Reproductive biology of the greyfin croaker <Emphasis Type="Italic">Pennahia anea</Emphasis> in the northern South China Sea

The greyfin croaker Pennahia anea is one of the most common croakers currently on retail sale in Hong Kong, but there are no regional studies on its biology or fishery. The reproductive biology of the species, based on 464 individuals obtained from local wet markets, was studied over 16 months (January 2008–April 2009) using gonadosomatic index (GSI) and gonad histology. Sizes used in this study ranged from 8.0 to 19.0 cm in standard length (SL). Both the larger and smaller size classes were missing from samples, implying that they are infrequently caught in the fishery. Based on GSI data, the approximate minimum sizes for male and female maturation were 12 cm SL. The size at 50% maturity for females was 14.3 cm SL, while all males in the samples were mature. Both GSI and gonad histology suggest that spawning activity occurred from March–April to June, with a peak in May. Since large croakers are declining in the local and regional fisheries, small species such as P. anea are becoming important, although they are mostly taken as bycatch. In view of unmanaged fishing pressure, and given the decline in large croakers and sizes of P. anea presently caught, proper management of the species is suggested.     

Reproductive cycle and gonad development of the Northern Argentinean <Emphasis Type="Italic">Mesodesma mactroides</Emphasis> (Bivalvia: Mesodesmatidae)

The reproductive cycle and gonad development of the yellow clam Mesodesma mactroides was studied over a period of 24 months (January 2005–December 2006) at the Argentinean sandy beach Santa Teresita. Histological examination of gonadal tissue revealed that sex ratios did not significantly deviate from the proportion of 1:1 and no case of hermaphroditism was found. The reproductive cycle of M. mactroides followed an annual cyclicality, which was significantly correlated to monthly mean sea surface temperatures (SST). Oocytes showed highest abundance in winter, indicating a process of gonadal development and sexual maturation. The mean oocyte size decreased significantly during spring. Modal oocyte sizes decreased significantly during winter and late spring of each year, suggesting spawning events. The condition index was not useful in describing the annual reproductive cycle of M. mactroides. Ash-free, shell-free dry mass was chosen to detect the condition of the specimens, and this significantly correlated with monthly mean SST and the gametogenic cycle. Annual recruitment patterns during summer–autumn indicated a 3-month-long planktonic phase of M. mactroides. The reproduction cycle and gonad development of M. mactroides showed only weak differences between data from the present study and those collected 40 years ago.

Gonadosomatic index and fecundity of Lower Missouri and Middle Mississippi River endangered pallid sturgeon estimated using minimally invasive techniques

Minimally invasive, non‐lethal methods of ultrasonography were used to assess sex, egg diameter, fecundity, gonad volume, and gonadosomatic index, as well as endoscopy to visually assess the reproductive stage of Scaphirhynchus albus. Estimated mean egg diameters of 2.202 ± 0.187 mm and mean fecundity of 44 531 ± 23 940 eggs were similar to previous studies using invasive techniques. Mean S. albus gonadosomatic indices (GSI) for reproductive and non‐reproductive females were 16.16 and 1.26%, respectively, while reproductive and non‐reproductive male GSI were 2.00 and 0.43%, respectively. There was no relationship between hybrid status or capture location and GSI. Mean fecundity was 48.5% higher than hatchery spawn estimates. Fecundity increased as fork length increased but did so more dramatically in the upper river kilometers of the Missouri River. By examining multiple fish over multiple years, the reproductive cycle periodicity for hatchery female S. albus was found to be 2–4 years and river dwelling males 1–4 years. The use of ultrasonic and endoscopic methods in combination was shown to be helpful in tracking individual gonad characteristics over multi‐year reproductive cycles.     

