 32 psu). The mean EP was highest for (and maximum EP similar among) 14, 20 and 26 psu cohorts when tested at their acclimation salinity whereas EP was lower at other salinities. For adults reared at 8 psu, a commonly encountered salinity in Baltic surface waters, EP was relatively low at all test salinities—a pattern indicative of osmotic stress. When incubated at 12°C and 15 different salinities between 0 and 34 psu, HS increased asymptotically with increasing S and was maximal (82.6–84.3%) between 24 and 26 psu. However, HS did depend upon the adult acclimation salinity. Finally, the 48-h survival of nauplii hatched and reared at 14 psu at one of six different temperatures (10, 12, 14, 16, 18 and 20°C) was measured after exposure to a novel salinity (either 7 or 20 psu). Upon exposure to 7 psu, 48-h naupliar mortality increased with increasing temperature, ranging from 26.7% at 10°C to 63.2% at 20°C. In contrast, after exposure to 20 psu, mortality was relatively low at all temperatures (1.7% at 10°C and ≤26.7% for all other temperatures). An intra-specific comparison of EP for three different T. longicornis populations revealed markedly different temperature optima and clearly demonstrated the negative impact of brackish (Baltic) salinities. Our results provide estimates of reproductive success and early survival of T. longicornis to the wide ranges of temperatures and salinities that will aid ongoing biophysical modeling examining climate impacts on this species within the Baltic Sea.     

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m⁻²) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).     

Ammonium regeneration and assimilation in eelgrass (Zostera marina) beds

Regeneration and assimilation of ammonium in the water column and in sediments of eelgrass (Zostera marina L.) beds of Izembek Lagoon and Crane Cove, Alaska, USA and Mangoku-Ura, northeastern Japan, were investigated by using a ¹⁵N isotope dilution technique. In the water column of Mangoku-Ura, ammonium was regenerated at a rate of 12 nmol l^-1 h^-1 and assimilated at a rate of 74 nmol l^-1 h^-1. The ammonium regeneration rate in sediments ranged from 2 to 150 nmol g^-1 h^-1, and with one exception, exceeded ammonium assimilation in sediments (0.3 to 77 nmol g^-1 h^-1). The ammonium regeneration in the water column was of little significance for the nitrogen supply to the eelgrass bed ecosystem. Net ammonium production (regeneration minus assimilation) in the sediment of Izembek Laggon met nitrogen demand for eelgrass growth, suggesting that ammonium regeneration in the sediments was very important for the nitrogen cycle in the eelgrass bed ecosystem.     

Salinity favorable for early development and gamete compatibility in two sympatric estuarine species of the genus Hediste (Polychaeta: Nereididae) in the Ariake Sea, Japan

Early development was examined, under various salinities, for two sympatric nereidid polychaetes, Hediste japonica and H. diadroma, which participated in a simultaneous reproductive swarming in an estuary of the Omuta-gawa River in Ariake Sea, Japan. The eggs of both species were isotonic to the medium of 27.5–30 psu salinity. The egg diameter in the isotonic salinity was 180–205 μm in H. japonica, and 130–160 μm in H. diadroma. Successful development of most embryos was observed in a salinity range of 22.5–30 psu in both species, while successful fertilization occurred in wider ranges of salinity, i.e., 10–34 psu in H. japonica and 10 to 30 psu in H. diadroma. In both species, free-swimming larval life started from the indistinct hatching of trochophores out of a jelly layer capsule. The lecithotrophic development appeared to run to the 4-setiger nectochaetes in H. japonica, while to 3-setiger nectochaetes in H. diadroma, resulting in a shorter pelagic larval life in H. japonica. In a comparison of larval morphology among Hediste species, we found a definite negative correlation between the prostomium width, which represents the larval size and depends on egg size, and relative length of chaetae to the prostomium width: the relative length of chaetae was the longest in H. diadroma (with the smallest egg size and long pelagic life), intermediate in H. japonica (intermediate egg size, short pelagic life), and the shortest in H. atoka (largest egg size, no true pelagic life). We also examined the possibility of hybridization between H. japonica and H. diadroma through cross-insemination experiments. The gametes of the two species were reciprocally compatible, and viable hybrid offspring were produced by the laboratory crosses. The hybrid larvae expressed intermediate phenotypes, but with a greater maternal influence in characteristics such as the relative length of chaetae and the lecithotrophic larval duration.     

