Upper temperature tolerances of some European molluscs. I Tellina fabula and T. tenuis

The upper temperature tolerances of two European species of Tellina from populations near the northern and southern limits of their geographical range are compared by means of median lethal temperature (LT₅₀), and median burial temperature (BT₅₀) determination for periods of exposure up to 96 h. The influence of previous acclimation temperature, and salinity within a limited range, on these determinations is also considered. T. fabula Gmelin, which is found in deeper water throughout its range, shows a lower thermal tolerance than T. tenuis da Costa. Both species display a significant effect of previous acclimation temperatures, the effect on the burrowing responses being greater than that shown by LT₅₀, and in both species the thermal tolerance of individuals from Mediterranean populations is greater than that of individuals from North Atlantic populations.     

Upper temperature tolerances of some European molluscs II. Donax vittatus,D. semistriatus and D. trunculus

The upper temperature tolerances of 3 European species of Donax [D. vittatus (da Costa), D. semistriatus Poli, D. trunculus L.] are compared by means of median lethal temperatures (LT₅₀) and median burial temperature (BT₅₀) determinations for periods of exposure up to 96 h. The influence of previous acclimation temperatures on these determinations is also considered. D. trunculus shows the greatest thermal tolerance, reflecting its distribution in shallower water; D. vittatus and D. semistriatus display much closer thermal tolerances, although D. semistriatus which has a more southerly geographical distribution has a slightly higher thermal tolerance. All 3 species exhibit a significant effect of previous acclimation temperature, the effect on the burrowing responses being greater than that on LT₅₀.     

Evolution of Cardium (Cerastoderma) edule,C. lamarcki and C. glaucum: studies of DNA-variation

Genetic relations between populations of Cardium (Cerastoderma) edule (L.) and C. lamarcki (Reeve) from Danish waters and C. glaucum (Bruguiére) from the Mediterranean Sea were studied using DNA analysis. Repetitive sequences of nuclear DNA were compared by Southern blotting of enzyme digested DNA fragments, separated on agarose gel by electrophoresis. Fragments were visualized by autoradiography using ³²P labelled probes. These were nick translated, randomly sheared fragments of the whole chromosomal genomes of two species of distant taxa — Mytilus edulis and Mya arenaria, respectively. The number of detected DNA bands was 18 for C. edule, 32 for C. lamarcki, and 24 for C. glaucum. Measurements of genetic distances, based on comparisons of these repetitive sequences using a simple dissimilarity coefficients, show the greatest distance, i.e., 0.84 between C. edule and C. lamarcki, 0.69 between C. edule and C. glaucum, and the smallest, 0.52 between C. lamarcki and C. glaucum. Comparisons of the three Cardium types, using gene spliced and cloned C. edule nuclear DNA (nDNA) fragments as probes, accorded with these findings. Also, analysis of mitochondrial DNA (mtDNA) from eggs and hepatopancreas, although not in itself conclusive, accorded with the results of the analyses of nDNA. MtDNA was measured by electron micrograph to 15 000 base pairs (bp), and sizes of restriction enzyme digested fragments were compared by gel electrophoresis. The results of the comparisons based on repetitive sequences supported by the two other studies suggest that C. edule resembles the ancestral form more than the two others, and that C. lamarcki is the most different and recently evolved form. This hypothesis is discussed in relation to the geographical distribution of the species complex.     

Biological studies on Cardium glaucum,based on some Baltic and Mediterranean populations

The recent separation of Cardium glaucum Bruguière from C. edule L. and the scarcity of reports of populations containing both species has necessitated further ecological study to demonstrate differences in habitat preferences. Some mixed and unispecific populations of the cockles Cardium edule and C. glaucum, situated in fjords around the Kattegat and Baltic coasts of Denmark were studied; it was demonstrated that some degree of tidal influece was essential for the survival of C. edule, and that C. glaucum was being restricted by the exposure of the site. A study of the cockle populations of some étangs on the Mediterranean coast of France demonstrated that, despite earlier reports of its presence, C. edule was not present in that area. A brief survey of the Bassin d'Arcachon (France) showed that it contained a mixed cockle population. It was concluded that C. edule was unable to occupy localities having a tidal amplitude of less than 0.2 m, and that high temperatures also restricted occupation of semitidal habitats; similarly, the exposure of the site restricted colonisation by C. glaucum.     

