# More than meets the eye: diversity and geographic patterns in sea cucumbers

## TL;DR

This study uses DNA barcoding to reveal hidden species diversity and geographic patterns in sea cucumbers, highlighting unique evolutionary dynamics in echinoderms.

## Contribution

The study reveals a high number of cryptic species in sea cucumbers and identifies unique patterns of reproductive isolation in echinoderms.

## Key findings

- Many cryptic species of sea cucumbers exist that are not currently recognized.
- Most sister species have non-overlapping geographic distributions.
- Closely related sympatric forms show little genetic differentiation in COI sequences.

## Abstract

Estimates for the number of species in the sea vary by orders of magnitude. Molecular taxonomy can greatly speed up screening for diversity and evaluating species boundaries, while gaining insights into the biology of the species. DNA barcoding with a region of cytochrome oxidase 1 (COI) is now widely used as a first pass for molecular evaluation of diversity, as it has good potential for identifying cryptic species and improving our understanding of marine biodiversity. We present the results of a large scale barcoding effort for holothuroids (sea cucumbers). We sequenced 3048 individuals from numerous localities spanning the diversity of habitats in which the group occurs, with a particular focus in the shallow tropics (Indo-Pacific and Caribbean) and the Antarctic region. The number of cryptic species is much higher than currently recognized. The vast majority of sister species have allopatric distributions, with species showing genetic differentiation between ocean basins, and some are even differentiated among archipelagos. However, many closely related and sympatric forms, that exhibit distinct color patterns and/or ecology, show little differentiation in, and cannotbe separatedby, COI sequence data. This pattern is much more common among echinoderms than among molluscs or arthropods, and suggests that echinoderms acquire reproductive isolation at a much faster pace than other marine phyla. Understanding the causes behind such patterns will refine our understanding of diversification and biodiversity in the sea.