Coypu (<Emphasis Type="Italic">Myocastor coypus</Emphasis>) in a Mediterranean remnant wetland: a pilot study of a yearly cycle with management implications

Following an apparent increase of local population density of coypu (Myocastor coypus) in a Mediterranean remnant wetland, we developed a pilot study aimed to evaluate a specific control program. Inside the study area, we performed three transects per month from August 2008 to July 2009, grouping data in bimonthly periods. The water level in the study area showed a maximum in December–January, significantly decreasing from late spring to summer and significantly increasing from late summer to winter. Sampled individuals mainly occurred in Phragmites reed beds and in rush beds (dominance of Carex sp., Juncus sp. Bolboschoenus sp.). The index of mean relative density of coypu individuals ranged between 1.40 (February–March) and 5.72 (October–November) with an evident increase in late summer–autumn. During this period, mean density of runways was higher in reed beds than in rush beds, with differences tending to significance. In summer, the network of channels in reed beds, locally used for fishery farm, may maintain a water level suitable for the coypu. These results (preference for reed beds and increase of coypu density in late summer–autumn) should be considered when coypu populations are under control program, at least in the Mediterranean region where there is a scarcity of available data.     

Reproductive cycle of the commercially harvested sea urchin (Paracentrotus lividus) along the western coast of Portugal

Harvested populations of the sea urchin (Paracentrotus lividus) from the northwestern (Carreço) and southwestern (Aljezur) coasts of Portugal were surveyed to describe the species reproductive cycle and assess possible relationships with geographical location and seawater temperature. Individuals were sampled monthly to analyze gonad histology, mean gonadal index (GI), and gonadosomatic index (GSI) during 2 consecutive years (November 2010–November 2012). Both populations presented an annual reproductive cycle, with synchronous gonad maturation and gamete release between sexes. Gonad maturation occurred throughout autumn–winter, followed by a single but prolonged spawning season during spring–summer. The duration of the spawning season displayed a latitudinal gradient likely related to the north–south increasing trend in seawater temperature, with the northwestern population (Carreço) exhibiting a shorter spawning period compared to the southwestern population (Aljezur). The timing and duration of the spawning season was compared with several populations throughout the distributional range of P. lividus in the Atlantic Ocean and Mediterranean Sea. In the population from Carreço, the size at first sexual maturity (test diameter = 35.9 mm) was considerably smaller than the minimum conservation reference size (MCRS) of 50 mm test diameter legally established for P. lividus. This study confirms that sustainable exploitation depends on harvesters’ awareness of and compliance with the MCRS and provides useful information for the eventual establishment of a closed season in the harvesting of P. lividus.     

Reproductive biology of caridean decapods from the Weddell Sea

Summary Data on reproductive biology are presented for five benthic caridean shrimps from the high Antarctic (Chorismus antarcticus, Notocrangon antarcticus, Nematocarcinus lanceopes, Lebbeus antarcticus and Eualus kinzeri). The first three species were very common on the Weddell Sea shelf and upper slope, whereas only a few individuals of the other two species were caught-but these did include some ovigerous females. Our measurements include size at first maturity, fecundity (total number and mass of eggs), individual egg mass, egg length, ovary indices, maximum size encountered and documentation of the reproductive cycle in spring and summer. Egg number generally increases with female size, and the largest species (N. lanceopes) also carries the highest number of eggs. The eggs of all high Antarctic species are large, the extreme being L. antarcticus with an egg length of up to 3.3 mm. For C. antarcticus and N. antarcticus, which have wide geographic distributions, a comparison is made with older published and unpublished data from the Subantarctic (South Georgia). High Antarctic representatives of these two species grow to a larger maximum size, attain sexual maturity later in their life cycle, and produce fewer and larger eggs in relation to both carapace length and female mass, than their Subantarctic counterparts.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Seasonal changes in condition, nutrition, gonad maturation and energy content in barbel, Barbus sclateri, inhabiting a fluctuating river

