Thermal dependency of burrowing in three species within the bivalve genus <Emphasis Type="Italic">Laternula</Emphasis>: a latitudinal comparison

The upper thermal limits for burrowing and survival were compared with micro-habitat temperature for anomalodesmatan clams: Laternula elliptica (Antarctica, 67°S); Laternula recta, (temperate Australia, 38°S) and Laternula truncata (tropical Singapore, 1°N). Lethal limits (LT₅₀) were higher than burrowing limits (BT₅₀) in L. elliptica (7.5–9.0 and 2.2°C) and L. recta (winter, 32.8–36.8 and 31.1–32.8°C) but the same range for L. truncata (33.0–35.0 and 33.4–34.9°C). L. elliptica and L. truncata had a BT₅₀ 0.4 and 2.4–3.9°C, respectively, above their maximum experienced temperature. L. recta, which experience solar heating during midday low tides, had a BT₅₀ 0.7–2.4°C below and a range for LT₅₀ that spanned their predicted environmental maximum (33.5°C). L. recta showed no seasonal difference in LT₅₀ or BT₅₀. Our single genus comparisons contrast with macrophysiological studies showing that temperate species cope better with elevated temperatures. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Very slow development in two Antarctic bivalve molluscs,the infaunal clam <Emphasis Type="Italic">Laternula elliptica</Emphasis> and the scallop <Emphasis Type="Italic">Adamussium colbecki</Emphasis>

Embryos of the large infaunal clam Laternula elliptica and the scallop Adamussium colbecki, from Antarctica, were cultured over an 18-month period. Their development rates were extremely slow, taking 240 and 177 h, respectively, to reach the trochophore stage. This is ×4 to ×18 slower than related clams and scallops from temperate latitudes. The relationship between temperature and development rate for bivalve molluscs shows the expected slowing with reduced temperature (Q ₁₀ in the range 2–4) for temperate and tropical species. However, the slowing at polar latitudes is much stronger than at warmer waters, and all of the limited data for Antarctic species are well above the Arrhenius plot for the overall bivalve data, and the Q ₁₀ value for Antarctic to cool temperate species is 11.8, well outside the expected range for biological systems. Either the relationships describing the effects of temperature on the kinetics of biological systems do not apply to Antarctic bivalve molluscs, or some other factor that cannot be compensated for becomes important at low temperature. In the laboratory, L. elliptica embryos stayed viable in very sticky egg capsules for up to 18 months without hatching. However, even the disturbance of removing eggs using a pipette ruptured some egg capsules allowing embryo release. Gametogenesis in Antarctic marine invertebrates is almost universally slowed compared to temperate species, with nearly all cases documented requiring more than 1 year to complete oogenesis. The only exception so far appears to be A. colbecki, which has a 1-year gametogenic cycle. The data here indicate that it has been unable to adapt embryonic development in a similar way, and we are not aware of any exceptions to the markedly slowed development at low temperature rule.     

Is the burrowing performance of a sandy beach surfing gastropod limiting for its macroscale distribution?

The size-specific burrowing capacity of Olivella semistriata, an extremely abundant surfing gastropod of exposed sandy beaches in the tropical East Pacific, was investigated in terms of sediment grain size. For all investigated sand classes, there was a significant increase in burial time with size. Burrowing was fastest in sand with grain size between 150 and 355 μm, as well as in native sediment (median grain size 209 μm) and in field conditions (median grain size 223 μm). Values of the burrowing rate index (BRI) were found to be between 3 and 7, rating the burrowing capacity of O. semistriata to be fast to very fast. Data from previous qualitative and quantitative sampling campaigns were used to identify the macroscale (i.e., between beaches) distribution of O. semistriata in terms of sediment grain size and swash conditions. Swash period was shorter than burial time, excluding this as a factor limiting the distribution. Swash standstill time (the time between uprush and backwash), however, was just long enough on the beaches, where O. semistriata was present to allow for securing firm anchorage. On reflective beaches, the swash standstill time is as short as 1 or 2 s, denying O. semistriata the time to burry itself before being swept away by the backwash. As such, swash standstill time is advocated as a valuable part of the swash exclusion hypothesis. A survey of the available literature on the burrowing of surfers shows that mole crabs are by far the fastest burrowers and the only surfers that burrow sufficiently fast to withstand the extremely short swash standstill time on reflective beaches. Burrowing ability of surfing gastropods is found in the same range as surfing bivalves, both being insufficient to cope with coarse sediment on steep beaches. Finally, we suggest that neither burial time nor BRI, yet rather the minimal burial time—the time needed to anchor securely in a certain sand at a given swash velocity—should be used to judge the limitations of burial in terms of sediment and swash conditions.