Suitability of estuarine nursery zones for juvenile weakfish (Cynoscion regalis): effects of temperature and salinity on feeding,growth and survival

Juvenile weakfish, Cynoscion regalis (Bloch and Schneider, 1801), exhibit significant spatial diffrences in growth rate and condition factor among estuarine nursery zones in Delaware Bay. The potential influence of temperature and salinity on the suitability of estuarine nursery areas for juvenile weakfish was investigated in laboratory experiments by measuring ad libitum feeding rate, growth rate and gross growth efficiency of juveniles collected in Delaware Bay in 1990 (40 to 50 mm standard length; 1.4 to 2.1 g) in 12 temperature/salinity treatments (temperatures: 20, 24, 28°C; salinities: 5, 12, 19, 26 ppt) representing conditions encountered in different estuarine zones during spring/summer. Feeding rates (FR) increased significantly with temperature at all salinities, ranging from 10 to 15% body wt d^-1 at 20°C to 33–39% body wt d^-1 at 28°C. Specific growth rates (SGR) ranged from 1.4 to 9.4% body wt d^-1 (0.3 to 1.5 mm d^-1) and gross growth efficiencies (K ₁) varied from 13.6 to 26.4% across temperature/salinity combinations. Based on nonlinear multiple regression models, predicted optimal temperatures for SGR and K ₁ were 29 and 27°C, respectively. Salinity effects on SGR and K ₁ were significant at 24 and 28°C where predicted optimal salinity was 20 ppt. At these warmer temperatures, SGR and K ₁ were significantly lower at 5 than at 19 ppt despite higher FR at 5 ppt. Therefore, maximum growth rate and growth efficiency occurred under conditions characteristic of mesohaline nurseries. This finding is consistent with spatial patterns of growth in Delaware Bay, implying that physicochemical gradients influence the value of particular estuarine zones as nurseries for juvenile weakfish by affecting the energetics of feeding and growth. Laboratory results indicate a seasonal shift in the location of physiologically optimal nurseries within estuaries. During late spring/early summer, warmer temperatures in oligohaline areas permit higher feeding rate and faster growth compared to mesohaline areas. By mid-late summer, spatial temperature gradients diminish and mesohaline areas provide more suitable physicochemical conditions for growth rate and growth efficiency whereas oligohaline areas become energetically stressful. Substantial mortality occurred at 5 ppt and 28°C, providing additional evidence that oligohaline conditions are stressful during late summer. Furthermore, juveniles provided a choice among salinities in laboratory trials preferred those salinities which promoted higher growth rates. The extensive use of oligohaline nurseries by juvenile weakfish despite the potential for reduced growth rate and growth efficiency suggests this estuarine zone may provide a substantial refuge from predation.     

Influence of environmental variables on the fish fauna of the deeper waters of a large Australian estuary

Fish were collected by gill nets from the deeper waters of the Entrance Channel, basins and rivers of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry (December to May) periods between August 1979 and July 1981. Simple-regression analysis showed that the number of species, abundance and biomass of fish in the rivers rose with increases in the salinity and temperature of both the surface and bottom of the water column. No such significant correlations were found in the Entrance Channel andbasins (Peel Inlet and Harvey Estuary), where salinity changes were far less marked. The number of species at sites throughout the estuary was inversely correlated with distance from the estuary mouth. Multiple-regression equations showed that, compared with the other environmental variables tested, bottom salinity had a greater influence on the nunber of species and abundance both in the rivers and in the system as a whole. These results indicate that salinity has a greater effect on the fauna in the deeper waters than in the shallows (cf. Loneragan et al., 1986). The larger fish which characterise the deeper waters may thus be less tolerant to low salinities than the smaller fish typically found in the shallows. Both classification and ordination separated the faunal composition of the rivers from those of the Entrance Channel and basins. The fauna of the two narrow and deeper sites in the rivers separated into wet- and dry-period components. Differences between the faunal composition of the riverine regions and those of the Entrance Channel and basins have been related to the much more variable and lower minimum salinities in the rivers. Species characteristic of the rivers included Amniataba caudavittatus, which is estuarine sensu stricto in south-western Australia, the semianadromous Nematalosa vlaminghi and the highly euryhaline Mugil cephalus. The indicator species for the Entrance Channel and basins were all marine species (Cnidoglanis macrocephalus, Hyporhamphus melanochir, Gerres subfasciatus and Pomatomus saltator).     

