Assessment of the Biogeography and Molecular Phylogenetics of Sea Cucumbers (Echinodermata: Holothuroidea) in South Africa

Abstract

Aim

The biogeography and evolution of South African sea cucumbers have been previously explored; however, these aspects lack detailed study. This study (1) explores the biogeography of sea cucumbers along the coastline of South Africa (SA) and (2) presents a preliminary molecular phylogeny based on the mitochondrial 16S marker.

Location

South Africa.

Taxon

Class Holothuroidea.

Methods

Distribution data obtained primarily from the literature and collections catalogues were used to determine species richness, diversity and endemism along equidistant sections of the SA coastline. Additionally, a phenetic cluster analysis was performed to identify marine biogeographic regions. Partial mitochondrial 16S sequences were used for a maximum likelihood (ML) analysis.

Results

Species richness was highest on the northeastern coast, whereas diversity indices (Shannon's exponential and Simpson's inverse indices) were highest on the southern coast, and endemism levels were highest on the western coast. The clustering pattern obtained from the phenetic analysis reflected, partly, the three main marine biogeographic provinces traditionally recognized. However, the biogeographic breaks observed between clusters and subclusters suggested two biogeographic regions: a west coast and a south-east coast region. A phylogenetic analysis found a basal grade and a derived clade. Grade A was composed of genera from the orders Holothuriida and Synallactida, whereas Clade B was composed of genera from the Order Dendrochirotida. The ML analysis allowed for the identification of Holothuria granosa from SA and showed that the generic limits of Roweia need revisiting.

Main Conclusions

The increase in species richness from the west coast towards the northern KwaZulu-Natal (KZN) coast may be due to variation in water temperature, and various factors and processes such as current direction and dispersal. The high richness and high levels of endemism on the west coast, especially False Bay, are likely a consequence of this area being especially well studied and an overlap region between the Benguela and the Agulhas currents. The combined biogeographic and phylogenetic results suggest a dispersal pattern of basal clades from the north-east Indian Ocean, towards the south coast, and then northwards along the Atlantic Ocean.