Some aspects of the thermal tolerance of Trichomya hirsuta (Mollusca: Bivalvia) of Eastern Australia

The upper temperature tolerance range for Trichomya hirsuta L. is described for acclimation temperatures of 10° to 32°C at 30% S. Changes in thermal resistance with time are described along with the incipient lethal temperatures. The eurythermicity for the species is 240.7 C°-squared. The ultimate upper incipient lethal temperature is 35.1°C. Definition of the incipient sublethal temperature is given, and is equal to 32°C. Diet effects are shown not to be significant in short-term lethal experiments, but indicate that unfed mussels reduce their resistance after prolonged exposure to high temperatures. Size was also shown to have no effect on resistance and tolerance. A comparison of the tolerance triangle of T. hirsuta with those of the temperate species Mytilus edulis and Mya arenaria revealed that the 24-h LT₅₀ minus 2.2 C° approximates the incipient lethal temperature. The eurythermicities of the temperate species are Mytilus edulis, 363 C°-squared; and Mya arenaria, 415 C°-squared.     

Genetic relations between the bivalves Cardium (Cerastoderma) edule,Cardium lamarcki and Cardium glaucum,studied by means of crossed immunoelectrophoresis

Rabbit antisera were raised against Cardium edule (L.) from the Waddensea locality, Skallingen, against Cardium lamarcki (Reeve) from the Danish locality, Dybsø Fjord, and against Cardium glaucum (Bruguière) from the French Mediterranean coast at Ile des Embiez. Allopatric and sympatric populations of C. lamarcki and C. edule from Swedish and Danish localities, and allopatric populations of C. glaucum from the Mediterranean Sea, Red Sea and Black Sea were studied by means of the described method and by starch gel electrophoresis of malate dehydrogenase (MDH). By means of crossed immuno-electrophoresis, using intermediary gels containing antibodies against one of the Cardium types and receiver gels containing antibodies against one of the others, type-specific and type-non-specific reactions were recorded. The results are compared with a similar investigation of Mytilus edulis, M. galloprovincialis and M. californianus. Based on immunological properties, it is shown that C. edule is a valid species different from the C. lamarcki/C. glaucum complex. It is further suggested that C. lamarcki and C. glaucum should still be referred to by these two names as significant genetic differences between them is revealed by this immunological study.     

Habitat selection of two congeneric bivalves,Cardium edule and C. glaucum in sympatric and allopatric populations

Habitat selection of two bivalve species, Cardium edule (L.) and C. glaucum (Bruguière) is described for sympatric and allopatric populations of both sibling species. It is demonstrated that C. edule grows faster than C. glaucum and is absent from localities with periodic low food-availability. The ability to burrow of C. glaucum is low compared with that of C. edule; the former species does not occur in very loose, well-sorted sand near the coast. Earlier studies which suggest that tidal range determine the species distribution could not be confirmed.     