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Settlement of <Emphasis Type="Italic">Macoma balthica</Emphasis> larvae in response to benthic diatom films

The role of multi-species benthic diatom films (BDF) in the settlement of late pediveliger larvae of the bivalve Macoma balthica was investigated in still-water bioassays and multiple choice flume experiments. Axenic diatom cultures that were isolated from a tidal mudflat inhabited by M. balthica were selected to develop BDF sediment treatments characterized by a different community structure, biomass, and amount of extracellular polymeric substances (EPS). Control sediments had no added diatoms. Although all larvae settled and initiated burrowing within the first minute after their addition in still water, regardless of treatment, only 48–52% had completely penetrated the high diatom biomass treatments after 5 min, while on average 80 and 69% of the larvae had settled and burrowed into the control sediments and BDF with a low diatom biomass (<3.5 μg Chl a g⁻¹ dry sediment), respectively. The percentage of larvae settling and burrowing into the sediment was negatively correlated with the concentration of Chl a and EPS of the BDF. This suggests higher physical resistance to bivalve penetration by the BDF with higher diatom biomass and more associated sugar and protein compounds. The larval settlement rate in annular flume experiments at flow velocities of 5 and 15 cm s⁻¹ was distinctly lower compared to the still-water assays. Only 4.6–5.8% of the larvae were recovered from BDF and control sediments after 3 h. Nonetheless, a clear settlement preference was observed for BDF in the flume experiments; i.e., larvae settled significantly more in BDF compared to control sediments irrespective of flow speed. Comparison with the settlement of polystyrene mimics and freeze-killed larvae led to the conclusion that active selection, active secondary dispersal and, at low flow velocities (5 cm s⁻¹), passive adhesion to the sediment are important mechanisms determining the settlement of M. balthica larvae in estuarine biofilms.     

Effect of the presence of the shore crab, <Emphasis Type="Italic">Carcinus maenas</Emphasis>, on burrowing behaviour and clearance rate of the common cockle, <Emphasis Type="Italic">Cerastoderma edule</Emphasis>

Bivalves demonstrate various morphological and behavioural adaptations to reduce the risk of being attacked by predators. This paper examines how the presence of the crab Carcinus maenas (L.), a natural predator of the cockle Cerastoderma edule (L.), affects its burrowing depth and clearance or feeding rate. Cockles were placed in experimental tanks and treated with three levels of predatory disturbance: (1) unfed crab loose inside the tank, (2) unfed crab inside a cage suspended in the water column and (3) no crab present. Cockles’ burrowing depth was measured in two sediment types: mud and sand. Cockles burrowed more deeply in treatments with no crabs. Burrowing depth in sand was significantly greater than in mud. Two factors may contribute to the reduction in burial depth of C. edule in the presence of C. maenas: the change in the vertical orientation of the cockle and the ‘cough response’. No significant difference was found in the cockles’ clearance rate among the different levels of predator threat.     

