Survival, fecundity and sex ratio of Dinophilus gyrociliatus (Polychaeta: Dinophilidae) under different dietary conditions

The influence of different diets (Tetramin dry fish food, frozen spinach, and Dieterba mixed cereals for babies) on survival, fecundity and sex ratio was examined in a population of the polychaete Dinophilus gyrociliatus. Specimens were collected in the harbour of Genoa and were studied under controlled laboratory conditions. In D. gyrociliatus sex determination is progamic; a single ovary produces eggs of two sizes, the small eggs develop into dwarf males and the large ones into females. Sex determination is chromosomal with male heterogamety of the XO/XX type, and therefore a 1:1 population sex ratio is expected. The results of the experiment revealed that diet significantly influenced survival, fecundity and also the sex ratio. With all diets the maximum lifespan was about the same, but after 10 weeks of experiments the percentage of survivors was 9% with Tetramin, 35% with spinach and 39% with cereals. Fecundity was different in relation to diet; females fed with Tetramin produced the maximum number of eggs. The greater and earlier mortality with the Tetramin diet could be related to the greater fecundity seen in this group, in that, the greater amount of energy allocated to the production of gametes may have reduced the percentage of individuals surviving to maximum age. In this species, diet affected not only fecundity but also the sex ratio; the worms fed with cereals showed a clear displacement of sex ratio towards males. Received: 10 March 1997 / Accepted: 22 September 1998     

Effects of diet and laboratory rearing on demography of Dinophilus gyrociliatus (Polychaeta: Dinophilidae)

Life table response experiments were performed to evaluate the demographic consequences of: (1) the dietary regimes and (2) the length of laboratory rearing in strains of Dinophilus gyrociliatus, a small infaunal polychaete. The first experiment was performed using animals recently collected from the natural environment and fed either on spinach or on Tetramin (artificial fish food with high caloric content). Starting from this original group, two distinct laboratory strains were established: the first raised only with spinach, the second only with Tetramin. In the first experiment, the group fed on Tetramin exhibited greater population growth rate (5), shorter generation time (T) and reduced expectation of life (e⁰) with respect to the animals fed on spinach. The second experiment took place 2 years later to evaluate the difference in life history traits between these two laboratory strains. In the case of the group fed on Tetramin, population parameters exhibited marked variations; in fact, 5 and the net reproductive rate (R₀) were significantly higher and T and e⁰ were shorter than the corresponding parameters observed in the first experiment. Conversely, the demographic variations induced by laboratory rearing on a spinach diet were limited to a reduction in the expectation of life. The decomposition analysis showed that the reduction in generation time and the increase in fecundity occurring during the first 4 weeks of life accounted for nearly all the differences in 5. During the long breeding period at constant temperature, photoperiod and salinity, a continuous selection of the most precocious and fecund individuals may have taken place as a consequence of the abundance of resources and the lack of predation.     

Karyotype and sex determination in <Emphasis Type="Italic">Dinophilus gyrociliatus</Emphasis> (Polychaeta: Dinophilidae)

The study of the male and female karyotypes of the polychaete Dinophilus gyrociliatus was performed with the aim of ascertaining the diploid number of chromosomes and confirming the existence of karyological differences between the two sexes. Our results evidenced that the female karyotype of D. gyrociliatus consists of 2n=24 chromosomes, while the male karyotype has 2n=23 chromosomes. X chromosomes are subtelocentric and much bigger than the autosomes. The size of the D. gyrociliatus chromosomes, which in the female embryos vary in length from little more than 2.8 wm to less than 0.7 wm, are relatively small by comparison with those of other polychaetes. The existence of maternal and environmental factors able to influence the sex ratio of D. gyrociliatus and chromosomal differences between the sexes confirms that: (1) sex determination is chromosomal and syngamic of the XX-X0 type and (2) control of the sex ratio is progamic and depends on genetic and environmental factors.     