Improved survival under heat stress in intertidal embryos (<Emphasis Type="Italic">Fucus</Emphasis> spp.) simultaneously exposed to hypersalinity and the effect of parental thermal history

Intertidal organisms exposed to thermal stress normally experience other stresses simultaneously, but how these combined stresses modify tolerance to heat, especially for embryos, is poorly understood. Tolerance of fucoid algal embryos to heat, with and without acclimation to a sublethal temperature and with simultaneous exposure to hypersaline media, was examined. Embryos of Fucus vesiculosus L. (mid-intertidal zone) were less tolerant than embryos of Fucus spiralis L. (upper intertidal zone); without acclimation and with a growth temperature of 14°C, about half of the embryos survived 3 h exposure to 33°C in F. vesiculosus and of 35°C in F. spiralis. Conditions experienced by parental thalli (4°C versus 14°C storage) significantly affected the heat tolerance of embryos grown for 24 h post-fertilization at 14°C in F. vesiculosus, a result that is important for biologists using fucoid algae as model systems. Acclimation to a sublethal temperature (29°C) or exposure to the LT₅₀ (33°C, F. vesiculosus; 35°C, F. spiralis) in 100 psu seawater (2850 mmol kg^–1 osmolality) resulted in 30–50% higher levels of embryonic survival. Higher levels of HSP60s were found in embryos exposed to 29–33°C than to 14°C; lower levels of HSP60s were present in embryos exposed to the LT₅₀ under hypersaline conditions than in normal seawater. Contemporaneous studies in 1995–1996 of substratum temperature and desiccation levels were made at Schoodic Point, Maine (USA) underneath F. spiralis and F. vesiculosus canopies and in Semibalanus balanoides patches. This study extends the bioindicator utility of heat-shock proteins in studies of intertidal organisms and demonstrates the importance of integrated stress responses in survival of a single stress factor (e.g. temperature).Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Studies on the reproductive cycles of Cardium edule and C. glaucum

Two allopatric populations of Cardium edule and two of C. glaucum were sampled over a period of 16 months and their gonad maturity cycles compared by histological methods. The onset of both spermatogenesis and oogenesis in C. edule took place between September and October; in C. glaucum it was slightly later, between October and November. Development of the gonads continued slowly throughout the winter, C. glaucum developing marginally faster than C. edule. Maturation accelerated in April and May, with the gonads of both species becoming fully ripe in late May and early June. Spawning of the two species approximately coincided at all the localities investigated. Spawning in C. edule was better coordinated than in C. glaucum. Transplanting cockles of one species to the typical habitat of the other did not appreciably alter the spawning period. The separation of spawning periods in sympatric associations of the two cockles is discussed.     

Phylogeography of Cerastoderma glaucum (Bivalvia: Cardiidae) across Europe: a major break in the Eastern Mediterranean

Molecular variation of the lagoon cockle Cerastoderma glaucum (Poiret, 1789) was examined across the species range along European coasts, from the northern Baltic to the Black Sea and the Caspian Sea. A major phylogeographic break in mitochondrial COI gene sequences (divergence 6.2%) separated a group of Ponto-Caspian and Aegean Sea haplotypes from those to the west of the Peloponnese peninsula in the Mediterranean and in the Atlantic-Baltic sector. A similar subdivision, yet not entirely geographically coincident, was found at 16 allozyme loci (Nei's distance D=0.15). The results imply a long-term isolation of populations in parts of the Eastern Mediterranean or Black Sea basins through the Pleistocene. The subdivision does not concur with previous views on the systematics of C. glaucum complex, but the pattern is notably similar to that described earlier from some fish species. Marked phylogeographic structuring was also found at lower level within the major Mediterranean-Atlantic phylogroup of C. glaucum, which was divided into six regional or local haplotype subgroups. Divergence of these groups may date back one or several major Pleistocene climatic cycles. Local mtDNA diversity was particularly high in a sample from the Ionian Sea, whose mitochondrial identity was of the Mediterranean-Atlantic type, while its nuclear characters were more strongly associated with the Ponto-Caspian type. Patterns of shallower, star-phylogeny type diversity within the Ponto-Caspian phylogroup and in the Baltic Sea area may represent more recent, post-glacial generation of variation.     

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.     