Habitat use,urea production and spawning in the gulf toadfish <Emphasis Type="Italic">Opsanus beta</Emphasis>

Field studies were conducted in Johnson Key Basin, Florida Bay, USA from September 2002 through September 2004 to examine physiological, ecological, and behavioral characteristics of the gulf toadfish, Opsanus beta (Goode and Bean in Proc US Natl Mes 3:333–345, 1880), in relation to nitrogen metabolism, habitat usage, and spawning. Fish collected 5 cm above sediments in experimental shelters (epibenthic) were compared with those collected by throw traps which were found on or burrowing within sediments. The relationship between microhabitat ammonia and urea excretion, as determined by the enzymatic activity of glutamine synthetase (GS), was examined. The hypothesis tested was that O. beta occupying epibenthic nests were less ureotelic with lower GS activities than non-nesting individuals on/in sediments, due to a decreased environmental ammonia burden. Porewater total ammonia (T _Amm) concentrations at a sediment depth of 5 cm, i.e., the approximate depth of burrowing toadfish, ranged from 0 to 106.5 μmol N l⁻¹ while the pH ranged from 7.48 to 9.14. There was a weak but significant correlation between environmental ammonia (NH₃) concentration and hepatic GS activity for epibenthic toadfish (P < 0.001, r ² = 0.10), but not for burrowing toadfish. Mean urea-N and T _Amm concentrations within shelters occupied by toadfish (n = 281) were 9.8 ± 0.83 μmol N l⁻¹ and 13.0 ± 0.7 μmol N l⁻¹, respectively. As predicted, hepatic GS activity was significantly lower in epibenthic toadfish captured in shelters (4.40 ± 0.24 μmol min⁻¹ g⁻¹; n = 281) as compared to individuals on/in sediments (6.61 ± 0.47 μmol min⁻¹ g⁻¹; n = 128). Glutamine synthetase activity generally peaked in March (spawning season) and was lowest in July. Gender differences in hepatic and branchial GS activity were also found during the spawning season, which is attributable to the fact that males brood and guard offspring in their epibenthic nests while females often rest on or burrow into the sediments. Finally, hepatic and branchial GS appeared to have different patterns of enzymatic activity suggesting functional differences in gene expression.     

Effects of water viscosity upon ventilation and metabolism of a flatfish,the common sole <Emphasis Type="Italic">Solea solea</Emphasis> (L.)

The French Atlantic coast contains large highly productive intertidal mudflats that are colonised by juveniles of numerous flatfish species, including the common sole (Solea solea, L.). These ecosystems are also heavily exploited by the shellfish farming industry. Intensive bivalve culture is associated with substantial biodeposition (1–6 t-dw ha⁻¹ day⁻¹), which directly or indirectly contributes to increase exopolysaccharide (EPS) concentrations at the interface between water column and seabed. EPS are long-chain molecules organised into colloids, which influence rheological properties of water, particularly viscosity. Increased water viscosity had consequences for ventilatory activity of juvenile flatfish, whereby the minimal pressure required to ventilate the medium increases directly with EPS concentration. Moreover, the critical EPS concentration ([EPS]_crit) at which water was no longer able to flow through the branchial basket ranged from almost nil to over 30 mg l⁻¹, depending on species and size. [EPS]_crit was lower in small individuals and individuals from species with high metabolic rates (turbot and plaice). These differences may depend upon gill and bucco-branchial cavity morphometrics. The ventilatory workload of sole increased with viscosity to a maximum at 2 mg EPS l⁻¹. Viscosity might, therefore, be a limiting factor for flatfish post larvae, which colonise the intertidal mudflats, depending upon their size and species. EPS concentrations in the field can reach 15 mg l⁻¹. A selective effect is conceivable but remains to be estimated in the field.     

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.     