The objective of this study was to investigate the seasonal cycle of condition, nutrition and gonad development, as well as the magnitude of seasonal variations in energy content of somatic and gonad tissues in juveniles, males and females of Sclater's barbel, Barbus sclateri, from the upper Guadalete River (south Spain). The influence of reproductive cycle on somatic changes was also investigated and discussed. Measurements of condition factor (K), somatic index (SI), gonadosomatic index (GSI) and somatic and gonad energy content (J g^-1 dw) were made in individual specimens taken from the Guadalete River monthly for 12 months. This freshwater ecosystem is characterised by strong seasonal fluctuations in water and flow level, temperature and food supply. It was found that in general juveniles, males and females of barbel exhibited a similar condition, nutrition and somatic energy cycle throughout the year, with the highest values in spring and the lowest in summer. Depletion of K, SI and somatic energy storage from spring until mid-summer seems to be associated with high metabolic demands during this period, and in adult fish also with spawning-related activity. The gonad index and energy content of the gonad were the highest in April and the lowest in summer for both sexes. Spawning took place during late spring – early summer, with fish quiescent by mid summer. The energy required for ovarian development (3970 J g^-1 dw) was greater than the one for testes development (2763 J g^-1 dw). Data on gonad energy content indicated a period (March to April) of intense energy accumulation (64% males and 37% females) which was related to the decline in the average somatic energy content in males and females. The somatic energy content was linearly related to K and SI. In the same way, GSI correlated positively with gonad energy. Linear trends were found between somatic parameters (K, SI and energy content) and gonad parameters (GSI and energy content). This revised version was published online in July 2006 with corrections to the Cover Date.     

The contribution of epipelon to total sediment microalgae in a shallow temperate eutrophic loch (Loch Leven, Scotland)

Benthic microalgae are known to perform important ecosystem functions in shallow lakes. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. We tested the hypothesis that the positioning (across a depth gradient of 2–22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. all viable microalgae in the surface sediments) in a shallow eutrophic loch. Total sediment microalgal Chl a concentrations (TS-Chl; range: 5–874 μg Chl a g⁻¹ dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. Epipelic Chl a concentrations (Epi-Chl; range: <0.10–6.0 μg Chl a g⁻¹ dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5–5.5 m) in all seasons except autumn/winter. Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Epi-Chl was associated with intermediate light and habitat disturbance during spring/summer and summer/autumn and varied positively with habitat disturbance, only, in autumn/winter and winter/spring. The epipelon community structure also varied with depth; diatoms dominated shallow water sediments, cyanobacteria dominated deep water sediments, and sediments at sites of intermediate depth returned the highest biovolume estimates and the most diverse communities. This study has strengthened the hypothesis that the structure and biomass of benthic microalgal communities in lakes are regulated by habitat disturbance and water clarity, both of which are expected to respond to climate change and eutrophication. The degree to which these structural responses reflect functional performance requires clarification.     

Life cycle,population dynamics,growth and production of <Emphasis Type="Italic">Abra segmentum</Emphasis> (Mollusca,Bivalvia) at low salinities in a Mediterranean lagoon

Aspects of the biology of Abra segmentum were investigated at low salinities in a Mediterranean coastal lagoon (Monolimni Lagoon, Northern Aegean Sea). Monthly samples were collected during the period from February 1998 to January 1999. Recruitment occurred from mid-spring to early autumn (0.3–5.7 psu) and recruits grew during summer and autumn (1.2–5.7 psu), while a major part vanished during next autumn, displaying a maximum life span of about 20 months. A positive correlation was found between the percentage of individuals having a shell length of ≤3.5 mm and temperature; age group 0 showed a growth rate of 0.97 mm per month, and the largest individual collected had a 19.76 mm shell length. The population density sharply increased during late spring (0.3–1.2 psu); this increase was followed by a decline during summer and, afterwards, a gradual increase up to late autumn. Secondary production calculated by the size–frequency method gave a mean annual density (n) of 3,357 individuals m⁻², a mean annual biomass (B) of 21.98 g DW m⁻², an annual production (P) of 73.72 g DW m⁻² and a P:B ratio of 3.35. A comparison of the present data with available data of A. segmentum populations from higher salinity habitats revealed that this bivalve in the study area showed a life history pattern similar to that of other populations of the species and a comparatively high growth rate, maximum body size, n, B, P and P:B ratio. Our findings suggest that the studied aspects of A. segmentum biology could not be markedly affected by low salinities.     