Sex ratio and sex change in wild and laboratory populations ofTyposyllis prolifera (Polychaeta)

Sex determination and sex differentiation in three local Istrian (Yugoslavia) populations of the polychaeteTyposyllis prolifera were studied both in the field and the laboratory. Significant differences in sex habits were revealed among the populations, the biological significance of which is unknown. (1) Sex ratios (% ) in natural populations from the Adriatic Sea amounted to 50% (Rovinj), 62% (Pore) and 77% (Pula). In the Pula-(but not in the Rovinj-)population, a correlation was found between population density on individualHalopteris scoparia thalli (the favorite habitat ofT. prolifera) and the respective sex ratio: The mean male protion was only a little more than 50% in densely populated thalli, and increased to more than 80% in sparsely populated thalli. Laboratory studies provided the information necessary to explain these field findings. (2) Laboratoryraised progenies showed an overall 1:1 primary (first sexual phase) sex ratio, independent of the local origin of the material and of whether worms were raised singly or under conditions of social contact. Nevertheless, the studied populations may differ with respect to the genetic mechanisms of sex determination: In the Pore- and Pulapopulation individual progenies often departed significantly from the 1:1 sex ratio, whereas individual progenies in the Rovinj- population never did so. (3) The populations proved to be partially protogynous. Sex differentiation in that half of the individuals which differentiated into males (primary males) was absolutely stable during sequential sexual phases. Worms which differentiated into females, however, often changed sex at an earlier or later stage of their life cycle (secondary males). Sex change was irreversible. As to the degree of lability of female differentiation under conditions of isolation, the findings suggest a genetic divergence among both populations and individuals of the same population. Female differentiation in the Rovinj-population was virtually stable, but Pula-females underwent rapid and complete masculinization. Pore-females were between these extremes. (4) In addition to genetic factors, exogenous conditions affected the incidence and time of sex change (Pula-population): Under conditions of social contact, sex change was delayed or suppressed as compared with isolated individuals. The degree of delay or inhibition was independent of the sex of the social partners, yet increased (to the point of saturation) with population density.     

Effects of food availability on dispersal and cub sex ratio in the Mednyi Arctic fox

Since the Pleistocene, Arctic foxes, Alopex lagopus, on Mednyi Island in the North Pacific have been isolated in a small area with rich food resources and no other terrestrial carnivores. This situation provides an unusually simple system within which the effect of food dispersion on demography and social organisation was examined. We studied the composition, location and dispersal of 67 Arctic fox groups and mapped their major food resources (seabird colonies) during 1994–2000 on Mednyi. We compared our observations with the predictions of models of sex-ratio determination. Our observations are most consistent with the predictions of Julliard's (2000) model, where mothers are expected to produce more offspring of the most dispersing sex in low-quality habitats, and more offspring of the most philopatric sex in high-quality habitats. The polygynous foxes on Mednyi Island lived where the principal food resources were patchily distributed (present on 11% of the shoreline), and cub survival to dispersal age or reproductive adult was higher in rich (25/45) than in poor (24/79) home ranges. Furthermore, dispersal was strongly sex-biased: most females (60%) remained on their natal ranges, whereas very few males (9%) did so. Significantly more female than male cubs (54 compared with 24) emerged from dens in resource rich ranges, whereas the sex ratio on poor ranges was approximately equal (51 females and 56 males). While our observations are also to some extent consistent with the local resource enhancement (LRE) hypothesis (which predicts a bias towards the sex most likely to cooperate with parents), this does not account for the observed spatial variability.     

Parental condition, brood sex ratio and differential young survival: an experimental study in gulls (Larus fuscus)

Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios. Received: 15 February 2000 / Revised: 7 August 2000 / Accepted: 26 August 2000     

Chromosomal complements of two forms of Neanthes japonica (Polychaeta,Nereididae) with evidence of male-heterogametic sex chromosomes

Karyotypes of the brackish-water polychaeteNeanthes japonica (Izuka) collected from five rivers in Japan from 1984 to 1988 were examined with air-drying and flame-drying methods using materials consisting of regenerating tails, clumps of spermatogonia and youngN. japonica specimens (embryos, larvae or juveniles). A diploid number of 28 was determined in well-spread metaphase chromosomes of mitotic cells. The presence of an XX-XY (male heterogametic) sex chromosome system was established for the first time in polychaetes. The Y chromosome was larger than the X chromosome. Slight differences in karyotype were found between two forms (the small- and large-egg forms), which are very similar in adult morphology but can be distinguished by reproductive and developmental characteristics.     

