Genetic variation in marine teleosts: High variability in habitat specialists and low variability in habitat generalists

Genetic variation was reviewed in 106 species of marine teleosts. Two heterozygosity estimates were used, one including all protein and enzyme loci and a second excluding the non-enzymatic protein loci. Mean heterozygosities are 0.055±0.036 based on all loci in 106 species and 0.060+0.038 based on enzymatic loci in 89 species. A significant negative correlation was noted between heterozygosity and the proportion of general protein loci included in the estimate. A comparison was made of heterozygosities among taxonomic orders and families, life zones, reproductive mode, geographical range and size. High levels of genetic variation are found in Clupeiformes, Atheriniformes, Pleuronectiformes, temperate pelagic, tropical, intertidal-sublittoral and wide-range species. Low levels of genetic variation are found in Gadiformes, Scorpaeniformes, temperate demersal, polar, and narrowrange species. The most striking differences in heterozygosities are between temperate demersal flatfishes and temperate demersal round fishes. It is suggested that much of the data can be explained by a habitat specialist-generalist model, with high heterozygosities in specialists and low heterozygosities in generalists, but that this is only one of a mosaic of factors which influence genetic variation.     

Variation in adult annual survival probability and remigration intervals of sea turtles

We analyzed a large dataset to quantify adult annual survival probability and remigration intervals for the Tortuguero, Costa Rica green turtle population. Annual survival probability was estimated at 0.85 (95% CI 0.75–0.92) using a recovery model and at 0.85 (95% CI 0.83–0.87) using an open robust design model. The two most common modes of remigration are 2 and 3 years. Annual survival probability is lower and remigration intervals are shorter than for other green turtle populations. Explanations for short remigration intervals include reproductive compensation due to historic population declines, availability of better quality food items, favorable environmental conditions, and short distance to the main foraging grounds. Variation in survival and remigration intervals have profound consequences for management and life history evolution. The short remigration intervals of Tortuguero green turtles partly offset mortality caused by turtle fishing in Nicaragua and mean that low juvenile survival represents a more urgent threat to the population than low adult survival. Low adult survival probability could result in selective pressure for earlier age at maturity.     

Light regulation of nitrate reductase in Ulya fenestrata (Chlorophyceae)

Observations were made on the behavior of nitrate reductase activity in the green alga Ulva fenestrata under controlled light:dark regimes. The activity of nitrate reductase (NR) was examined in response to normal seasonal photoperiods as well as in response to shortened or extended periods of darkness. NR activity exhibits a light-dependent diurnal rhythm under both normal summer and winter photoperiods, with a maximum in the early morning (2 to 2.5 h after the start of illumination). This peak of activity is followed by a lower steady-state level of activity which is sustained throughout the light period. There is a sustained minimal level of activity in darkness. The morning peak in activity is always observed as long as tissue is illuminated, irrespective of the previous light or dark treatments. As such, it appears that nitrate reductase activity in U. fenestrata is under circadian control. There is no major difference in the NR activity pattern between summer and winter plants, except that the peak activity values in winter plants are consistently much higher (5 times) than in summer plants. The study also suggests that illumination prior to the normal start of photoperiod triggers a different set of regulatory mechanisms, indicating that the physiological state of plants is important in dictating the NR activity response to illumination.Please address all correpondence and requests for reprints to G. J. Smith at his present address (Hopkins Marine Station)     

Benthic invertebrate larval response to substrate characteristics at settlement: Shelter preferences of the American lobsterHomarus americanus

The factors controlling shelter selection by the first post-larval stage of the American lobsterHomarus americanus were studied in the laboratory in 1988 using an experimental approach. The relative importance of shelter size, light level within shelters, number of shelter entrances, and presence of algal-film cover in and around them were investigated. Post-larvae showed a strong preference for opaque versus transparent shelters. No difference in post-larval selectivity was observed either for shelters of different widths or heights or between shelters with different numbers of entrances. Post-larvae showed no preference for microhabitats colonized by microorganisms compared to others without colonization. The intensity of the response measured suggests a possible hierarchical organization of the different cues involved in shelter selection, the light level within the shelter apparently being the most important. Simple linear regression was used to assess the effect of shelter area on the number of post-larvae occupying a shelter. Up to four post-larvae were observed simultaneously in a shelter of 40 cm² basal area, indicating that they can cohabit at settlement. The minimum shelter area necessary to the first post-larval stage was estimated to be about 3.25 cm². The results are discussed with reference to recent hypotheses concerning the mechanisms controlling larval settlement in other benthic species with relatively weak mobility during the larval and the post-larval phases.Contribution to the programmes of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec) and of the Institut Maurice-Lamontagne     