Larval growth in the estuarine crab Chasmagnathus granulata: the importance of salinity experienced during embryonic development, and the initial larval biomass

The importance of salinity experienced during embryonic development and initial larval biomass on larval growth was studied in the South American estuarine crab Chasmagnathus granulata. Ovigerous females were maintained at three salinities (15, 20, and 32‰) from egg laying to hatching of zoea l. Larvae from all treatments were reared under constant conditions of photoperiod (12∶12), temperature (18°C), and salinity (first instar at 20‰, subsequent instars at 32‰). Biomass was measured as dry weight, carbon, and nitrogen content per individual at egg laying, hatching of zoea l, premoult zoea l, and zoea 4, and in 8-day-old megalopa. From hatching to premoult zoea 4, biomass was higher for larvae from prehatching salinities of 15 and 32‰. There was a significant positive correlation between biomass at hatching and at premoult zoea l and zoea 4. Accumulated biomass during zoeal stages tended to be higher for larvae from broods with higher biomass at hatching, although this trend was not always significant. Zoea 4 either directly metamorphosed to megalopa or moulted to zoea 5, following, respectively, a short or long developmental pathway. The proportion of zoea 4 that followed the long pathway was negatively correlated with biomass of zoeal stages. Biomass at hatching was correlated with biomass of megalopae developed through the short pathway, although it was not correlated with the accumulated biomass at this stage. Megalopae developed through the long pathway (i.e. metamorphosed from zoeae 5) had higher biomass than those from the short pathway. The present results suggest that prehatching salinity and initial egg and larval biomass can be very important for larval growth. Published online: 9 August 2002     

Physiological responses of horseshoe crab (<Emphasis Type="Italic">Limulus polyphemus</Emphasis>) embryos to osmotic stress and a possible role for stress proteins (HSPs)

We tested the effects of osmotic stress on survival, developmental rate, and level of HSPs on American horseshoe crab (Limulus polyphemus) embryos. Animals were maintained in the laboratory at an ambient salinity of 20 ppt and then exposed to 4-h osmotic shocks at salinities of 10, 30, 40, 50, and 60 ppt, with a control group at 20 ppt. Horseshoe crab embryos had 100% developmental success (defined as individuals reaching the first instar or trilobite larval stage) at all salinities. However, osmotic stresses, especially hyperosmotic conditions, slowed the rate of development. Embryos subjected to osmotic stress showed higher levels of HSP70 and HSP90 than control animals kept at a salinity of 20 ppt. HSPs are of value to horseshoe crab embryos in surviving the fluctuating salinities that are typical of estuarine beach habitats.     

Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong

The phenology and primary productivity of a population of Zostera japonica (Aschers. & Graebn.) threatened by the construction of Hong Kong's new international airport were studied over a 12-month period. The need to conserve the population, and the small leaf size of Z. japonica rendered traditional destructive or marking techniques inapplicable for percentage cover and biomass estimation. A nondestructive method based on image analysis techniques was therefore devised for repeated estimation of percentage cover, biomass and leaf area index. This technique, which involved random quadrat sampling, photographic recording and image analysis, was able to provide data on the three parameters with acceptable precision and was cost-effective in the field. Z. japonica demonstrated a strongly seasonal cycle of vegetative growth, with different patterns for leaf density (peak in March) and overall bed area (peak in June). Total (above- and below-ground) net primary productivity was estimated at between 344 and 688 g AFDW m⁻² yr⁻¹. Percentage cover of Z. japonica was negatively correlated with total suspended solids (TSS) in the water column while total bed area was negatively correlated with water salinity. Increased sedimentation associated with the new airport project was identified as one important factor affecting the growth of the seagrass, as TSS reached the high level of ≈1 g DW l⁻¹ during the first half of the study period. Sediment traps set in the beds also recorded potential sedimentation rates at between 2.89 and 14.5 mg cm⁻² d⁻¹. This high turbidity resulted in a sharp decrease in the density of Clithon spp., the dominant grazers of epiphytic algae on Z. japonica. Effects of sedimentation and shading on growth of Z. japonica were investigated by field manipulative experiments. Experimental increase of sedimentation rate and shade both resulted in larger decreases in percentage cover and above-ground AFDW compared with the control. Received: 3 March 1997 / Accepted: 14 March 1997