Temperature tolerance of the polychaete worms Diopatra cuprea and Clymenella torquata

The resistance to high temperatures of two species of intertidal tube-dwelling polychaete worms has been tested to show seasonal and geographic variations. The summer 50% survival temperature at Beaufort, North Carolina, was 42.5 °C for Diopatra cuprea (Bosc) and 40.5 °C for Clymenella torquata (Leidy). Winter 50% survival temperatures for both species at Beaufort were approximately 4.0 C° lower. Both species showed a geographic difference in 50% survival temperature of more than 4.0 C° between North Carolina and Massachusetts in summer. D. cuprea from the Mississippi coast showed a lower survival temperature, probably due to combined temperature and salinity effects. Laboratory acclimation of C. torquata from Beaufort at low temperatures, during summer months, produced experimental results similar to those from winter animals. The seasonal differences in temperature tolerance are phenotypic expressions of a physiological response which can be related to environmental temperature patterns.     

The nature of heat coma in Littorina littorea (Mollusca: Gastropoda)

 The nature of heat coma was examined in the edible periwinkle Littorina littorea (L.). Duration of acclimation did not influence heat-coma temperature at 12 °C, although other acclimation temperatures were important in influencing thermal tolerance, with positive shifts in coma temperature observed in response to elevated temperatures. Previous thermal history also influenced heat-coma temperatures. Individuals subjected to repeat heat-coma events on a daily basis showed significant declines (P < 0.05) in coma-temperature; in contrast individuals exposed to repeat heat-coma events on a weekly basis showed no decline in thermal tolerance. Size-effects occurred at selected sites, where decreased heat-coma temperatures were recorded in large individuals. Received: 16 August 1999 / Accepted: 13 June 2000     

Temperature adaptation in sea anemones: Physiological and biochemical variability in geographically separate populations of Metridium senile

Latitudinally separate populations of the sea anemone Metridium senile (L.) are very similar genetically by electrophoretic criteria, yet respond differently to temperature. Anemones from southern and northern California (USA) have different oxygen consumption patterns in response to acclimatory and acute changes in temperature. Northern anemones show a pronounced increase in Q₁₀ at temperatures just above the normal environmental range, but southern anemones do not. The two populations also differed in the extent of metabolic compensation to temperature following several weeks of acclimation. This acclimation regime resulted in changes in the activities of several enzymes of intermediary metabolism, yet the extent and direction of these changes did not display a consistent trend with regard to acclimation temperature or population. The biochemical concomitants of acclimatory and acute temperature effects were studied further by measuring the concentrations of adenylates (ATP, ADP, and AMP) in anemones from the two populations exposed to different temperature regimes. During cold acclimation for several weeks, total adenylate concentrations (A_T) increased in both the southern and northern populations, possibly due to metabolic rate compensation, since A_T is positively correlated with tissue metabolic rate in many species. Moreover, the extremely low weight-specific oxygen consumption rates of M. senile are probably related to its very low A_T values. Acute temperature decreases had no effect on adenylate concentrations and adenylate energy charge (AEC); in contrast, acute temperature increases led to large changes in adenylate concentrations. The effects of starvation on adenylate concentrations are pronounced, and the effect is temperature-dependent. In starved individuals held at 20°C, AEC values fell to 50% of normal values after 8 d, while those held at 10°C maintained normal AEC values.     

Salinity tolerance,salinity preference and temperature tolerance in the high-shore harpacticoid copepod Tigriopus brevicornis

Tigriopus brevicornis (O. F. Müller) were collected in 1992 from rock pools close to U.M.B.S. Millport, Isle of Cumbrae, U.K. and acclimated to various combinations of salinity and temperature for at least 1 wk prior to laboratory experiments. Higher salinities of acclimation enhanced tolerance to high salinity stress, while tolerance of low salinities was hardly affected by acclimation salinity. Acclimation to low temperature (10°C) extended the survivable salinity range for T. brevicornis. High-salinity acclimation enhanced the survivable temperature range. Copepods acclimated to 60 survived significantly lower and higher temperatures than did 34-acclimated individuals. At high temperature, 75-acclimated female copepods had the highest median lethal temperature, 38.9°C. Females were significantly more resistant to high temperatures than males. The copepods were seen to have a very low median lethal temperature when frozen into solid ice for 2 h; 50% mortality occurred at-16.9°C in 10°C, 34-acclimated T. brevicornis. Salinity preference experiments demonstrated an ability to discriminate between salinities differing by as little as 3. Copepods acclimated to 34 chose salinities near their acclimation salinity; individuals acclimated to 5 favoured slightly higher salinities, while copepods acclimated to 60 chose rather lower salinities.     

