Sexual reproduction and external brooding by the Caribbean gorgonianBriareum asbestinum

The reproductive biology of the gonochoricBriareum asbestinum (Pallas), a common Caribbean gorgonian, was studied in the San Blas Islands (Panama), La Paguera (Puerto Rico) and the Bahamas. Of 721 colonies examined at the height of the reproductive season, during the summers of 1985 to 1988, all were either male or female. No hermaphroditic colonies or polyps were observed. The sexratio of Panama populations was significantly biased in favor of males (2.1:1,n=454) over all four years of the study. A significant male bias was also found in populations sampled in Puerto Rico (2.3:1,n=143) and in the Bahamas (2.5:1,n=124). The consistency of this geographically widespread skewed sex-ratio is unique among octocorals and scleractinians. Ooyctes appear annually in September and reach maximum diameter (600 to 900µm) in June and July of the following year. The 3 to 5 mo developmental cycle of spermaries is shorter than that of females and begins in March. Spawning in males is synchronous and occurs following the full moons of June and July. Fertilization is internal. Embryos are released from polyps very early in development, often whilst still in the process of cleaving. Embryos remain attached to the outside of the colony for 3 to 5 d, many becoming entrapped in mucous sheets secreted by the colonies. Larvae are negatively buoyant and settle rapidly once displaced from the colony. Males become reproductive at smaller branch sizes than females. Reproductive effort (gonads polyp^–1 and total gonad volume polyp^–1) for both males and females increased with increasing branch size and in polyps away from the tip and base of the branch.Please address all correspondence and requests for reprints to Dr Brazeau at his present address: Department of Biology, University of Houston, Houston, Texas 77204-5513, USA     

Feeding preferences of periwinkles among four species ofFucus

A widely accepted view of intertidal community organizatiton in the NW Atlantic proposes that fucoid vegetation is maintained by the actions of predators which remove species competitively superior toFucus species. Herbivory is an important component of these predatory interactions, but has been studied largely with reference to the interaction betweenF. vesiculosus andLittorina littorea. There are many species of fucoids and herbivorous invertebrates on the shores of the NW Atlantic and this paper reports field and laboratory experiments performed in 1987 (in or near Halifax, Canada) on the effects of grazing by three species ofLittorina on adults and juveniles of four species ofFucus. In laboratory experiments, portions ofFucus species were presented singly (no choice) or together (multiple choice) to single species ofLittorina. AdultF. distichus was grazed to only a small extent.F. evanescens andF. vesiculosus adult tissues were heavily grazed in most laboratory experiments.F. spiralis adults were heavily grazed in no choice experiments, but were grazed only slightly in multiple choice tests. When adult fucoid tissues were exposed to a mixed suite of grazers in the field, onlyF. vesiculosus was grazed heavily. Tests done on adult tissues showed clearly thatFucus species are highly variable in their vunerability to grazers. Moreover, we found variability among periwinkle species in grazing rates on adultFucus. We also found a great disparity in the feeding rates of different periwinkle species on juveniles ofFucus. Therefore, it is not possible to extrapolate from a single periwinkle species/single fucoid species interaction when attempting to identify vegetation structuring processes.     

Assessing Risks to Biodiversity from Future Landscape Change

We examined the impacts of possible future land development patterns on the biodiversity of a landscape. Our landscape data included a remote sensing derived map of the current habitat of the study area and six maps of future habitat distributions resulting from different land development scenarios. Our species data included lists of all bird, mammal, reptile, and amphibian species in the study area, their habitat associations, and area requirements for each. We estimated the area requirements using home ranges, sampled population densities, or genetic area requirements that incorporate dispersal distances. Our measures of biodiversity were species richness and habitat abundance. We calculated habitat abundance in two ways. First, we computed the total habitat area for each species in each landscape. Second, we calculated the number of habitat units for each species in each landscape by dividing the size of each habitat patch in the landscape by the area requirement and summing over all patches. Species richness was based on presence of habitat. Species became extinct in the landscape if they had no habitat area or no habitat units, respectively. We then computed ratios of habitat abundance in each future landscape to habitat abundance in the present for each species. We also computed the ratio of future to present species richness. We then calculated summary statistics across all species. Species richness changed little from present to future. There were distinctly greater risks to habitat abundance in landscapes that extrapolated from present trends or zoning patterns, however, as opposed to landscapes in which land development activities followed more constrained patterns. These results were stable when tested using Monte Carlo simulations and sensitivity tests on the area requirements. We conclude that this methodology can begin to discriminate the effects of potential changes in land development on vertebrate biodiversity.     

