Learn About Seastars: Population & community ecology

Genetics

Topics relating to population & community ecology include GENETICS, considered here, and COMPETITION, KEYSTONE PREDATOR, and POPULATION BIOLOGY dealt with in other sections.

Research study 1

sea star Evasterias troschelliIn comparison with species that brood their young, asteroids with planktotrophic larvae would be expected to have greater opportunity for gene flow and, thus, much broader geographic distribution. To test this idea scientists in Alaska analyse the genetic structure of 5 species of sea stars from 6 locations to determine levels of intra- and inter-population variation. Surprisingly, the results show higher heterozygosity in Leptasterias epichlora, a brooder, than in either of the free-spawners Pisaster ochraceus and Evasterias troschelli, or in L. polaris, a brooder with circumpolar distribution. Additionally, the authors demonstrate: 1) that hybridisation between L. epichlora and L. hexactis can occur in some situations, 2) that there is no subspecies of P. ochraceus, as has been proposed in earlier literature, and 3) that E. troschelli is a single panmictic species. The authors conclude that genetic variation in sea stars is unrelated to mode of reproduction. Stickle et al. 1992 Can J Zool 70: 1723.

NOTE allozyme variation at 16-25 loci (overall, 33 different enzymes are examined)

NOTE unstructured population with random mating

Evasterias troschelli exposed at low tide 0.6

Research study 2

In a similar larger-scale follow-up investigation the same research groups investigates the genetic structure of Leptasterias at 37 intertidal locations between Alaska and California. By use of both morphological and genetic criteria, 4 species are confirmed: L. epichlora, L. hexactis, L. aequalis, and L. aspera. No more than 2 species are found at any given location, and hybridisation is rare (only 0.9% of 1085 specimens). All species are morphologically and genetically similar, but could usually be separated by at least one diagnostic allozyme locus. A 5th species, tentatively identified as L. pusilla, may be present at some sites in central California where it overlaps with the similar L. hexactis at the latter's southern limits of distribution. Foltz & Stickle 1994 p. 291 In, Echinoderms through time (David et al., eds) Balkema, Rotterdam; see also Foltz 1998 p. 235 In, Echinoderms (Proc 9th Int. Echinoderm Conf., San Francisco) (Mooi & Telford, eds.) Balkema, Rotterdam.

Research study 3

How useful are oceanographic models for understanding genetic connectivity between populations of marine invertebrates with free-living larvae of known pelagic duration? This is not a new question, but is investigated afresh by researchers from Canadian universities and federal fisheries institutions using an up-to-date high-resolution ocean-current model. The model as used here simulates larval dispersal in the bat star Patiria miniata with a free-swimming period of about 10wk, and for which genetic structure along a section of the British Columbia coast is known from several other investigations. These earlier studies disclose major breaks in genetic connectivity between Haida Gwaii and Vancouver Island and elsewhere. Also, the species is absent from the coasts of Washington and Oregon, but is plentiful south of Fort Bragg, California. Results from 70d runs of the model show that predictions of larval-vectored gene flow closely match what is known from these earlier map showing modelled movement of larvae of bat stars Patiria miniata along the British Columbia coast and also from populations in Fort Bragg, Californiastudies (see map). A more fine-scale genetic sampling by the researchers of 50 individuals within the previously unstudied phylogeographic break between southern Haida Gwaii and northern Vancouver Island (at Bella Bella, see orange cluster of points on map) also shows that gene flow predicted in the oceanographic model matches that actually found. The insert on the map shows a predicted northward movement of particles from Fort Bragg, California, the northermost distribution of P. miniata in the state, but not far enough to reach northern sites within the 70d lifespan of the larvae. With respect to this, the authors acknowledge that their simulation does not take into account known strengthening of northward coastal currents from California during El Nino years (up to 4-fold intensity). Finally, the authors remark on the surprisingly high level of population genetic structure in this species with such a long-lived dispersal stage. Sunday et al. 2014 Molec Ecol 23 (20): 5036.

NOTE see LEARNABOUT/MOLLUSCA/whelDisp.php#RS5 for a brief summary of one of these investigations

 

This 70d run of the model shows the start of movement of
simulated larvaefrom Vancouver Island northwards but not
quite reaching Haida Gwaii. The general northern flow owes
to the westward flowing Kuroshio Current acrossthe Pacific
Ocean splitting into the northward flowing Alaska Current

 

Research study 4

map showing distributions of 4 haplotypes of the 6-arm seastar LeptasteriasMembers of the Leptasterias species-complex continue to receive research attention of the phylogeographic kind. This owes largely to their brooding mode of development leading to low dispersal capability and high potential genetic subdivision. As an example, a recent study by scientists at San Francisco State University using mitochondrial DNA-sequence data has disclosed a previously undescribed genotype inhabiting intertidal sites near to and at the mouth of the San Francisco Bay estuary. This specialised ecotype (light orange colour on map) is genetically similar but separate from clades located to the north and south (green, black, & reddish). The authors propose a theory of glacial refuge, isolating a clade that over time adapted to the specialised conditions of estuarine outflow. In this way the Bay estuary may be acting a phylogeographic barrier to north-south dispersal of other clades. Melroy et al. 2017 Mar Biol 164: 152.

NOTE the taxonomic status of genus Leptasterias is presently unresolved. Recent molecular analyses identify 6 species complexes from Alaska to California: aequalis, alaskensis, polaris, aleutica, hexactis, and leptodoma. The putative aleutica species is intermixed with another clade camtschatica, while the aequalis complex in central California is partitioned into 4 distinct clades, with further splits between some of these


Distribution of 4 clades of Leptasterias aequalis in the immediate
vicinity of the mouth of San Francisco Bay estuary. Circle
size reflects sample numbers genotyped (in brackets)