Does fish larval dispersal differ between high and low latitudes?

Several factors lead to expectations that the scale of larval dispersal and population connectivity of marine animals differs with latitude. We examine this expectation for demersal shorefishes, including relevant mechanisms, assumptions and evidence. We explore latitudinal differences in (i) biological (e.g. species composition, spawning mode, pelagic larval duration, PLD), (ii) physical (e.g. water movement, habitat fragmentation), and (iii) biophysical factors (primarily temperature, which could strongly affect development, swimming ability or feeding). Latitudinal differences exist in taxonomic composition, habitat fragmentation, temperature and larval swimming, and each difference could influence larval dispersal. Nevertheless, clear evidence for latitudinal differences in larval dispersal at the level of broad faunas is lacking. For example, PLD is strongly influenced by taxon, habitat and geographical region, but no independent latitudinal trend is present in published PLD values. Any trends in larval dispersal may be obscured by a lack of appropriate information, or use of ‘off the shelf’ information that is biased with regard to the species assemblages in areas of concern. Biases may also be introduced from latitudinal differences in taxa or spawning modes as well as limited latitudinal sampling. We suggest research to make progress on the question of latitudinal trends in larval dispersal.     

Theoretical limits to the correlation between pelagic larval duration and population genetic structure

Increasing dispersal duration should result in increasing dispersal distance, facilitating higher gene flow among populations. As such, it has long been predicted that genetic structure (e.g. F(ST) ) among populations of marine species should be strongly correlated with pelagic larval duration (PLD). However, previous studies have repeatedly shown a surprisingly poor correspondence. This result has been frequently interpreted as evidence for larval behaviours or physical oceanographic processes that result in larvae failing to reach their dispersal potential, or error inherent in estimating PLD and F(ST) . This study employed a computer modelling approach to explore the impacts of various uncertainties on the correlation between measures of genetic differentiation such as F(ST) and PLD. Results indicate that variation resulting from PLD estimation error had minor impacts on the correlation between genetic structure and PLD. However, variation in effective population size between species, errors in F(ST) estimation and non-equilibrium F(ST) values all had major impacts, resulting in dramatically weaker correlations between PLD and F(ST) . These results suggest that poor correlations between PLD and F(ST) may result from variation and uncertainty in the terms associated with the calculation of F(ST) values. As such, PLD may be a much stronger determinant of realized larval dispersal than suggested by the weak-to-moderate correlations between PLD and F(ST) reported in empirical studies.     

Early life history and settlement of the slender filefish, <Emphasis Type="Italic">Monacanthus tuckeri</Emphasis> (Monacanthidae), at Calabash Caye,Turneffe Atoll,Belize

Synopsis We examined early life history traits and patterns of settlement of the slender filefish, Monacanthus tuckeri, at Calabash Caye, Turneffe Atoll, Belize. A settlement peak was evident at the new moon, and no settlement occurred at the full moon. However, settlement rates at the quarter moons could not be estimated due to sampling gaps. Many reef fishes show new moon settlement peaks, so M. tuckeri shares some characteristics with the primarily perciform species on coral reefs. Pelagic larval duration was long (mean = 42 days) and variable, suggesting that dispersal patterns might be diverse. Size at settlement was large (mean = 32 mm total length) and also variable. Larval duration and size at settlement were outside of the average values exhibited by reef fishes, but are not beyond the extreme end of the range, and might be explained by association with pelagic debris prior to settlement. There were no differences in overall settlement rates on reef and seagrass habitats, and fish settling to either habitat did not differ in larval duration, size at settlement, or larval growth rate. This suggests that settlement to alternative habitats may be random, or driven by availability of suitable microhabitat, rather than habitat quality or individual traits.     

Does mating behaviour affect connectivity in marine fishes? Comparative population genetics of two protogynous groupers (Family Serranidae)

Pelagic larval duration (PLD) has been hypothesized to be the primary predictor of connectivity in marine fishes; however, few studies have examined the effects that adult reproductive behaviour may have on realized dispersal. We assessed gene flow (connectivity) by documenting variation in microsatellites and mitochondrial DNA sequences in two protogynous species of groupers, the aggregate spawning red hind, Epinephelus guttatus, and the single‐male, harem‐spawning coney, Cephalopholis fulva, to ask whether reproductive strategy affects connectivity. Samples of both species were obtained from waters off three islands (Puerto Rico, St. Thomas and St. Croix) in the Caribbean Sea. Despite the notion that aggregate spawning of red hind may facilitate larval retention, stronger signals of population structure were detected in the harem‐spawning coney. Heterogeneity and/or inferred barriers, based on microsatellites, involved St. Croix (red hind and coney) and the west coast of Puerto Rico (coney). Heterogeneity and/or inferred barriers, based on mitochondrial DNA, involved St. Croix (coney only). Genetic divergence in both species was stronger for microsatellites than for mitochondrial DNA, suggesting sex‐biased dispersal in both species. Long‐term migration rates, based on microsatellites, indicated asymmetric gene flow for both species in the same direction as mean surface currents in the region. Red hind had higher levels of variation in microsatellites and lower levels of variation in mitochondrial DNA. Long‐term effective size and effective number of breeders were greater for red hind; estimates of θ_f, a proxy for long‐term effective female size, were the same in both species. Patterns of gene flow in both species appear to stem in part from shared aspects of larval and adult biology, local bathymetry and surface current patterns. Differences in connectivity and levels of genetic variation between the species, however, likely stem from differences in behaviour related to reproductive strategy.     