Effects of temperature and salinity acclimation of adults on larval survival,physiology, and early development of Lytechinus variegatus (Echinodermata: Echinoidea)

Larval survival and developmental rates of Lytechinus variegatus (Lamarck) were determined as a function of temperature and salinity in two experiments by: (1) directly transferring fertilized eggs to 35, 30, 27.5, 25, 20, 15, and 10S seawater at 18 and 23°C, and (2) acclimation of adult sea urchins to the conditions described above for 1 to 4 wk prior to spawning. Developmental rates and percent survival of larvae prior to metamorphosis decreased at salinities below 35 (Q₁₀ values for metamorphosis=0.380 to 0.384). Temperature and salinity significantly (P<0.05) affected metabolic rates of L. variegatus plutei. These results show that L. variegatus larvae are stenohaline when compared to larvae of other echinoderm species. LC₅₀ values (S), developmental rates, and survival to metamorphosis indicate that acclimation of adult sea urchins to lower salinity prior to spawing and fertilization does not enhance development or survival of embryos exposed to low salinity.     

Mate choice,fecundity and sexual dimorphism in two pipefish species (Syngnathidae)

Summary In order to understand the causes of sexual dimorphism, mate choice and size-related fecundity were studied in two pipefish species, Syngnathus typhle and Nerophis ophidion. Sexual dimorphism is more pronounced in N. ophidion; females are larger, have sexual colourings, and are more active during courtship. In S. typhle the sexes are alike in all these respects. Males brood their offspring in both species. In N. ophidion fecundity was positively correlated with both body size and the amount of sexual colouring in females. In males no correlation between body size and fecundity, or between body size and embryo size existed. Predictably, in mate choice experiments with equal-sized females, males chose females with more extensive sexual colourings. We explain sexual dimorphism in this species as a consequence of both natural selection (fecundity increases with size in females but not in males) and sexual selection (males prefer larger females). We argue that sexual size dimorphism did not evolve by selection minimizing overlap in food niches between the sexes, because food production is high in the Zostera beds where the fishes live, and no size dimorphism was found in the sympatrically occurring S. typhle. Furthermore, in N. ophidion dimorphism is not greater in a particular mouth character than in overall body size. In S. typhle egg size and the average number of eggs transferred per spawning were positively correlated with female body size. Apparently more energy per offspring was provided by larger males than by smaller males, and larger males also carried more offspring. As predicted, large mates were preferred by both sexes in mate choice experiments. This is explicable in terms of both natural selection (fecundity increases with size in both sexes) and sexual selection (both sexes prefer large mates). As a consequence of selection acting in the same direction in both sexes, sexual dimorphism is absent in S. typhle.     

The effects of operational sex ratio and food deprivation on copulation duration in the water strider (Gerris remigis Say)

Summary A North American water strider, Gerris remigis (Hemiptera) copulates for prolonged periods of time, apparently beyond the time necessary for insemination (Rubenstein 1984; Wilcox 1984; Clark 1987), I tested predictions from two different hypotheses of the adaptive significance of this behavior. Manipulations of the operational sex ratio in artificial streams revealed that water striders copulate significantly longer when the sex ratio is male-biased. This result is consistent with the hypothesis that prolonged copulation functions as a type of contact mate guarding, which reduces sperm competition by preventing the female from remating prior to oviposition. The finding is also consistent with the hypothesis that copulation is prolonged to enhance female foraging efficiency. In a second experiment, sex ratio and total population density were held constant, and the period of food deprivation of females was manipulated. Contrary to the predictions of the female foraging hypothesis, starved females did not copulate longer than fed females. Of the two hypotheses considered, only the copulatory guarding hypothesis explains both results. This does not mean, however, that there is no selective benefit from the enhanced foraging efficiency resulting from prolonged copulation. Nor does it exclude the possibility that this benefit has been necessary for the evolution of prolonged copulation in water striders.     

