Short-lived Larvae Research Articles

Outcrossing in field populations of two species of self-fertile ascidians

Self-compatible gametes and short-lived larvae in the sessile solitary ascidians Corella inflata Huntsman and Chelyosoma productum Stimpson suggest that field populations might be characterized by limited dispersal and self-fertilization. Because C. inflata in the laboratory spawns gametes into its atrium and retains offspring until they are competent to settle, it was expected to show an almost complete lack of outcrossing in the field. C. productum, in the laboratory, spawns streams of gametes, sometimes eggs and sperm simultaneously, into the water and is self-compatible. Embryos develop for an additional day in the plankton before hatching. Thus, some setting was also expected for C. productum, though it appears to have a far greater potential for gene flow with both outcrossing and more extended larval dispersal possible. Genetic population structure in these species was examined using protein electrophoresis to determine frequencies of heterozygotes at the polymorphic locus glucosephosphate isomerase (GPI). Heterozygotes were found in both species indicating that outcrossing occurs. At three locations, C. inflata genotype frequencies did not differ significantly from Hardy-Weinberg expected frequencies and indicated from 41 to 100% outcrossing. Another measure of breeding system (S), calculated from the same data, gave a pooled location outcrossing estimate of 82%. Also, the three locations did not differ significantly from each other in allele frequencies, indicating a lack of genetic differentiation on a scale ≈ 100 km. Brooded embryos and short-lived larvae do not appear to be limiting gene flow on this scale in this species.

Why Life Histories Evolve Differently in the Sea

Synopsis. Marine life histories differ from terrestrial life histories because seawater is denser and more viscous than air, because desiccation is not a problem for organisms in water, and because food is abundant in suspension and solution. (1) Mating and competition for paternity in the sea often differs. Female gametes are often spawned freely. Passively dispersed spermatophores could in some cases provide single paternity to an entire clutch of offspring. Penises of sessile animals reach far for copulation. There are no pollinators. (2) In many clades of benthic marine animals, greater dispersal of offspring is associated with large adult size, and greater parental care of offspring and reduced planktonic larval periods are associated with small adult size. (3) Many benthic marine animals are colonies with modular construction, and these also commonly brood embryos and have short-lived larvae, in contrast to related solitary forms. (4) Unlike dispersal of terrestrial animals, larval dispersal of marine animals is often obligate with sexual reproduction and often includes a precompetent period during which larvae cannot settle at good sites. Unlike terrestrial seeds, marine larvae have no clear adaptations for dispersal, often grow during dispersal, and often leave bad sites. Feeding planktonic larvae are common among marine animals and rare among other aquatic animals, perhaps because of persistent aquatic routes between habitable sites for marine animals. Peculiarities in marine life histories may influence many aspects of evolution in the sea. Closely related sedentary marine animals can differ greatly in larval dispersal with consequences for recruitment to populations, genetic exchange between benthic populations, adaptation to local conditions, sex allo? cation, interaction with kin, speciation, and extinction.

Comparison of larval bioenergetics of two marine gastropods with widely differing lengths of planktonic life, Thais haemastoma canaliculata (Gray) and Crepidula fornicata (L.)

Larvae of many shallow-water near-shore tropical gastropod species appear to have planktonic durations of several months or more and therefore may have great dispersal capabilities. To investigate physiological factors possibly related to length of planktonic life, this study documented the larval bioenergetics and carbon budgets for one such teleplanic (long-distance) species, Thais haemastoma canaliculata (Gray), in comparison with the relatively short-lived larvae of the temperate species, Crepidula fornicata (L.). Growth, ingestion, and respiration rates were measured in the laboratory during the first 4 wk of larval life for T.h. canaliculata and the first week of larval life of C. fornicata, up to shell lengths of ≈900 and 700 μm, respectively. Larvae of both species were reared at 24°C on a diet of Isochrysis sp. (clone T-ISO) at 18 × 10 4 cells · ml −1. Tissue growth increased at a constant rate for C. fornicata, but at an exponential rate for T.h. canaliculata. Ingestion rates (cells eaten · larva −1 · h −1) for C. fornicata were ≈1.25× higher than rates for T.h. canaliculata at equivalent shell lengths. Respiration rates (μl O 2 consumed · larva −1· h −1) for T.h. canaliculata were generally 1.25 × higher than rates for C. fornicata at similar shell lengths. Estimated assimilation efficiencies were similar for both species and declined during development (from 44 to 28% for C. fornicata and from 33 to 25% for T.h. canaliculata). However, while gross and net growth efficiencies (growth · ingestion − · 100% and growth (growth + respiration) −1 100%) declined during larval development for C. fornicata, growth efficiencies were constant or increased during early development for T.h. canaliculata.

POPULATION GENETICS OF TIGRIOPUS CALIFORNICUS. II. DIFFERENTIATION AMONG NEIGHBORING POPULATIONS.