Growth of two Antarctic lamellibranchs: Adamussium colbecki and Laternula elliptica

The growth of two Antarctic lamellibranchs, Adamussium colbecki (E.A. Smith) and Laternula elliptica (King and Broderip), has been investigated. A. colbecki is a pectinid that grows to a shell height of approximately 70 mm in 6 to 7 years; L. elliptica is a deep-burrowing form, ecologically similar to the genus Mya and grows to a shell length of approximately 90 mm in 12 to 13 years. The growth pattern of both species is accurately described by the Bertalanffy growth equation and both have smaller values of the Bertalanffy K coefficient than comparable temperate species (A. colbecki K=0.24, L. elliptica K=0.16).     

Clearance rates of the great scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) and blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) at low natural seston concentrations

Great scallop, Pecten maximus, and blue mussel, Mytilus edulis, clearance rate (CR) responses to low natural seston concentrations were investigated in the laboratory to study (1) short-term CR variations in individual bivalves exposed to a single low seston diet, and (2) seasonal variations in average CR responses of bivalve cohorts to natural environmental variations. On a short temporal scale, mean CR response of both species to 0.06 μg L⁻¹ chlorophyll a (Chl a) and 0.23 mg L⁻¹ suspended particulate matter (SPM) remained constant despite large intra-individual fluctuations in CR. In the seasonal study, cohorts of each species were exposed to four seston treatments consisting of ambient and diluted natural seston that ranged in mean concentration from 0.15 to 0.43 mg L⁻¹ SPM, 0.01 to 0.88 μg L⁻¹ Chl a, 36 to 131 μg L⁻¹ particulate organic carbon and 0.019 to 0.330 mm³ L⁻¹ particle volume. Although food abundance in all treatments was low, the nutritional quality of the seston was relatively high (e.g., mean particulate organic content ranged from 68 to 75%). Under these low seston conditions, a high percentage of P. maximus (81–98%) and M. edulis (67–97%) actively cleared particles at mean rates between 9 and 12 and between 4 and 6 L g⁻¹ h⁻¹, respectively. For both species, minimum mean CR values were obtained for animals exposed to the lowest seston concentrations. Within treatments, P. maximus showed a greater degree of seasonality in CR than M. edulis, which fed at a relatively constant rate despite seasonal changes in food and temperature. P. maximus showed a non-linear CR response to increasing Chl a levels, with rates increasing to a maximum at approximately 0.4 μg L⁻¹ Chl a and then decreasing as food quantity continued to increase. Mean CR of M. edulis also peaked at a similar concentration, but remained high and stable as the food supply continued to increase and as temperatures varied between 4.6 and 19.6°C. The results show that P. maximus and M. edulis from a low seston environment, do not stop suspension-feeding at very low seston quantities; a result that contradicts previous conclusions on the suspension-feeding behavior of bivalve mollusks and which is pertinent to interpreting the biogeographic distribution of bivalve mollusks and site suitability for aquaculture.     

Effects of mussel (Perna canaliculus) biodeposit decomposition on benthic respiration and nutrient fluxes

Suspension-feeding bivalves increase the quantity and quality of sedimenting organic matter through the production of faeces and pseudofaeces that are remineralised in coastal sediments and thus increase sediment oxygen demand and nutrient regeneration. Bivalves are intensively cultivated worldwide; however, no bivalve biodeposit decay rates are available to parameterise models describing the environmental effects of bivalve culture. We examined sediment biogeochemical changes as bivalve biodeposits age by incubating coastal sediments to which we added fresh mussel (Perna canaliculus) biodeposits and measured O₂ and nutrient fluxes as well as sediment characteristics over an 11-day period. Biodeposits elevated organic matter, chlorophyll a, phaeophytin a, organic carbon and nitrogen concentrations in the surface sediments. Sediment oxygen consumption (SOC) increased significantly (P=0.016) by ∼1.5 times to 1,010 μmol m⁻² h⁻¹ immediately after biodeposit addition and remained elevated compared to control cores without additions for the incubation period. This increase is in the range of observed in situ oxygen demand enhancements under mussel farms. To calculate a decay rate for biodeposits in sediments we fitted a first-order G model to the observed increase in SOC. The significant model fit (P=0.001, r ²=0.72) generated a decay rate of 0.16 day⁻¹ (P=0.033, SE=0.05) that corresponds to a half-life time of 4.3 day. This decay rate is 1–2 orders of magnitude higher than published decay rates of coastal sediments without organic enrichment but similar to rates of decaying zooplankton faecal pellets. NH₄⁺ release increased rapidly on the day of biodeposit addition (P=0.013) and reached a maximum of 144 μmol m⁻² h⁻¹ after 5 days which was 3.6 times higher compared to control cores. During this period NH₄⁺ release was significantly (P<0.001 to P=0.043) higher in the cores with biodeposit additions than in control cores.     