Decomposition and microbial dynamics for standing,naturally positioned leaves of the salt-marsh grass Spartina alterniflora

Decomposition of leaves of smooth cordgrass (Spartina alterniflora Loisel.) was monitored for two cohorts of leaves from September 1984 to May 1985 (autumn and winterspring) at Sapelo Island (31°23 N; 81°17 W). The leaves were tagged in plance at the ligule, rather than cut and placed in litterbags. Dead leaves were not abscised from shoots. Loss of organic mass from the attached leaves was at least 60 to 68% of the orginal values. Fungal mass, as measured by an enzyme-linked immunosorbent assay, formed > 98% of the microbial standing crops in two of three autumn samples, and in all samples for the colder, drier, winterspring cohort. Fungal mass was probably mostly in the form of the mycelium and pseudothecia of an ascomycete, Phaeosphaeria typharum (Desm.) Holm. Fungal dominance of microbial standing crops declined when autumn leaves bent downward and acquired a large sediment content (ash=35% of dry matter); the bacterial crop then rose to 7% of the total microbial crop. Microphotoautotrophic mass was always measurable, but was never more than 2% of the microbial crop. Carbon-dioxide fixation was much lower than carbon-dioxide release, and a substantial portion of the fixation may have been anaplerotic fungal fixation. Threeto 8 wk net fungal productivity (average per day) was much greater (16 to 26 times) than measured instantaneous bacterial productivity (extrapolated to per-day values) early in each decay period. Fungal productivity was negative late in the decay period. Fungal productivity was negative late in the decay period for autumn leaves, and was approximately equal to bacterial productivity late for winter-spring leaves. Net nitrogen immobilization was observed only late in the decay period for autumn leaves, implying that nearly all dead-leaf nitrogen was scavenged into fungal mass after the first sampling interval. Flux estimates for dead-leaf carbon indicated a flow of 11–15% of the original to fungal mass, 2% to bacterial mass, 15–21% to carbon dioxide, 10–12% to dissolved leachage, and 34–36% to small particles; 32–39% remained attached as shreds at the end of the study periods. Salt-marsh periwinkles (Littorina irrorata Say) appeared to be the major shredders of dead leaves and conveyors of leaf-particulate material to the marsh sediment, at least in those parts of the marsh where the snails are densely concentrated (usually areas of short- and intermediateheight cordgrass shoots).     

The breeding cycles of two species of grapsid crabs (Crustacea: Decapoda) from the North Coast of Kyushu,Japan

The incidence of ovigerous females in populations of two grapsid crabs, Hemigrapsus penicillatus (de Haan, 1835) and Sesarma (Parasesarma) pictum (de Haan, 1835) were followed from August 1975 to November 1976. H. penicillatus, which inhabits the lower intertidal region near the mouth of Tatara-Umi Estuary, breeds from March to November. S. pictum, which inhibits crevices and abandoned holes of other species and is abundant at and above the upper intertidal region of the estuary, breeds from May to September. H. penicillatus matures when the female reaches 6 to 7 mm carapace width, whereas S. pictum becomes mature when the carapace width is 12 to 13 mm, although the maximum size attained by females of both species is almost the same. H. penicillatus produces 5 to 6 broods, S. pictum 2 to 3 broods, during a breeding season. The size of a newly laid egg of H. penicillatus is smaller than that of S. pictum. The number of eggs produced by a female H. penicillatus is greater than that of S. pictum during a breeding season. In both species the peak of the breeding season is during summer, at which time the ovarian activity is also apparently accelerated. The major environmental factor which controls the breeding in these crabs appears to be temperature. H. penicillatus is submerged at every high tide, and is relatively inactive from late November to February when the ambient water and air temperatures are rather low. S. pictum is dormant in crevices or understones of the splash zone from November to March. The length of the breeding season of these crabs appears to be inversely proportional to the period of their winter dormancy.     

