Robert Black

PATTERN BENEATH THE CHAOS: THE EFFECT OF RECRUITMENT ON GENETIC PATCHINESS IN AN INTERTIDAL LIMPET

Gene exchange among widely separated areas characterizes many marine organisms with planktonic dispersal. From an evolutionary perspective, the essential feature of such dispersal is that recruits to local populations come from somewhere else. Thus, localized adaptation is not accumulated over time, and changes in the genetic composition of adults reflect single-generation effects of selection and recruitment. The most obvious effect of planktonic dispersal is the reduction of geographic variation in genetic composition (e.g., Scheltema, 1971, 1978), and low variances of allelic frequencies have been found to be associated with planktonic dispersal (e.g., Berger, 1973; Levinton and Suchanek, 1978; Winans,

Chaotic genetic patchiness in an intertidal limpet, Siphonaria sp.

Variation of 4 polymorphic enzymes was studied for 2 yr (1978 and 1979) in an undescribed species of Siphonaria, a pulmonate limpet, from a rocky shore at Rottnest Island, Western Australia. Depending on the locus, significant genetic differences were found among sites along 50 m of shore, between high and low portions of the shore within sites, between adults and recruits, and between recruits in the 2 yr. This genetic heterogeneity does not follow a simple, consistent pattern, but forms a shifting, ephemeral genetic patchiness best described as chaotic. This patchiness may result from temporal variation of numbers and genotypes of recruits, which leads to the proposal that planktonic dispersal, although causing uniformity on a large scale, can give rise to fine-scale genetic patchiness.

Effects of recruitment on genetic patchiness in the urchinEchinometra mathaei in Western Australia

Enzyme polymorphisms in the sea urchinEchinometra mathaei were examined to test the relative influences of population turnover and patchiness in recruitment on genetic heterogeneity. We found that the total variance in allelic frequency among three populations separated by approximately 4 km at Rottnest Island, Western Australia (collected in February 1985) is as large as that among five additional samples collected over a distance of 1 300 km along the Western Australian coast in August 1987. This suggests that the forces causing genetic differentiation act on a local scale and occur in a single generation. A comparison of sites with different histories of recruitment indicates that the observed genetic differences among age groups are the result of prerecruitment effects, and that differences among sites reflect their individual histories of recruitment.

The Wahlund effect and the geographical scale of variation in the intertidal limpet Siphonaria sp.

Samples of Siphonaria sp. were collected between 1978 and 1982 from sites covering its known geographic range, from Kalbarri, Western Australia to Port Robe, South Australia. Geographic variation of 7 polymorphic enzymes was examined in this intertidal pulmonate limpet, and was found to be consistently small, indicating a large-scale influence of gene flow due to planktonic dispersal. Despite this large-scale uniformity, there is fine-scale genetic patchiness, which is repeated, rather than accumulated, on the larger scale. Throughout its geographic range, Siphonaria sp. shows deficits of heterozygotes for all 7 loci. The consistency among loci indicates that the causes of the deficits are populational, rather than locus-specific. A Wahlund effect, the departure from Hardy-Weinberg equilibrium due to mixing of individuals from groups with different allelic frequencies, is the simplest explanation of such deficits. The limited geographic variation of allelic frequencies, however, is grossly inadequate to produce these deficits through a Wahlund effect. Similarly, temporal variation in allelic frequencies in recruits does not explain the deficits. The largest contributor to a Wahlund effect appears to be binomial sampling variance among small local breeding groups. Thus, mixing of larvae on a scale of metres, rather than among geographical areas, apparently produces the deficits of heterozygotes.

Asexual viviparity and population genetics of Actinia tenebrosa

The intertidal anemone Actinia tenebrosa is viviparous. An electrophoretic study of 3 polymorphic enzymes in Western Australian populations has confirmed genetic identity of adults and their brood young, indicating asexual reproduction. The population effects of this clonal reproduction are seen as linkage disequilibrium and departures from Hardy-Weinberg equilibrium within populations, and large differences between populations. The data also suggest occasional sexual reproduction, and the likelihood of a mixture of reproductive modes in this species.