Aerobic metabolic rates of swimming juvenile mako sharks, <Emphasis Type="Italic">Isurus oxyrinchus</Emphasis>

The shortfin mako shark, Isurus oxyrinchus, is a highly streamlined epipelagic predator that has several anatomical and physiological specializations hypothesized to increase aerobic swimming performance. A large swim-tunnel respirometer was used to measure oxygen consumption (MO₂) in juvenile mako sharks (swimming under controlled temperature and flow conditions) to test the hypothesis that the mako shark has an elevated maintenance metabolism when compared to other sharks of similar size swimming at the same water temperature. Specimen collections were conducted off the coast of southern California, USA (32.94°N and 117.37°W) in 2001-2002 at sea-surface temperatures of 16.0–21.0°C. Swimming MO₂ and tail beat frequency (TBF) were measured for nine mako sharks [77–107 cm in total length (TL) and 4.4 to 9.5 kg body mass] at speeds from 28 to 54 cm s⁻¹ (0.27–0.65 TL s⁻¹) and water temperatures of 16.5–19.5°C. Standard metabolic rate (SMR) was estimated from the extrapolation to 0-velocity of the linear regression through the LogMO₂ and swimming speed data. The estimated LogSMR (±SE) for the pooled data was 2.0937 ± 0.058 or 124 mg O₂ kg⁻¹ h⁻¹. The routine metabolic rate (RMR) calculated from seventeen MO₂ measurements from all specimens, at all test speeds was (mean ± SE) 344 ± 22 mg O₂ kg⁻¹h⁻¹ at 0.44 ± 0.03 TL s⁻¹. The maximum metabolic rate (MMR) measured for any one shark in this study was 541 mg O₂ kg⁻¹h⁻¹ at 54 cm s⁻¹ (0.65 TL s⁻¹). The mean (±SE) TBF for 39 observations of steady swimming at all test speeds was 1.00 ± 0.01 Hz, which agrees with field observations of 1.03 ± 0.03 Hz in four undisturbed free-swimming mako sharks observed during the same time period. These findings suggest that the estimate of SMR for juvenile makos is comparable to that recorded for other similar-sized, ram-ventilating shark species (when corrected for differences in experimental temperature). However, the mako RMR and MMR are apparently among the highest measured for any shark species.     

<Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> blooms and clonal isolates from the northwest Atlantic coast cause rapid mortality in larvae of multiple bivalve species