Response of zooxanthellae in symbiosis with the Mediterranean corals Cladocora caespitosa and Oculina patagonica to elevated temperatures

Scleractinian symbiotic corals living in the Ligurian Sea (NW Mediterranean Sea) have experienced warm summers during the last decade, with temperatures rapidly increasing, within a few days, to 3–4°C above the mean value of 24°C. The effect of elevated temperatures on the photosynthetic efficiency of zooxanthellae in symbiosis with temperate corals has not been well investigated. In this study, the corals, Cladocora caespitosa and Oculina patagonica were collected in the Ligurian Sea (44°N, 9°E), maintained during 2 weeks at the mean summer temperature of 24°C and then exposed during 48 h to temperatures of 24 (control), 27, 29 and 32°C. Chlorophyll (chl) fluorescence parameters [F _v/F _m, electron transport rate (ETR), non-photochemical quenching (NPQ)] were measured using pulse amplitude modulated (PAM) fluorimetry before, during the thermal increase, and after 1 and 7 days of recovery (corals maintained at 24°C). Zooxanthellae showed a broad tolerance to temperature increase, since their density remained unchanged and there was no significant reduction in their maximum quantum yield (F _v/F _m) or ETR up to 29°C. This temperature corresponded to a 5°C increase compared to the mean summer temperature (24°C) in the Ligurian Sea. At 32°C, there was a significant decrease in chl contents for both corals. This decrease was due to a reduction in the chl/zooxanthellae content. For C. caespitosa, there was also a decrease in ETR_max, not associated with a change in F _v/F _m or in the non-photochemical quenching (NPQ); for O. patagonica, both ETR_max and F _v/F _m significantly decreased, and NPQ_max showed a significant increase. Damages to the photosystem II appeared to be reversible in both corals, since F _v/F _m values returned to normal after 1 day at 24°C. Zooxanthellae in symbiosis with the Mediterranean corals investigated can therefore be considered as resistant to short-term increases in temperature, even well above the maximum temperatures experienced by these corals in summer.     

Human trampling as short-term disturbance on intertidal mudflats: effects on macrofauna biodiversity and population dynamics of bivalves

The effect of physical disturbance in the form of trampling on the benthic environment of an intertidal mudflat was investigated. Intense trampling was created as unintended side-effect by benthic ecologists during field experiments in spring and summer 2005, when a mid-shore area of 25 × 25 m was visited twice per month by on average five researchers for a period of 8 months. At the putatively-impacted location (I) (25 × 25 m) and two nearby control locations (Cs) (25 × 25 m each), three sites (4 × 4 m) were randomly selected and at each site, three plots (50 × 50 cm) were sampled after 18 and 40 days from the end of the disturbance. Multivariate and univariate asymmetrical analyses tested for changes in the macrofaunal assemblage, biomass of microphytobenthos and various sediment properties (grain-size, water content, NH₄ and NO₃ concentrations in the pore water) between the two control locations (Cs) and the putatively-impacted location (I). There were no detectable changes in the sediment properties and microphytobenthos biomass, but variability at small scale was observed. Microphytobenthos and NH₄ were correlated at I to the number of footprints, as estimated by the percentage cover of physical depressions. This indicated that trampling could have an impact at small scales, but more investigation is needed. Trampling, instead, clearly modified the abundance and population dynamics of the clam Macoma balthica (L.) and the cockle Cerastoderma edule (L.). There was a negative impact on adults of both species, probably because footsteps directly killed or buried the animals, provoking asphyxia. Conversely, trampling indirectly enhanced recruitment rate of M. balthica, while small-sized C. edule did not react to the trampling. It was likely that small animals could recover more quickly because trampling occurred during the growing season and there was a continuous supply of larvae and juveniles. In addition, trampling might have weakened negative adult-juvenile interactions between adult cockles and juvenile M. balthica, thus facilitating the recruitment. Our findings indicated that human trampling is a relevant source of disturbance for the conservation and management of mudflats. During the growing season recovery can be fast, but in the long-term it might lead towards the dominance of M. balthica to the cost of C. edule, thereby affecting ecosystem functioning.     

