Evaluating edge-of-range genetic patterns for tropical echinoderms, <I>Acanthaster planci</I> and <I>Tripneustes gratilla</I>, of the Kermadec Islands, southwest Pacific

Abstract

AbstrAct.-Edge-of-range populations are often typified by patterns of low genetic diversity and high genetic differentiation relative to populations within the core of a species range. The "core-periphery hypothesis," also known as the "central-marginal hypothesis," predicts that these genetic patterns at the edge-of-range are a consequence of reduced population size and connectivity toward a species range periphery. It is unclear, however, how these expectations relate to high dispersal marine species that can conceivably maintain high abundance and high connectivity at their range edge. In the present study, we characterize the genetic patterns of two tropical echinoderm populations in the Kermadec Islands, the edge of their southwest Pacific range, and compare these genetic patterns to those from populations throughout their east Indian and Pacific ranges. We find that the populations of both Acanthaster planci (Linnaeus, 1758) and Tripneustes gratilla (Linnaeus, 1758) are represented by a single haplotype at the Kermadec Islands (based on mitochondrial cytochrome oxidase c subunit I). such low genetic diversity concurs with the expectations of the "core-periphery hypothesis." Furthermore, the haplotypic composition of both populations suggests they have been founded by a small number of colonists with little subsequent immigration. Thus, local reproduction and self-recruitment appear to maintain these populations despite the ecologically marginal conditions of the Kermadec Islands for these tropical species. Understanding rates of self-recruitment vs reliance on connectivity with populations outside of the Kermadec Islands has implications for the persistence of these populations and range stability of these echinoderm species.Population attributes are expected to differ according to range position. Differences in abundance, reproduction, and dispersal (connectivity) can often be observed across a species range, and over evolutionary timescales these population attributes are expected to lead to predictable neutral genetic patterns across their range. For

Swept away: ocean currents and seascape features influence genetic structure across the 18,000 Km Indo-Pacific distribution of a marine invertebrate, the black-lip pearl oyster Pinctada margaritifera

“…This observation suggests that while surface ocean currents permit sufficient gene flow across the Pacific Ocean to ensure populations retain a high degree of connectivity, circulation patterns and IBD may also facilitate regional larval retention, that stabilises population genetic structure. Because even low levels of gene flow [101, 102] are able to prevent population divergence, it is conceivable that standing genetic diversity and structure are maintained by a “founder takes all” density-dependent effect [103], where individuals arriving after an initial colonisation event may be “blocked” by established conspecifics [11, 103]. …”

“…While several studies have examined C-M genetic patterns in terrestrial taxa [5, 10], comparatively few investigations have involved marine species [8], and marine invertebrates in particular [11]. Marine systems present several challenges for range-wide studies, as >70% of invertebrates and many vertebrates are characterised by large population sizes, high fecundity, external fertilisation and larvae that typically remain in the plankton for several weeks, although this may vary anywhere from a few minutes to years [12–16].…”

Larval connectivity across temperature gradients and its potential effect on heat tolerance in coral populations

“…Thus, the role of connectivity in determining heat tolerance depends on how strong a role the genetic factor plays. In addition, locations with the greatest connectivity to other reefs within a population of a species are more likely to have greater genetic diversity than more isolated locations (Liggins et al, 2014). If that genetic diversity includes different levels of adaptation to heat stress, then the bleaching response should reflect that.…”

Color diversity and distribution of sea urchin Tripneustes gratilla in Cenderawasih Bay ecoregion of Papua, Indonesia

“…All samples shows 99-100% similarity with the COI sequences of T. gratilla (accession number KF012821.1-KF012824.1.) from Southwest Pacific (Liggins et al, 2014). The data indicated that the samples were collected from Cenderawasih Bay ecoregion is T. gratilla species.…”