Reproductive energy investment in corals: scaling with module size

In colonial modular organisms, differences in module size and colony growth patterns among species have the potential to impose varying constraints on reproductive investment. Here, we compare reproductive output among seven morphologically different species of spawning reef corals, and analyse the relationship between reproductive output and module (polyp) size. Reproductive output ranged between 132 and 384 J cm^–2, with lipid constituting the key indicator of energy investment. Lipid decreased by 85–100%, whereas protein and carbohydrate were relatively invariant between pre- and post-spawning tissues in all species, representing 1–15% and <1%, respectively, of the energy investment to reproductive output. The ratio of energy content in reproductive to somatic tissues (gonadosomatic index, GSI) varied among species from 0.20 (Symphyllia recta) to 1.31 (Acropora tenuis), the latter being the highest value reported for any iteroparous marine invertebrate. Surprisingly, small-polyped species (Acropora, Montipora) had 2- to 6-fold higher GSIs than large-polyped ones (Lobophyllia, Symphyllia). Energy equivalents of tissues increased with the 1.50–1.76 power of polyp diameter for somatic tissues and with the 1.42–1.80 power of polyp diameter for reproductive output. In both cases, increases in energy equivalents with polyp diameter were less than the scaling exponent of 3 predicted for an isometric relationship between tissue volume (or mass) and polyp diameter, indicating significant constraints of space, design or physiological energetics with increasing polyp size. We hypothesise that such constraints have played a key role in the evolution of modularity in cnidarians.     

The maturity and breeding season of the bellybarred pipefish, <Emphasis Type="Italic">Hippichthys spicifer</Emphasis>, in Okinawa-jima Island rivers

We investigated the breeding season and size at maturation and described the morphology of newly released Hippichthys spicifer larvae collected from the estuaries of four rivers on northern Okinawa-jima Island, southern Japan. The minimum size of brooding males was 108 mm standard length (SL). The smallest mature female, as estimated from gonadosomatic index (GSI) analysis and histological observations of gonads, was about 100 mm SL. Histological observations showed the gonad of H. spicifer to be a cylindrical tube with a sequential pattern of follicle development and a single germinal ridge. We surmised that the breeding season is year-round, as shown by monthly changes in female GSI, gonad histology, and monthly changes in the occurrence of brooding males. The monthly changes in female GSI and proportions of brooding males were small in winter. The number of eggs in the male brood pouch ranged from 114 to 1,764 (604.4 ± 322.8, mean ± standard deviation; n = 25). The SL of the released larvae was 9.9 mm. All fins except the pectoral fins were formed, the body was elongated, and the developmental stage was similar to that of other Urophori species. The smallest individual present in the mangrove areas of estuaries was 78.0 mm SL.     

Photosynthetic activity of epilithic algal communities in sections of the Torna stream (Hungary) with natural and modified riparian shading

The photosynthetic activity of epilithon grown in the Torna-stream (Hungary) was studied. Samples were collected from natural sections with closed canopy and natural stream bed and from modified sections with trapezoid river bed and shores lacking riparian vegetation. We assumed that human modifications would have a significant impact on epilithic photosynthesis, and therefore expected to observe corresponding changes in photosynthesis–irradiance (P–I) parametric. The laboratory P–I measurements were carried out monthly between April 2008 and September 2009 with epilithon after 3 weeks of in situ colonization. The maximum rate of the photosynthesis (P _max) correlated positively with the chlorophyll a content of the samples. Natural sites had lower biomass and P _max values than modified sites, and the biomass differed substantially in spring and summer, the P _max differed significantly only in spring. Natural sections had higher biomass-specific photosynthesis values than modified sections in all seasons, but the difference was significant only in summer–autumn: in summer the samples from the natural sections had significantly higher photosynthetic efficiency. In spring and summer, the photoadaptation parameters (I _k ) of communities grown in modified sections were substantially higher than in natural sections. Light availability appeared to be the major factor regulating the seasonal photosynthetic activity of epilithic communities.     