Effects of salinity on food intake,absorption and conversion in the rainbow trout Salmo gairdneri

In rainbow trout (Salmo gairdneri Richardson) which had been acclimatised to the experimental salinity and temperature, food intake was maximal in the intermediate salinities of 15.0 and 28.0%, less in fresh water and 7.5% and minimal, by a statistically significant margin, in 32.5%. There were marked day-to-day fluctuations in food intake. When salinity was abruptly increased by 7.5 or 13.0%, there were decreases in growth rate which were related to decreases in food intake. Recovery of food intake and growth rate to pre-increase levels was complete within 14 days. Absorption efficiency, in terms of total dry matter, total energy and total nitrogen, was negatively related to salinity. Total nitrogen was absorbed considerably more efficiently than either energy or dry matter. Conversion efficiency (K ₁ and K ₂) was estimated, also in terms of dry matter, energy and total nitrogen, in trout of the O+ and 1+ year groups weighing from 50 to 150 g. There was a tendency for conversion efficiency to decline with salinity, especially between 28.0 and 32.5%. Dry matter and energy conversion were significantly lower than nitrogen conversion efficiency.     

Effect of temperature,salinity and food level on sexual and asexual reproduction in Brachionus plicatilis (Rotifera)

The reproductive response of sexual and asexual female Brachionus plicatilis (Muller) was examined over temperatures ranging from 20° to 40°C, salinities from 5 to 40 S, and food levels from 0.25 to 20 g Chlorella vulgaris dry-weight per ml. Reduced food levels, as well as temperature and salinity extremes, reduced reproduction of both sexual and asexual females, but did so differentially. Reproduction by sexual females was reduced to a greater extent at environmental extremes than asexual females. The broad, flat reproductive response curve of asexual females extended beyond the limits of the narrower, more sharply peaked curve of sexual females. Thus zones of exclusively asexual reproduction exist at environmental extremes where sexual reproduction is physiologically restricted. These results are corroborated by a comparison of the lifetime fecundity of individual sexual and asexual females over a 20°C temperature range. No differences in lifetime fecundity occurred between sexual and asexual females at 18° and 28°C. At 38°C, however, asexual female fecundity reached its highest level, while sexual female fecundity declined 15%. The appearance of sexual females in rotifer populations in the result of both inducible and repressible factors.     

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     

Mating frequency, colony size, polyethism and sex ratio in fungus-growing ants (Attini)

The mating frequency of queens was estimated for eight attine ant species, Myrmicocrypta ednaella, Apterostigma mayri, Cyphomyrmex costatus, C. rimosus (four lower attines), Trachymyrmex isthmicus, Serico-myrmex amabalis, Acromyrmex octospinosus and Atta colombica (four higher attines), and correlated to colony size, worker polyethism, and sex ratio. Mating frequency was calculated from within-colony relatedness estimated by CAP-PCR DNA fingerprinting. Most queens of lower attines and T. isthmicus mated with only one male, while those of the three higher attines mated with multiple males. Mating frequency was positively correlated with colony size. Polyethism among workers was dependent on worker age in lower attines but on body size in higher attines, suggesting some correlation between mating frequency (i.e., within-colony gene diversity) and caste complexity. The sex ratio was biased toward females in species where the mating frequency equaled one, but toward males in species where the mating frequency was greater than two. Changing in nest site from ground surface to deep underground may have facilitated the evolution of large colony size in Attini, and this may have resulted in the evolution of polyandry (a queen mates with multiple males). With the evolution of polyandry in higher attines, Atta and Acromyrmex in particular have generated high genetic diversity within their colonies and complex social structures. Received: 26 October 1999 / Revised: 25 May 2000 / Accepted: 24 June 2000     

Effects of temperature on oocyte growth in the Mediterranean terebellid Eupolymnia nuebulosa (Annelida: Polychaeta)