Globally, many commercial bivalve populations have declined in recent decades. In addition to overharvesting and habitat loss, the increasing frequency and intensity of harmful algal blooms (HABs) are likely to contribute to bivalve losses, particularly in cases where blooms negatively impact larval stages. This paper reports on the lethal effects of clonal cultures and blooms of Cochlodinium polykrikoides from the US Atlantic coast on the larvae of three species of commercially and ecologically valuable bivalves: the Eastern oyster (Crassostrea virginica), the bay scallop (Argopecten irradians), and the Northern quahog (hard clam; Mercenaria mercenaria). Both cultures and blooms of C. polykrikoides were highly toxic to all three species of bivalve larvae causing 80–100% mortality during 24- to 72-h exposures at concentrations of 1–2 × 10³ cells ml⁻¹. Toxicity was dependent on cell densities, growth stage of C. polykrikoides (i.e. cultures in exponential stage growth were more toxic than later stages), exposure time of larvae to cells (i.e. longer exposure caused higher mortality), the age of larvae (i.e. younger larvae were more sensitive), and the relative abundance of C. polykrikoides (i.e. the presence of other microalgae decreased toxicity). Free radical-scavenging enzymes (peroxidase and catalase) and the removal of C. polykrikoides cells (i.e. culture filtrate) significantly increased larval survival suggesting toxicity is maximized by contact with live cells and may involve labile toxins bound by these compounds including e.g. reactive oxygen species. The toxicity of C. polykrikoides to bivalve larvae was generally more severe than other HAB species (e.g. Karenia brevis, Karlodinium veneficum, Alexandrium tamarense, Prorocentrum minimum). Since the bivalves in this study spawn in the months when C. polykrikoides blooms on the east coast of North America, these results suggest that these blooms may have detrimental effects on efforts to restore these already diminished populations.     

Transfer of brevetoxins to a tellinid bivalve by suspension- and deposit-feeding and its implications for clay mitigation of <Emphasis Type="Italic">Karenia brevis</Emphasis> blooms

Blooms of the brevetoxin-producing Karenia brevis in the Gulf of Mexico cause massive fish kills, food poisoning and adverse respiratory effects in humans. Sedimentation of toxic cells following inert clay application could reduce toxin incorporation by commercially important suspension-feeding bivalves and thus prevent direct public health impacts, but could potentially lead to brevetoxin (PbTx) accumulation by benthic deposit-feeders. The goal of this study was therefore to compare suspension- and deposit-feeding as pathways for brevetoxins. We investigated: (1) the effect of toxic K. brevis on both feeding modes using a facultative deposit-suspension feeding tellinid bivalve, the clam Macoma balthica, as a model species and (2) the relative effectiveness of brevetoxin transfer via suspension- and deposit-feeding over 24-h exposure. Sedimentation of K. brevis was achieved by treatment with 0.25 g phosphatic clay l⁻¹ and brevetoxin concentrations were measured by ELISA. Karenia brevis reduced both suspension- and deposit-feeding activity. This study demonstrates that brevetoxins can be rapidly accumulated by a surface deposit-feeding bivalve from sedimented K. brevis cells and that comparable toxin levels can be attained by both suspension- and deposit-feeding modes [1.2–1.6 μg PbTx (g tissue wet weight)⁻¹]. Deposit-feeding clams generally do not pose a direct threat to humans but may provide a pathway for brevetoxin food web transfer.     

Burrowing by the bivalve mollusc Lithophaga curta in the living reef coral Montipora berryi and a hypothesis of reciprocal larval recruitment

The burrowing bivalve Lithophaga curta is abundant (8 to 9 per 100 cm²) in the encrusting, hermatypic coral Montipora berryi at Enewetak; this is the first report of L. curta colonizing living coral. Conversely, larvae of M. berryi appear to settle in empty bivalve burrows, tentatively identified as those of L. curta, located in dead M. berryi. Several hypotheses are suggested on the adaptive significance of substrate selection by larvae of both species and an overall hypothesis of reciprocal recruitment, describing the dynamic aspects of this apparently co-evolved relationship, is proposed.     

Deleterious effects of a nonPST bioactive compound(s) from <Emphasis Type="Italic">Alexandrium tamarense</Emphasis> on bivalve hemocytes