Some effects of temperature on the growth and metabolic rate of juvenile blue crabs,Callinectes sapidus,in the laboratory

Juvenile blue crabs, Callinectes sapidus Rathbun, were grown in the laboratory at different temperatures, and metabolic-rate determinations were made. Growth is shown to be dependent upon temperature. Crabs kept at high temperatures (34° and 27°C) grow faster than those kept at lower temperatures (13°, 15°, and 20°C). Increase in size per molt is less at higher temperatures than at lower ones. Mortality is directly proportional to temperature between 13° and 34°C and is very high during ecdysis at elevated temperatures. Metabolic rate increases with temperature, but various degrees of acclimation are seen after 4 weeks exposure. No acclimation of general activity to temperature was found. The findings are applied theoretically to crabs living in the region of heated discharge canals of electrical generators: the motile blue crab could extend its growing season without decreasing size at maturity by active selection of thermal surroundings.In part based on a thesis submitted in partial fulfillment of the requirements for the Degree of Master of Science at the University of Florida, USA.     

Tolerance to low oxygen and sulfide in Amphiura filiformis and Ophiura albida (Echinodermata: Ophiuroidea)

The brittle stars Amphiura filiformis (Müller) and Ophiura albida (Forbes) were exposed to different oxygen saturations (100, 10, 5, 3, and <1% oxygen saturation) and to physiological anoxia (<1% oxygen saturation) at different total sulfide concentrations (0, 2, 20, 200 μM). The mortality was followed during experiments and the median survival time (LT₅₀) was determined. The infaunal A. filiformis had a significantly higher tolerance to both hypoxia and sulfide than did the epibenthic O. albida. After exposure to 10% oxygen saturation for a month, only 2.0% A. filiformis and 0% O. albida were dead. In oxygen saturations <1% A. filiformis and O. albida had a LT₅₀ of 7.5 and 2.5 d, respectively. The presence of even very small concentrations of sulfide decreased the survival significantly. Sulfide is shown to be the key factor for the survival of the two species. Received: 11 October 1996 / Accepted: 12 November 1996     

Upper temperature tolerances of Tellina tenuis and T. fabula

The upper lethal tolerances of Tellina tenuis da Costa and T. fabula Gmelin were studied under a variety of temperature regimes in which both short-term and long-term mortality were noted; these included steady temperatures, acclimation at a steady temperature then exposure to a steady tempeature, and acclimation at gradually increasing temperatures. T. tenuis alone was also tested for variation in mortality rate according to the date of collection or to the size (i.e., age) of the mollusc. The results suggested that T. tenuis was consistently more tolerant than T. fabula, and that the difference in tolerance was around 5 C°. Both species readily acclimated to higher temperatures after steady temperature acclimation, but not under a regime of gradually increasing temperature. There was no significant, variation in the mortality rate of T. tenuis with season or age. It is suggested that, since the difference in temperature tolerance is not reflected in their respective geographical distributions, there may be some other stage in their life cycle, e.g. the larvae, which is equally susceptible to temperature extremes. The difference in tolerance is, however, reflected in their respective littoral and sublittoral distributions.