Sperm competition or sperm selection: no evidence for female influence over paternity in yellow dung flies Scatophaga stercoraria

Recent studies of non-random paternity have suggested that sperm selection by females may influence male fertilization success. Here we argue that the problems originally encountered in partitioning variation in non-random mating between male competition and female choice are even more pertinent to interpreting patterns of non-random paternity because of intense sperm competition between males. We describe an experiment with the yellow dung fly, Scatophaga stercoraria, designed to partition variance in the proportion of offspring sired by the second male, P ₂, between males and females, and to control for sperm competition. Large males were shown to have a higher P ₂ than small males but P ₂ was independent of the size of the female’s first mate. This result might suggest an absolute female preference for large males via sperm selection. However, large males have a higher constant rate of sperm transfer and displacement. After controlling for this effect of sperm competition, large males did not achieve higher paternity than small males. We argue that a knowledge of the mechanism of sperm competition is essential so that male effects can be controlled before conclusions are made regarding the influence of sperm selection by females in generating non-random paternity. Received: 4 April 1995 / Accepted after revision: 17 October 1995     

Patterns of selective predation by juvenile,benthivorous fish on estuarine macrofauna

Benthic feeding on macrofauna was studied in juveniles of the sparids Lithognathus lithognathus and Rhabdosargus holubi in the upper reaches of the Gamtoos Estuary, South Africa. Fish and benthic macrofauna were sampled simultaneously, and the selection of invertebrate prey assessed. Both fish species strongly selected for corophioid amphipods and consumed other benthic taxa in low numbers. R. holubi also exploited aquatic autotrophs, while L. lithognathus had a narrow prey-spectrum, feeding almost exclusively on the tube-dwelling amphipod Grandidierella lignorum. G. lignorum was the most abundant prey species, both in the benthos and the fish's diet. This species also showed prominent behavioural differences between the sexes; males were markedly more active on the sediment surface than females, who rarely left their tubes during the day. Males switched from an infaunal to epifaunal microhabitat in search of receptive females, concurrently increasing their exposure to fish predators. Consequently, L. lithognathus selected significantly more males than female amphipods, causing a marked bias towards females in the sex ratio and age-structure of the amphipod population. Juvenile amphipods were less preyed upon, presumably as a result of lower prey-detection or capture efficiency by the predators. Accepting current notions about predation as an important structuring element for benthic communities, our data also stress the prominence of size-and sex-selective predation in structuring individual prey populations.     

Genetic differentiation in three Sepia species (Mollusca: Cephalopoda) from Galician waters (north-west Iberian Peninsula)

The systematics of the genus Sepia is not yet clear. Morphological evidence has led to S. officinalis Linnaeus, 1758 being considered as belonging to the subgenus Sepia sensu stricto, and S. orbignyana Férussac, 1826 and S. elegans Blainville, 1827 as belonging to the subgenus Rhombosepion. Samples of 30 individuals of S. officinalis and S. orbignyana from both sides of an oceanographic boundary off the north-west Iberian Peninsula, and a sample of S. elegans from the northern side, were collected in 1993–1994. Allozyme electrophoresis for 32 presumptive loci revealed low levels of genetic variability for the three Sepia species (mean expected heterozygosity estimates were <0.052). No significant differences in allozyme frequencies were detected among populations of either S. officinalis or S. orbignyana. The genetic identities (I) of S. officinalis and S. orbignyana (I=0.12) and of S. elegans (I=0.13) were significantly different from that of S. orbignyana and S. elegans (I=0.49). The former are typical of values for confamilial genera, and a new generic status is proposed for the latter two species, which become Rhombosepion orbignyana (Férussac, 1826) and R. elegans (Blainville, 1827).     