Neighbourhood size and the importance of barriers to gene flow in an intertidal snail

The littorine gastropod Bembicium vittatum has direct development from benthic egg masses, and shows high levels of genetic subdivision in the Houtman Abrolhos Islands, Western Australia. Parallel studies of dispersal and the scale of variation of allozyme frequencies were used to estimate the extent of the neighbourhood, the area of complete genetic mixing. Estimates based on the variance of dispersal distances were 58 to 69 m, very similar to the estimate of 62 m from the median dispersal distance. The allozymes indicated mixing over greater distances, up to 150 to 300 m. Combined with estimates of population density at two sites, these values suggest neighbourhood sizes of 990 to 37 200 adults. However, estimation of effective population size based on a model of isolation by distance gave very low values of 22 to 38, raising doubts about the applicability of this method. Nevertheless, there was an overall pattern of isolation by distance along about 11 km of nearly continuous suitable habitat, with smooth clinal variation in allelic frequencies over distances of 2 to 6 km at individual loci. In contrast, there was no pattern of isolation by distance, and allelic frequencies showed less smooth patterns, along a discontinuous series of islands over the same distance. Within this discontinuous series, genetic subdivision, as measured by GST, was twice as large at distances of 1 km as for populations on continuous shores, demonstrating the importance of even relatively short water gaps as barriers to gene flow in this directly developing intertidal species.

Genetic subdivision of the intertidal snail Bembicium vittatum (Gastropoda: Littorinidae) varies with habitat in the Houtman Abrolhos Islands, Western Australia

The littorine gastropod Bembicium vittatum has benthic egg masses and is thought to have little or no period of planktonic development. A study of allozyme polymorphisms in populations from the Houtman Abrolhos Islands, Western Australia, permitted examination of genetic subdivision within and among island groups, and among populations from contrasting habitats which occur throughout the Abrolhos archipelago. Across all populations, the average FST for five polymorphic loci was 0.163, which is very large for a marine species but similar to values found for other gastropods that lack planktonic development. The level of genetic subdivision varied dramatically with habitat. Over similar distances, the average FST among lake populations was 0.277, compared with 0.138 among populations from protected lagoonal shores, and only 0.022 among populations from relatively exposed shores. This variation emphasizes the need to consider local conditions when attempting to determine the genetic structure of a species.

Preliminary evidence for progressive sestonic food depletion in incoming tide over a broad tidal sand flat

Sequential sampling of a water mass flooding over a broad intertidal sand flat at Shark Bay, Western Australia demonstrated a pattern of progressively declining chlorophyll concentrations. After 3 h 25 min of movement up over the intertidal flat, the already low concentration (0·06 μg l−1) of chlorophyll a had declined by 25%. If representative of average conditions, this observed rate of sestonic food depletion, when combined with reductions in feeding time during low tide, would predict a 59% lower growth for mid-intertidal vs. subtidal suspension-feeding bivalves. Previously published reductions in bivalve growth with increased elevation at this site are not much larger (57, 61, 86 and 92%) than predicted, implying that food depletion in incoming tides may help constrain growth of suspension feeders on broad intertidal sand flats.

The functional significance of the relative size of Aristotle's lantern in the sea urchin Echinometramathaei (de Blainville)☆

Abstract Echinometra mathaei(de Blainville) from different locations at Rottnest Island, Western Australia, had Aristotles lanterns of different sizes relative to the size of their tests. Urchins with relatively larger lanterns also had heavier tests and muscles of the lantern relative to the size of the test, and smaller relative weights of spines and gonads. These patterns of variation indicate the possibility of a trade-off in allocation of resources between the relative size of the lantern and the amount of gonads and spines. To provide direct experimental evidence of the feeding performance by urchins with relatively small and relatively large lanterns (jaws), we transplanted urchins into the same microhabitat. Because urchins with relatively larger lanterns had more material in their guts, and removed benthic algae from larger areas than urchins with relatively smaller lanterns, these experiments clearly demonstrated the functional significance of the relative size of Aristotles lantern in E. mathaei.

Responses of growth to elevation fail to explain vertical zonation of suspension-feeding bivalves on a tidal flat

SummaryFive species of suspension-feeding bivalves were transplanted to each of two elevations on a tidal flat at Shark Bay, Western Australia, at six replicate locations spaced at 1-km intervals along the shore. Four species exhibited greatly reduced growth at the higher elevation, while the fifth species did not respond to elevation. The magnitude of the % reductions in growth with increased elevation was 2–3 times the % reduction in average daily submergence, confirming a previous suggestion that differences in feeding time alone are insufficient to explain completely the reduced growth of suspension-feeding bivalves at higher tidal elevatios. All four species that responded showed the same pattern of higher growth lower on the shore, even though transect sampling showed that two were normally abundant only high on the shore while the other tow were naturally restricted to elevations low on the shore. Consequently, knowledge of how individual growth within species varies with tidal elevation fails to explain observed zonation patterns with elevation in this guild of suspension-feeding bivalves. The paradoxical distribution pattern of those two species that were rare at the lower tidal elevations, where they actually grew more rapidly, implies that some biological agent(s) of mortality not physiological stress set(s) their lower distributional limit on the shore. Biological rather than physical factors commonly, although not universally, set lower distributional limits of invertebrates in rocky intertidal zones, but this study provides the first experimental data to explore this concept in marine soft sediments.