Further experiments on larval behaviour of the tubicolous polychaete Spirorbis inornatus L'Hardy & Quiévreux

Laboratory experiments indicate that there are many ecologically isolated populations of Spirorbisinornatus L'Hardy & Quiévreux, each tending to select the algal substratum which supported their parents. In some algae certain parts of the thallus are particularly favourable for settlement and, in some areas, this involves adaptation of the local population, e.g. to settle on fronds of Laminaria in sheltered localities. Other parts are unfavourable and these are generally the parts which are most illuminated in their natural orientation (e.g. the upper sides of Himanthalia buttons and the outer sides of Laminaria holdfast branches). These parts are active in producing antibiotics, which may thus have an antifouling role, deterring settlement of larvae whether or not those larvae come from populations adapted to settling upon other parts of the thallus.Most larvae from populations on the turf-alga Chondrus settle without any pelagic phase, whereas the majority from populations on Himanthalia and Laminaria holdfasts become pelagic. The proportions choosing holdfasts in laboratory experiments are high, however, suggesting that the need for skill in such choice, by these pelagic populations, more than balances any effect from outcrossing.     

Predictability of littoral-zone fish communities through ontogeny in Lake Texoma,Oklahoma-Texas,USA

Synopsis We sampled larval, juvenile and adult fishes from littoral-zone areas of a large reservoir (Lake Texoma, Oklahoma-Texas) (1) to characterize environmental factors that influenced fish community structure, (2) to examine how consistent fish–environment relationships were through ontogeny (i.e., larval vs. juvenile and adult), and (3) to measure the concordance of larval communities sampled during spring to juvenile and adult communities sampled at the same sites later in the year. Larval, juvenile and adult fish communities were dominated by Atherinidae (mainly inland silverside, Menidia beryllina) and Moronidae (mainly juvenile striped bass, Morone saxatilis) and were consistently structured along a gradient of site exposure to prevailing winds and waves. Larval, juvenile and adult communities along this gradient varied from atherinids and moronids at highly exposed sites to mostly centrarchids (primarily Lepomis and Micropterus spp.) at protected sites. Secondarily, zooplankton densities, water clarity, and land-use characteristics were related to fish community structure. Rank correlation analyses and Mantel tests indicated that the spatial consistency and predictability of fish communities was high as larval fishes sampled during spring were concordant with juvenile and adult fishes sampled at the same sites during summer and fall in terms of abundance, richness, and community structure. We propose that the high predictability and spatial consistency of littoral-zone fishes in Lake Texoma was a function of relatively simple communities (dominated by 1–2 species) that were structured by factors, such as site exposure to winds and waves, that varied little through time.     

Phylogenetic relationships and ecomorphological divergence in sympatric and allopatric species of Paragalaxias (Teleostei: Galaxiidae) in high elevation Tasmanian lakes

Four species of Paragalaxias (Galaxiidae) inhabit lakes of submontane Tasmania. P. dissimilis and P. eleotroides are sympatric in Great Lake and Shannon Lagoon; P. mesotes occurs in Arthurs and Woods lakes; P. julianus is found in lakes of the Western Plateau. Phylogenetic analysis shows the genus to be monophyletic, and indicates that P. julianus is the sister-species of the other three species, and that P. dissimilis is the sister-species of P. mesotes and P. eleotroides. Morphological comparisons show that the two sympatric species have diverged from the others, with P. dissimilis becoming limnetic/pelagic (terminal mouth, eyes more lateral with convex interorbital, symmetrical paddle-shaped pectoral fins with rays divided once, forked tail, many, long gill rakers, large swimbladder), while P. eleotroides has become benthic (downturned mouth, eyes high on head with interorbital convex, rhomboidal pectoral fins with upper rays longest, rays divided twice, truncated tail, few, short gill rakers, small swim bladder). This character divergence is consistent with the tenets of character displacement except that it remains unproved that it has been driven by resource competition. The ecomorphological divergence parallels that described for species pairs in northern cool temperate lakes with fish faunas of low species richness, particularly in threespine stickleback, Gasterosteus aculeatus.