Eupolymnia nebulosa (Montagu) is a widely distributed terebellid polychaete that builds its tubes on the coastal shelf in areas with mixed soft and hard bottoms. From a long-term survey in the Bay of Banyuls, France (NW Mediterranean), we found an advancement of the timing of the spawning period coincident with a delayed breakdown of the thermocline. We postulate that persistent high temperatures can influence gamete development by stimulating oocyte growth, resulting in earlier spawning. During 1992 and 1993, we used a between-individuals experimental approach to assess the possible effect of temperature on oocyte growth based on: (1) determination of the growing fraction of the oocyte population (i.e. oocyte net growth); (2) identification of differences in oocyte growth-rate among females; (3) comparison of non-significantly different size-distributions of the growing oocyte fraction (net oocyte size-distributions) at the beginning of the experiments with those at the end. No effect of temperature on oocyte growth was detectable at the population level, but a positive individual response to prolonged high temperature was evident. Thus, the lack of a significant response by the population to prolonged high temperature does not imply a lack of individual response. We propose a model of oocyte-growth dynamics based on temperature that incorporates previous observations of extended oogenesis and oocyte growth during periods of both increasing and decreasing temperature and on the scattered pattern of oocyte size-distributions at the onset of spawning. Received: 4 November 1996 / Accepted: 18 January 1997     

Feeding by larvae of two different developmental modes in Streblospio benedicti (Polychaeta: Spionidae)

Females of the spionid polychaete Streblospio benedicti (Webster) produce either small eggs (60–70 μm diameter) and planktotrophic larvae, or large eggs (100–200 μm) and lecithotrophic larvae that reportedly do not feed. This intraspecific polymorphism, a form of poecilogony, is potentially useful in studies of larval ecology and evolution, but necessary data on larval form and function are lacking. This study describes the morphology and nutritional biology of larvae obtained from Atlantic (South Carolina) and Pacific (California and Washington) populations from 2003 to 2005. The two types of larvae produced by Atlantic S. benedicti differed greatly in length (229±22 μm SD for planktotrophs vs. 638±40 μm for lecithotrophs) and chaetiger number (2–5 vs. 10–11) at release from the female’s brood pouch. Planktotrophic larvae bore long provisional chaetae on their first chaetiger; provisional chaetae were absent in lecithotrophic larvae. Larvae from Pacific populations were all of the lecithotrophic form, and were similar to their Atlantic counterparts in all respects. High-speed video microscopy revealed that both types of larvae used opposed bands of cilia to capture suspended particles and transport them to the mouth, where they were often ingested. Lecithotrophic larvae reared with suspended phytoplankton (Rhodomonas sp., 10⁴ cells ml⁻¹) for 2 days grew significantly faster than sibling larvae reared without added food, indicating that these larvae can digest and assimilate ingested food. Larvae of S. benedicti that develop from large eggs are thus facultative planktotrophs instead of obligately non-feeding lecithotrophs, a result that affects the interpretation of comparative studies of the ecology and evolution of larvae in S. benedicti and certain other marine invertebrates.     

Influence of social factors on sex ratio at birth,maternal investment and young survival in a prosimian primate

Summary In order to determine whether social factors influence sex ratio at birth in lesser mouse lemurs, experiments were conducted during 5 successive breeding periods on 51 females. At the beginning of the breeding season, females were either isolated (I) or grouped (G) in heterosexual groups with an increasing number of females (2, 3 or 4). To ensure mating, I females were introduced in a group only during the oestrous period. After mating, both I and G females were isolated during pregnancy and lactation. Reproductive capacities of females in terms of oestrus occurrences (n = 324), impregnations (n–210), pregnancies (n = 136) or abortions (n = 38) or litter sizes (1–3 young) were affected neither by age and parity of females nor by group housing prior to conception. G females produced significantly more sons than daughters (67% males for 189 newborn) while females living alone except during the mating period demonstrated a significant inverse tendency (39.6% males for 96 newborn). Distribution of sexes in litters was statistically different from random and varied according to the shift of sex ratio at birth. In G females, the shift in the sex ratio towards males was consistent across the different groups, independent of the number of females living together, suggesting that the presence of only 1 female is sufficient to induce a bias in the sex ratio. No correlation was found between infant survival at weaning and age, parity or group housing of the mother. The maternal investment allocated to male or female newborn was similar provided the litter contained at least 1 male. In litters without males, growth and survival of female infants were significantly less. These results on sex ratio bias in captive female mouse lemurs agree with directions of bias predicted by the local resource competition model for facultative sex ratio adjustment (Clark 1978). Nevertheless, the pattern observed in mouse lemurs appears to be independent of the nutritional state of the female and of differential maternal investment.     