Pelagic fish distribution and ontogenetic vertical migration in common mesopelagic species off Lützow-Holm Bay (Indian Ocean sector,Southern Ocean) during austral summer

The horizontal and vertical distributions of fish were examined off Lützow-Holm Bay in the Indian Ocean sector of the Southern Ocean during midnight sun in January 2005. Fish were sampled from six discrete depth layers (0–2,000 m). The most abundant fish in layers from the surface to 200 m were larval stages of Electrona antarctica and Notolepis coatsi. In layers from 200 to 2,000 m, fish assemblages were relatively uniform among all stations and were dominated by E. antarctica (juvenile–subadult), Cyclothone microdon, and Bathylagus antarcticus. Cluster analysis revealed three epipelagic communities related to water temperature and salinity. An ontogenetic habitat shift to deeper layers was apparent for E. antarctica, N. coatsi, and B. antarcticus. Preferences for warm waters were observed in E. antarctica (larvae) and N. coatsi (preflexion to flexion larvae), although they were distributed across a broad range of temperature and salinity in epipelagic zones.     

Influence of age,nutritional status and season on the reproductive system in wild male mink (<Emphasis Type="Italic">Neovison vison</Emphasis>)

The objectives of this study on the reproductive system of wild male mink (Neovison vison) were to determine the influence of age, nutritional status and season on reproductive traits and to produce baseline data that may be used in environmental monitoring programmes. The 117 male mink included in the study were killed by hunters all over Sweden (from latitude 56° N to 67° N) during August to the end of April from 2005 to 2008. The weights of penis, testes, and epididymides were lower, the anogenital distance was shorter and the diameter of seminiferous tubules was less in juveniles compared with adult mink (p < 0.0001–p = 0.0024). There was a positive effect (p < 0.05) of nutritional status on the baculum length and on the anogenital distance, but not on any of the other reproductive traits. The season of sampling influenced the penis, testes and epididymes weights and the diameter of seminiferous tubules (p = 0.008–p < 0.0001). During spring, 86% of the mink had sperm in their epididymides, whereas the corresponding figure for autumn was 3%; in addition, when the average diameter of the tubules reached 138 μm, sperm were present in the epididymides. Sperm morphology was analysed in 29 of the mink, and the percentage of morphologically defect sperm were generally low. Taken together, this study suggests that when assessing male reproductive data in wild mink populations, season must be taken into account as well as whether the individuals are juvenile or adult. In contrast, the nutritional status seems not to be of any major significance.     

Reproductive cycle of Triplophysa stenura (Herzenstein, 1888) (Balitoridae: Nemacheilinae) from the Yarlung Tsangpo River in the Tibetan Plateau,China

The reproductive biology of Triplophysa stenura, an indigenous cyprinid species from the Qushui County in the middle of the Yarlung Tsangpo River in the Tibetan Plateau, China, was investigated by bimonthly sampling from May 2014 to April 2015 using electrofishing. The gonadosomatic index (GSI) of T. stenura differed significantly between males and females over the seasonal trend, indicating that spawning extends from March (shortly after the ice melts) to May, as observed for indigenous species in response to environmental conditions on the cold plateau. Average oocyte diameter was 0.66 ± 0.13 mm, whereby the oocyte diameter distribution exhibited a clear seasonal pattern as the temporal pattern of GSI. Absolute fecundity of T. stenura was 3999 eggs/individual, which increased significantly with the increase in fish size and gonad weight, demonstrating the relationship of an exponential equation between them.     

