Clonal structure through space and time: High stability in the holothurianStichopus chloronotus(Echinodermata)

Abstract

Sea cucumbers are increasingly exploited for human consumption and for their curative properties, and many wild populations are now depleted or in danger of extinction. While aquaculture is seen as an alternative to fisheries and as a mean to restore wild populations, more knowledge is needed on their reproductive strategies to render this practice efficient, notably for fissiparous holothurians, which are some of the mobile animals able of asexual reproduction by transverse fission. Little information is available on their population genetic diversity and structure. Here, the clonal structure of populations of the fissiparous sea cucumber Stichopus chloronotus has been investigated using nine microsatellite loci and a random sampling, at different spatial (intra‐reef and inter‐reef) and temporal (inter‐season and inter‐year) scales. Our findings highlight the importance of asexual reproduction in maintaining these populations, and the prevalence of the “initial seedling recruitment” strategy (ISR), leading to a high stability of clonal composition over seasons and years. It also seemed that clonal propagation was limited to the reef scale (<10 km) while reefs were connected by sexual dispersal. This is the first time that clonal structure in sea cucumbers has been studied at such a fine scale, with a specific sampling strategy. It provides key findings on the genetic diversity and structure of fissiparous sea cucumbers, which will be useful for the management of wild populations and aquaculture.

Isolation and characterization of 21 microsatellite loci for the sea cucumber Holothuria (Halodeima) atra (Echinodermata, Holothuroidea) reveal low asexual propagation through time in Reunion Island (southwestern Indian Ocean)

“…The global FST between both seasons (0.019 NS ) suggests no genetic differentiation and thus, temporal stability in this H. atra population. The stability observed through time for H. atra was also found for S. chloronotus [14].…”

“…The absence of identical MLGs in our dataset may be due to the high number of loci used decreasing mathematically the probability to find two identical MLGs over the 42 alleles, and also their inherent high polymorphism. Indeed, reducing the number of loci used, to be able to compare clonal richnesses among studies (1,000 sub-datasets from the random selection of nine microsatellites over the 21 specific for H. atra), the mean clonal richness in H. atra reached 0.91 ± 0.002 [± SE; min: 0.63 (28 MLGs); max: 1 (44 MLGs)] and remained extremely high comparatively to the one found in S. chloronotus populations (R = 0.09) [14]. Indeed, in S. chloronotus, each population (based on 64 sampled individuals per site × season) was comprised of one to three dominant MLGs [14], although the percentage of S. chloronotus individuals issued from asexual reproduction estimated based on body shape few decades ago was quite low (6.5%) [24].…”

“…MLGs sharing a certain number of alleles, considered close enough to be part of the same lineage) was also investigated. Furthermore, to compare the results obtained with other studies, notably to compare the clonal richness estimated in the sea cucumber Stichopus chloronotus at the same site using nine specific microsatellite loci [14], nine out of 21 loci were randomly selected 1,000 times. From these new 1,000 sub-datasets, MLGs were then identified as above (missing data kept and considered as potentially identical alleles, as above) and R was also estimated and compared to previous studies.…”

Sex or Fission? Genetics Highlight Differences in Reproductive Strategies of Two Sympatric Fissiparous Sea Cucumber Species in Reunion Island (Southwestern Indian Ocean)

“…Two additional sites with low densities (<0.1 ind•m −2 , personal observations) were studied: CAP (Cap Méchant) and PTE (Petit Trou d'Eau) (Figure 1). Individuals of S. chloronotus were sampled in the same high density sites as in a previous study [31]: PAS (Passe de l'Hermitage; corresponding to HIGH1 in [31]; with a density of 0.…”

“…The number of individuals of S. chloronotus performing fission has been estimated to 17% at Reunion Island [24], using the same method as for H. leucospilota [23]. However, two decades later, the clonal richness was analysed using nine microsatellite markers [31] and authors concluded that it was extremely low (R = 0.09), meaning that many individuals of S. chloronotus are clones, and therefore, have participated or participate to asexual reproduction. Visual census is not a good predictor to estimate clonal propagation, as concluded by a study on Holothuria atra [32].…”