Monitoring the genetic diversity in holothurian populations from several Mediterranen sea regions with mitochondrial DNA sequences
Sea cucumbers are deposit feeding members of marine benthic communities that play a key role in sea floor dynamics by processing and bioturbating the sediment. Many of these species are considered economically relevant as luxury food and traditional medicine in Asia. This importance has led to an over-exploitation of many natural stocks, especially in the Mediterranean area where excessive fishing has caused a loss of biodiversity among populations with a negative impact on the marine ecosystem. For these reasons, a few initiatives are now evaluating sea cucumbers as novel aquaculture species. Moreover, some legal actions are taking from Italian Ministry of Agriculture, Food Sovereignty and Forests to protect and preserve these important marine resources. The aim of this study was to evaluate the level of genetic diversity within two sympatric Holothuria sea cucumber species, H. polii and H. tubulosa, across ten Mediterranean areas along Italy. Sea cucumbers have been sampled from different substrates seabed typologies (sand, rock, Posidonia meadows). We sequenced informative portions of the COI mitochondrial gene in a total of 441 holothurians (251 H. polii, 177 H. tubulosa and other holothurian species). Genetic divergence (p-distance, haplotype and nucleotide diversity) within and between species were calculated and a Neighbour Joining (NJ) phylogenetic tree was obtained using the dataset, including outgroup sequences retrieved from Genbank (H. scabra, H. forskali, H. mammata, H. arguinensis and Ocnus glacialis). Genetic diversity analyses suggest that H. polii and H. tubulosa species are genetically different, while are quite homogeneous among and within populations. H. polii showed a lower genetic distance than H. tubulosa, maybe due to the different length of the planktonic larval period before they begin the benthonic life. The phylogenetic analyses showed a clear differentiation between the two species. The outgroup H. mammata clustered as a sister group of H. tubulosa clade, as expected. Whitin populations, the NJ trees showed no differentiation between site populations, indicating active gene flows among all Mediterranean areas investigated. Interestingly, the sequence obtained from six specimens morphologically assigned to a species clustered within the other species, indicating a possible occurrence of hybridization events between these two sympatric species. These data will be also useful in establishing cultivated stocks for the aquaculture with a high level of genetic variability and on the other hand to implement conservation actions for these two holothurian species.