Gonopore development and sex change in the Antarctic shrimp Chorismus antarcticus (Caridea: Hippolytidae)

Chorismus antarcticus is the only protandrous hermaphrodite decapod species found on the Antarctic continental shelves. The morphological structures of the male and female gonopores were described and used to determine sex and to fix the size range at which the transition from male to female takes place. The parallel occurrence of male and female gonopores was found in all specimens. The presence of open gonopore flaps at both the third and fifth pair of pereiopods in individuals between 9 and 12 mm of carapace length (CL) is discussed as the morphological indicator of sex change. Few females occurred from 9 mm CL onwards, whereas all shrimps larger than 13 mm CL only had open female gonopores. This size corresponds exactly with the CL at which onset of oocyte development and female gonad production starts. Secondary sexual characters, the gonopore structures, seem to allow an accurate prediction of the size range at which hermaphrodite caridean decapods change sex. Received: 23 February 1996/Accepted: 15 July 1996     

Seasonal and inter-individual variation in testosterone levels in badgers <Emphasis Type="Italic">Meles meles</Emphasis>: evidence for the existence of two endocrinological phenotypes

Elevated testosterone levels can lower condition and increase parasites. We analysed testosterone in 84 blood samples of wild European badgers Meles meles collected at regular intervals (winter = mating season; spring = end of mating season; summer = minor mating peak; autumn = reproductive quiescence), and related variation to body condition, subcaudal gland secretion, parasite burden, and bite wounding. All males showed elevated levels in winter and low levels in autumn. In neither season did testosterone correlate with fitness-related parameters. However, two different endocrinological phenotypes existed in spring and summer. Whilst some males lowered their testosterone to levels comparable to autumnal quiescence (Type 1), others maintained elevated levels comparable to those during winter (Type 2). In spring and summer high levels were correlated with lower body condition and increased parasite burden, and Type 2 males tended to suffer higher mortality rates than Type 1. No animals older than 6 years adopted phenotype 2, indicating that males either switch phenotypes with age or that Type 2 results in lower life expectancy, evidencing the costs of male reproduction in badgers.     

Age composition,growth, and reproductive biology of yellow catfish (<Emphasis Type="Italic">Peltobagrus fulvidraco</Emphasis>, Bagridae) in Ce Lake of Hubei Province,Central China

This study presents fundamental information on the reproductive biology, size and age composition, and growth of yellow catfish, Peltobagrus fulvidraco. Sampling was carried out monthly from Ce Lake in Hubei province from January 2004 and December 2004. Overall female/male (F/M) sex ratio was 1.08, not significantly different from expected 1.0 ratio. But sex ratio (F/M) of the spawning group was 1.14:1 with skewness towards females (p < 0.05). The smallest sexually mature male and female massed 20.51 and 16.93 g, and measured 100.1 and 94.3 mm in total length, respectively. Reproductive period for this species is from April to August when GSI is considerably higher than the rest months (p < 0.05). The highest GSI values were measured from May to July for both sexes and this period was presumably the peak spawn. First maturation occurred at age 1⁺ for both sexes. Fish of age 1⁺ and 2⁺ comprised the greatest portion of spawners. The absolute fecundity (AF) of P. fulvidraco ranged from 657 to 11,696 eggs per fish, with mean AF value of 3,570 ± 122 (S.D.) eggs per fish, varying considerably at given length, weight and age. The fecundity relative to total length (F/L) fluctuated from 58 to 680 eggs/mm, with a mean value of 246 ± 34 (SD) eggs/mm, and fecundity relative to total weight (F/W) varied from 29–133 eggs/g, with a mean value of 67 ± 7 (SD) eggs/g. P. fulvidraco is a single spawner, with low fecundity compared with other Bagrid fish.     