The known negative effects of shellfish toxin-producing dinoflagellates on feeding, burrowing and survival of some bivalve mollusks has prompted questions concerning whether they might also impair the internal defense system of affected bivalves and make them more susceptible to disease agents. The primary components of the cellular defense system are hemocytes. Many toxic dinoflagellates are too large to be ingested whole by hemocytes and would most likely be exposed to intracellular toxins only after the algae are consumed, broken down, and the water-soluble toxins, released. Therefore, we conducted a series of experiments in which hemocytes from two suspension-feeding bivalves—the Manila clam, Ruditapes philippinarum, and the softshell clam, Mya arenaria—were exposed in vitro to filtered extracts of one highly toxic paralytic shellfish toxin (PST)-producing and one nonPST-producing strain of Alexandrium tamarense (isolates PR18b, 76 ± 6 STXeq cell⁻¹ and CCMP115, with undetectable PST, respectively). We measured adherence and phagocytosis, two hemocyte attributes known to be inhibited by bacterial pathogens and other stressors. We found no measurable effect of a cell-free extract from a highly concentrated suspension of the PST-producing strain on hemocytes of either bivalve species. Instead, extract from the nonPST-producing strain had a consistent negative effect on both clams, resulting in significantly lower adherence and phagocytosis compared to strain PR18b and filtered seawater controls. The bioactive compound produced by strain CCMP115, which has yet to be characterized, may be similar to the PST-independent allelopathic compounds described for Alexandrium spp., which act on other plankters. These compounds and those produced by other harmful algae are known to cause immobilization, cellular deformation and lysis of co-occurring target organisms. Thus, nonPST producing Alexandrium spp., which do not cause paralysis and burrowing incapacitation of clams, may still produce a compound(s) that has negative effects not only on hemocytes, but on other molluscan cell types and their functions, as well.     

Population structure,mortality and growth of <Emphasis Type="Italic">Pinna nobilis</Emphasis> Linnaeus, 1758 (Mollusca,Bivalvia) at different depths in Moraira bay (Alicante,Western Mediterranean)

An investigation to characterize the causes of Pinna nobilis population structure in Moraira bay (Western Mediterranean) was developed. Individuals of two areas of the same Posidonia meadow, located at different depths (A1, −13 and A2, −6 m), were inventoried, tagged, their positions accurately recorded and monitored from July 1997 to July 2002. On each area, different aspects of population demography were studied (i.e. spatial distribution, size structure, displacement evidences, mortality, growth and shell orientation). A comparison between both groups of individuals was carried out, finding important differences between them. In A1, the individuals were more aggregated and mean and maximum size were higher (A1, 10.3 and A2, 6 individuals/100 m²; A1, x = 47.2 ± 9.9; A2, x = 29.8 ± 7.4 cm, P < 0.001, respectively). In A2, growth rate and mortality were higher, the latter concentrated on the largest individuals, in contrast to A1, where the smallest individuals had the higher mortality rate [A1, L = 56.03(1 − e^−0.17t ); A2, L = 37.59(1 − e^−0.40t ), P < 0.001; mean annual mortality A1: 32 dead individuals out of 135, 23.7% and A2: 16 dead individuals out of 36, 44.4%, and total mortality coefficients (z), z _A1(−30) = 0.28, z _A1(31–45) = 0.05, z _A1(46−) = 0.08; z _A2(−30) = 0.15, z _A2(31–45) = 0.25]. A common shell orientation N–S, coincident with the maximum shore exposure, was observed in A2. Spatial distribution in both areas showed not enough evidence to discard a random distribution of the individuals, despite the greater aggregation on the deeper area (A1) (A1, χ ² = 0.41, df = 3, P > 0.5, A2, χ ² = 0.98, df = 2 and 0.3 < P < 0.5). The obtained results have demonstrated that the depth-related size segregation usually shown by P. nobilis is mainly caused by differences in mortality and growth among individuals located at different depths, rather than by the active displacement of individuals previously reported in the literature. Furthermore, dwarf individuals are observed in shallower levels and as a consequence, the relationship between size and age are not comparable even among groups of individuals inhabiting the same meadow at different depths. The final causes of the differences on mortality and growth are also discussed.     