Effect of food type and ration on growth of juvenile Capitella sp. I (Annelida: Polychaeta): macro- and micronutrients

Growth rates of juvenile Capitella sp. I were determined on different rations of six food types: Gerber's mixed cereal, TetraMin fish flakes, benthic diatoms, Ulva sp., spring detritus, and summer detritus. A simple growth model estimated maximum growth rate and maintenance ration for each food. There were differences in the growth response among foods relative to nitrogen content. As juveniles increased in size, differences in growth between foods became more pronounced. For all juveniles, growth rates were correlated with levels of the amino acids histidine, phenylalanine, threonine, and valine, and the polyunsaturated fatty acid 20:5w3; correlations with histidine and phenylalanine levels were the most significant. Regressions of growth rates as a function of these two amino acids suggest a daily maintenance ration of 300 pg histidine and phenylalanine mg^-1 nitrogen biomass. Juvenile worms grew on spring detritus but not on summer detritus, indicating the probable importance of micronutrients (polyunsaturated fatty acids, amino acids) for growth of juvenile Capitella sp. I in the field.     

Effects of temperature and salinity on larval development ofNecora puber (Brachyura: Portunidae)

Temperature and salinity affected both length of larval development and mortality inNecora puber collected in the Ría de A Coruña during December 1984 and January 1985. Development time decreased considerably with increased temperature. This decrease was sharper when temperature increased from 15° to 20°C than when it increased from 20° to 25°C. At 35S, average development took 48, 32 and 28 d at 15°, 20° and 25°C, respectively. At the three salinities tested (25, 30 and 35), larval development was completed only at 15°C, at 20°C/30 and 35S, and at 25°C/35S. Development times at 15° and 20°C were highly significantly different at both 35 and 30S (P 0.01). However, there were no significant differences between development times at 20° and 25°C (P > 0.05). Within any one specific temperature series, no significant difference was observed between the salinity values tested (P > 0.05). The duration of each of the five zoeal stages was similar within each and the same temperature/salinity combination, whereas the duration of the megalop was twice as long as any of the zoeal stages. The combination of the lowest temperature (15°C) and the highest salinity (35) tested resulted in the greatest larval survival of 28%. Highest mortality occurred at 25°C, at which temperature development was completed only at 35S. A sharp drop in larval survival was observed in the transition period Zoea V — megalop in all combinations of temperature and salinity tested. Within the limits of tolerance to temperature and salinity, the former effected more pronounced differences in the duration of larval development, while salinity appeared to constitute a limiting factor for survival.     

Effects of salinity on endogenous rhythm of the Manila clam, Ruditapes philippinarum (Bivalvia: Veneridae)

The Manila clam Ruditapes philippinarum, an intertidal bivalve, was exposed to different salinity regimes (from 31.0–31.7‰ down to 20‰, 15‰, 10‰, 5‰), and the endogenous rhythm in its oxygen consumption was studied using an automatic intermittent-flow respirometer. When exposed to salinities reduced from 31.5‰ to 20‰ and 15‰ under otherwise constant conditions, the clams recovered a clear endogenous circatidal rhythm in their oxygen-consumption rate after having dampened periods of 12 h and 48 h, respectively. At salinities less than 10‰, however, the oxygen-consumption rate was depressed greatly at the beginning of the experiment for about 36 h and then increased to a level higher than normal, but the rhythm of oxygen consumption was not recovered. The results of this study indicate that the Manila clam, a euryhaline organism, cannot maintain a normal metabolic activity at a salinity lower than 15‰. All clams were dead after exposure at a salinity of 5‰ for 7 days. Received: 28 February 2000 / Accepted: 26 August 2000