Adaptations for the maintenance of water balance by three species of Antarctic mites

Three species of Antarctic mites, Alaskozetes antarcticus, Hydrogamasellus antarcticus and Rhagidia gerlachei, are abundant in the vicinity of Palmer Station, Antarctica. No single mechanism for reducing water stress was shared by all three species. A. antarcticus and R. gerlachei (both ca. 200 μg) are over twice as large as H. antarcticus (ca. 90 μg), but all had similar body water content (67%) and tolerated a loss of up to 35% of their body water before succumbing to dehydration. All imbibed free water and had the capacity to reduce water loss behaviorally by forming clusters. Alaskozetes antarcticus was distinct in that it relied heavily on water conservation (xerophilic classification) that was largely achieved by its thick cuticular armor, a feature shared by all members of this suborder (Oribatida), and abundant cuticular hydrocarbons. In comparison to the other two species, A. antarcticus was coated with 2–3× the amount of cuticular hydrocarbons, had a 20-fold reduction in net transpiration rate, and had a critical transition temperature (CTT) that indicates a pronounced suppression in activation energy (E _a) at temperatures below 25°C. In contrast, H. antarcticus and R. gerlachei lack a CTT, have lower amounts of cuticular hydrocarbons and have low E _as and high net transpiration rates, classifying them as hydrophilic. Only H. antarcticus was capable of utilizing water vapor to replenish its water stores, but it could do so only at relative humidities close to saturation (95–98 %RH). Thus, H. antarcticus and R. gerlachei require wet habitats and low temperature to counter water loss, and replace lost water behaviorally through predation. Compared to mites from the temperate zone, all three Antarctic species had a lower water content, a feature that commonly enhances cold tolerance.     

Effects of body-size and season on digestive organ size and the energy balance of cockles fed with a constant diet of phytoplankton

Seasonal variation in size-dependence of seawater clearance rate, absorption efficiency, oxygen consumption, gill area, length of the crystalline style and dry weight of digestive gland was analyzed in cockles Cerastoderma edule from the Mundaka Estuary, Spain. Experimental determinations were performed monthly (from July 1998 to November 1999) in cockles being fed with Tetraselmis suecica (organic content: 87.84 ± 1.95%) at a concentration of 3 mm³/l for 3 days. Analysis of covariance reveals no seasonal differences in both size-dependence of seawater clearance rate and oxygen consumption, which were found to scale to dry body weight with mass-exponents of 0.56 and 0.62, respectively. No significant correlation was found between absorption efficiency and body weight. Mass-exponents for gill area, dry weight of the digestive gland and length of the crystalline style remained constant among seasons showing values of 0.62, 0.34 and 0.82, respectively. Seasonal trends for every physiological determination were calculated for a standard size (200 mg) cockle: standardized clearance rates and oxygen consumptions followed a similar trend with minimum values in winter (≈0.5 l/h and ≈100 μl O₂/h, respectively) and maximum values during spring–summer (≈1.7 l/h and ≈250 μl O₂/h, respectively), whereas absorption efficiency and food throughput time showed both the opposite pattern with highest values corresponding to winter months (≈50–60% and ≈5–6 h, respectively), and lowest (≈30% and ≈3–4 h, respectively) to summer–autumn. Scope for growth exhibited minimum values in winter followed by a rapid increase along the winter–spring transition, maximum values being attained in spring (May) and summer (July). Exponential decline of seasonal values of absorption efficiency associated to rising ingestion rates of organic matter presented an asymptotic minimum at 0.35. Absorption efficiency was positively related to food throughput time, whereas the latter fell to a minimum of 3.548 h with increasing food intake. So, maintenance of throughput time—and consequently absorption efficiency—along with enhanced filtering activity provided cockles with higher absorption rates improving scopes for growth registers during spring and summer. These dynamics might be explained as the consequence of the seasonal digestive adjustments in cockles, which, in fact, were found to increase the size of the digestive organs during that period.