UV- and salinity-induced oxidative effects in the marine diatom <Emphasis Type="Italic">Cylindrotheca closterium</Emphasis> during simulated emersion

The diatom Cylindrotheca closterium was exposed to transient light- and osmotic conditions as occur during its tidal emersion. The objective was to analyze how this simulated emersion contributes to the production of active oxygen species (AOS) and via this, to oxidative cell damage. Light- and salinity conditions were varied in factorial combination: low light (no UVB) or high light (unweighted UVB-dose rates of respectively 0.01; 0.07; 0.24; 1.03 W m⁻²) at normal (30 psu) or high salinity (60 psu). UVB (0.01–0.24 W m⁻²) and high salinity had a significant, negative effect on the photosynthetic efficiencies ΔF/F _m’ (steady-state quantum yield) and F _v/F _m (maximum yield). UVB at 1.03 W m⁻² (15 kJ m⁻² d⁻¹) almost arrested electron transport. At ecologically relevant UVB levels, i.e. below 0.24 W m⁻² (≈3.4 kJ m⁻² d⁻¹) with UVB:PAR<0.4:100 (PAR photosynthetically active radiation) only dynamic photoinhibition was observed (protection via heat dissipation). Non-photochemical quenching was positively correlated with the de-epoxidation of diadinoxanthin (DD) to diatoxanthin (DT). A decreasing ratio DT/(DD+DT) after 4 h of UVB at >0.07 W m⁻² and at 60 psu indicated a reversal of the diatom xanthophyll cycle (diminished photoprotection) which may be caused by an enhanced AOS production. Oxidative stress and -damage to C. closterium cells were assessed applying fluorescent indicator dyes, via confocal microscopy and quantitative image analysis. AOS production rates (cellular DCF fluorescence) were stimulated by UV, and were ~50% higher at 60 psu. AOS production decreased with an increasing pre-exposure (0–4 h) to normal UVB (0.24 W m⁻²), which indicated a stimulation of the antioxidative defence. Non-protein thiols (indicator CMF) and glutathione pools (HPLC-analyzed) decreased with UVB-dose rates (0.01–0.24 W m⁻²), most likely due to AOS-mediated thiol oxidation. Hypersalinity (60 psu) and UVB (0.01–0.24 W m⁻²) caused membrane depolarization (dye DIBAC₄(3)) and phospholipid hydrolysis (phospholipase A₂ dye: bis-BODIPY FL-C₁₁-PC). AOS production may have diminished the membrane polarity, and peroxidized the membrane lipids (HPLC-analyzed malondialdehyde) which enhanced PLA₂ activity. The dyes indicated an increased oxidative (lipid) damage at a 15% inhibition of photosynthesis in this diatom, at UVB levels and salinities that can be expected in situ during its periodic tidal emersion.     

Two different proteases produced by a deep-sea psychrotrophic bacterial strain,<Emphasis Type="Italic"> Pseudoaltermonas</Emphasis> sp. SM9913

A psychrotrophic bacterial strain, Pseudoaltermonas sp. SM9913, was isolated from deep-sea sediment collected at 1,855 m depth. Two proteases produced by Pseudoaltermonas sp. SM9913 were purified, MPC-01 and MCP-02. MCP-01 is a serine protease with a molecular weight of 60.7 kDa. It is cold-adapted with an optimum temperature of 30–35°C. Its K_m and E_a for the hydrolysis of casein were 0.18% and 39.1 kJ mol^–1, respectively. It had low thermostability, and its activity was reduced by 73% after incubation at 40°C for 10 min. MCP-02 is a mesophilic metalloprotease with a molecular weight of 36 kDa. Its optimum temperature for the hydrolysis of casein was 50–55°C. The K_m and E_a of MCP-02 for the hydrolysis of casein were 0.36% and 59.3 kJ mol^–1, respectively. MCP-02 had high thermostability, and its activity was reduced by only 30.5% after incubation at 60°C for 10 min. At low temperatures, Pseudoaltermonas sp. SM9913 mainly produced the psychrophilic protease MCP-01.Communicated by O. Kinne, Oldendorf/Luhe