Many species of intertidal marine invertebrates are restricted in their geographic distribution to discrete patches of habitat which occur irregularly along the shoreline. The rocky intertidal fauna of the Pacific coast of North America exhibits such restriction. Though locally abundant, rocky habitat is irregularly interrupted by stretches of sandy beach which vary in length from a few meters to tens of kilometers. Since many of the species inhabiting the rocky intertidal are relatively sedentary as adults and will not leave rock substrate, sandy beaches result in geographic subdivision of adult populations. Dispersal, primarily via planktonic larval stages, is expected to be important in the ecology and evolution of these species not only because it permits recolonization of habitat patches following local extinction, but also because it can maintain the genetic continuity of geographically separated populations. Despite its general importance in the biology of marine invertebrates, we currently have few data addressing the extent to which larval (or adult) dispersal effectively homogenizes gene pools of isolated coastal populations within a species. In the past decade, the discovery of extensive protein polymorphism in a variety of marine invertebrate species has contributed to our understanding of the genetic structure of natural populations of these organisms. Littorinid gastropods, for example, have been the focus of several studies. Using enzyme polymorphisms as genetic markers, Berger (1973, 1977) showed that the extent of population differentiation in three species of littorinids is correlated with differences in larval dispersal patterns among the species. Littorina littorea, a mid-intertidal species with planktonic larval development, showed little differentiation among geographically separated populations, while L. saxatilis (high intertidal) and L. obtusata (low intertidal), both without planktonic larvae, were significantly more differentiated. Snyder and Gooch (1973) obtained similar results in comparing population differentiation in L. saxatilis to Nassarius obsoletus, a species with long-lived planktonic larvae. Gooch (1975) and Crisp (1978) have reviewed some additional data concerning the relationship between dispersal ability (as measured by length of planktonic larval life) and genetic differentiation and have found it to be generally consistent with the inverse correlation observed by Berger: species with longlived planktonic larvae tend to be genetically homogeneous over broad geographic ranges, while species with short-lived larvae or direct development show some degree of genetic differentiation among neighboring populations. Exceptions to the above relationship have been well documented (e.g., Struhsaker, 1968; Koehn et al., 1980; others). Two factors can account for such apparent exceptions: (1) Strong differences in natural selective forces among populations arising from habitat heterogeneity can result in genetic differentiation even where there is significant inter-population dispersal. (2) Species which appear to have high dispersal ability may not always experience high levels of gene flow. Scheltema (1975) has pointed out that knowledge of the length of larval life is insufficient for understanding the dispersal capabilities of invertebrate species. He suggests that in order to estimate the ex-

The analysis of pattern in communities of bryozoa. I. Discrete sampling methods

Hitherto, discrete sampling methods for the analysis of spatial dispersion patterns have always been based on a unit ( e.g. a quadrat) of standardized dimensions. A new technique is described, of use when the sampling units (as with Fucus fronds) cannot have a constant area. Based on computer simulation, the method permits the comparison of an observed frequency distribution with a Poisson series. By its use, it is shown that the colonies of Electra pilosa (L.) (larvae planktotrophic), on a randomly collected sample of Fucus serratus L., occur in aggregations - though they are not necessarily ‘gregarious’. Aggregation perhaps facilitates cross ( i.e. inter-colony) fertilization. Bryozoa having short-lived larvae are well known to occur in dense aggregations. Thus, in ‘rapids’ systems, several species are found in great abundance on the fronds of Laminaria digitata (Huds.) Lamour and intermingled with Spirorbis spp. Such a community, dominated by Spirorbis corallinae de Silva & Knight-Jones, Hippothoa hyalina (L.) and Scrupocellaria reptans (L.), has been sampled using random quadrats. Local clumping was generally apparent and, under such conditions, it is concluded that analysis based on quadrat sampling is unlikely to detect any spacing out which may be present. The abundance of Spirorbis and bryozoa are positively correlated, except at very high densities of the former.

GENE FREQUENCIES IN A MARINE ECTOPROCT: A CLINE IN NATURAL POPULATIONS RELATED TO SEA TEMPERATURE.

The concept that gene flow is continuous and widespread within a species has recently been questioned (Ehrlich and Raven, 1969), but no data could be cited for marine organisms. Indeed, the spatial and temporal distribution of gene frequencies is unknown for all but a minute fraction of the three million existing animal species. Hitherto, field information on genic differentiation in animals that lack phenotypic polymorphism has been very difficult to obtain. The development of high-resolution zone electrophoresis coupled with staining techniques for specific proteins has now made it routinely possible to identify marker genes and to determine gene and genotype distributions (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966). Factual information on this question of gene flow among populations and the degree of genic differentiation is especially critical for marine ectoprocts because the adults of several species with short-lived larvae (hours) are unable to be distinguished phenotypically over distances of hundreds to thousands of kilometers. Results presented below support the notion that selection adjusts genotype frequencies over very short distances for these sessile marine animals with short-lived larvae. The change in frequency of alleles in populations can be observed over 10-20 kms distance even though populations are nearly continuous over the intervening area. We analyzed the distribution of allele frequencies along a coastal and island transect of 35 kms along southern Cape Cod. From 29 to 47 colonies of the widespread encrusting ectoproct Schizoporella unicornis (see Appendix) were collected from 1/? to 3 m depth from pilings or floating docks at each of five localities, Cape Cod, Massachusetts (Fig. 1). Local populations were estimated to contain several hundred colonies. Irregular aggregations of colonies of S. unicornis have been recorded nearly every kilometer of the way from one end o,f our transect to the other (Sumner et al., 1913) and our impression from examining dredged material is that they are even more closely spaced. Elsewhere along the Atlantic coast of the United States, S. unicornis is also an abundant local species.