Individual and combined toxicity of some petroleum aromatics to the marine amphipod Elasmopus pectenicrus

Toxicity of 4 components of petroleum oils to the marine amphipod Elasmopus pectenicrus (Bate) has been assessed. Two ephemeral aromatic hydrocarbons, naphthalene (A) and 1, 2, 4-trimethylbenzene (B) were more toxic than two persistent aromatics, o-cresol (C) and o-toluidine (D). The acute toxicity concentrations obtained for individual aromatic compounds were always greater than the actual concentrations found in the water-soluble fractions (WSF) of fuel oils. Results from mixtures of 2 or more components indicated that the LC₅₀ levels were primarily determined by the more toxic substances, A and B. Naphthalene and 1, 2, 4-trimethylbenzene became more toxic to the E. pectenicrus when present in a mixture of more than 2 components, and the toxicity increased with increasing numbers of components present. Synergistic effects, therefore, possibly occur in the whole WSF. No antagonistic effects were observed among the 4 petroleum aromatics.University of Texas, Marine Science Institute Contribution No. 290     

Survival, growth and reproduction of the salt-marsh amphipod Uhlorchestia spartinophila reared on natural diets of senescent and dead Spartina alterniflora leaves

The talitrid amphipod Uhlorchestia spartinophila lives in close association with standing-dead leaves of the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the Atlantic coast of North America. This study probed the strength of the trophic link between the amphipod population and the decomposition process in this detrital-based ecosystem. We measured survival, growth and reproductive output in groups of amphipods reared for 6 wk on five diets derived from sheath and blade portions of S. alterniflora leaves just prior to (senescent) and during (dead) decomposition. In unfed treatments, the daily specific mortality rate was 0.391 and starved amphipods survived no longer than 11 d. Among the fed treatments, a diet of senescent sheaths resulted in the lowest survival (20%) and yielded no offspring. Groups fed senescent blades, dead sheaths, dead blades and unwashed dead sheaths had survival rates of 56 to 84% and produced 5.0 to 12.5 offspring replicate⁻¹. Sex ratio usually favored females, but approached unity in treatments with high overall survival, suggesting that quality of available food resources may influence sex ratio in this species. Mean specific growth rates (mm mm⁻¹ d⁻¹) ranged from 0.013 to 0.016, and matched previous estimates of growth from field populations. Overall ecological performance (survival + growth + reproduction) was similar for all food treatments, except senescent sheaths, which yielded a final mean (±SD) dry biomass (0.4 ± 0.42 mg replicate⁻¹) of amphipods significantly lower than that of other diets (1.7 ± 0.81 to 2.6 ± 0.69 mg replicate⁻¹). Natural diets derived from decomposing cordgrass leaves can fulfill the nutritional requirements of U. spartinophila populations, but variation in initial amounts of living fungal biomass among the five experimental diets only partially explained the responses of amphipods in our experiment. Structural characteristics and variation in rates of fungal occupation within different portions of cordgrass leaves may affect the amphipod's ability to access plant production made available by decomposers. Received: 12 December 1996 / Accepted: 18 December 1996     

Studies on the carbohydrases in the digestive tract of the milkfish chanos chanos

Crude extracts from various regions of the digestive tract of pond grown milkfish were tested forttheir ability to catalyze the hydrolysis of various carbohydrates. The most active carbohydrases were those involved in the hydrolysis of -glucosidic bonds. Maltose, trehalose, dextrin, starch and glycogen were rapidly hydrolyzed in the presence of crude extracts from the intestines and the pyloric caeca. High amylase activity was observed in extracts from the intestines, pancreas, pyloric caeca and liver. The intestinal amylase had optimum activity at pH 6.2 and at a temperature of about 50°C. It was active at a chloride concentration of 10 to 40 ppt. The amylase activity in the intestines consistently peaked daily at about noon when the milkfish gut was full. In contrast, enzyme activity was significantly lower at 0030 hrs when the gut was empty. These results are consistent with earlier observations that the milkfish is a daytime feeder and suggest further that intestinal amylase secretion is in phase with the feeding activity of the milkfish. Although the fishes used in this study fed mostly on the naturally occurring algae in the ponds, no cellulase activity was detected in any region of the digestive tract. Less active carbohydrases that were detected include a -glucosidase and -galactosidase, both of which were of limited substrate specificity.SEAFDEC Contribution No. 71