Quantitative genetic analysis of growth in larval scallops (Placopecten magellanicus)

We estimated the broad-sense heritability of larval size in 20 full-sib families of the giant scallop, Placopecten magellanicus (Gmelin, 1791) grown in laboratory culture in August and September 1991. The goal was to compare scallops with other bivalves which have been shown to have significant heritabilities for larval growth. Secondly, we estimated the lipid content of occytes from female parents, since this is hypothesized to affect larval growth and survival. Finally, we estimated the among-family variation in mortality from 4 to 21 d as a test of possible genetic variation for viability among larval scallops. Significant genetic variation (h²=1.10 to 1.24) was estimated for larval shell length at 4, 14, and 21 d. There was a significant correlation (r=0.66) between larval size at 4 d and lipid content of oocytes, but only when two females with high levels of lipid oocyte^–1 were excluded as outliers. There was no correlation between larval size at 14 and 21 d and lipid content of oocytes. Mortality among families from 4 to 21 d was high (69 to 97%), and was significantly different among families. These results indicate that there is significant heritability for larval growth which is largely independent of the lipid content of the oocytes. A high heritability for larval growth may indicate that this trait is only weakly correlated with fitness.     

Exploitative compensation by subordinate age-sex classes of migrant rufous hummingbirds

Summary The three age-sex classes of rufous hummingbirds (Selasphorus rufus) that directly interact on southward migratory stopovers in our California study system differ in territorial ability and resource use. Immature males are behaviorally dominant to adult and immature females and defend the richest territories. Here, we test the hypothesis that the territorially subordinate age-sex classes compensate exploitatively for their exclusion from rich resources. Our results show that females were able to accumulate energy stores at rates comparable to males despite their subordinate territorial status. Territorial females gained body mass at the same rate and in the same pattern as males, and resumed migration at the same body masses. Moreover, during periods when birds were nonterritorial and used dispersed resources, adult and immature females maintained or gained body mass, whereas immature males lost mass. We suggest that females may be energetically compensated by (1) lower costs of flight incurred during foraging and defense, resulting from their lower wing disc loading, and (2) greater success at robbing nectar from rich male territories, resulting from duller coloration (immature females), experience (adult females), and, possibly, hormonal differences. In the future, experiments will be necessary to distinguish the various hypotheses about the mechanisms involved in compensation. Correspondence to: F.L. Carpenter     

Sexual reproduction in the corallimorpharian sea anemone Corynactis californica in a central California kelp forest

The seasonal cycle of sexual reproduction in the corallimorpharian sea anemone Corynactis californica (Calgren, 1936) was studied for 18 consecutive months (July 1987 to December 1988) at a subtidal area in the Hopkins Marine Refuge (HMLR), Pacific Grove, Monterey Bay, California. Samples were collected, histological sections were prepared, and gametocytes were examined and measured. C. californica grows by multiple or longitudianl fission to form single-sexed clones of various color and size. In female clones, oocytes appeared in late August and early September; they increased in size steadily in fall, and peaked in early December in both years. In male clones, spermatogenesis was synchronous with the female oogenic cycle, and motile sperm were observed in most testes in early December. Spawned gonads were found in both sexes from late November to early December. Spawning was induced in the laboratory in early December, and external fertilization was followed by development of free-swimming larvae. Gametogenesis and spawning are correlated with seasonal increases in seawater temperature and phytoplankton abundance, and we discuss and compare timing of sexual reproduction in this corallimorpharian to those in several actiniarian and scleractinian species.