Morphological variations and dysmorphic features in the sea cucumber <i>Cucumaria frondosa</i>

Studying morphological abnormalities provides valuable insights into the developmental processes, species adaptability, gene expression, and the potential effect of environmental pressures. Dysmorphic features are well known within several taxa like arthropods but has much less been investigated in others like echinoderms, and specially sea cucumbers (Holothuroidea). The dendrochirotid Cucumaria frondosa (Gunnerus, 1767), which is the most studied and economically important holothuroid species in the North Atlantic, is known to present different colour morphotypes but anatomical variations have rarely been reported. This study tallied dysmorphic features across a sample of more than 900 individuals of C. frondosa examined over five years. About 1.3% and 15.9% of individuals presented non-standard external or internal anatomical characteristics, respectively. Individuals with unusual body plans, including two posterior ends and two cloaca/anuses, or an abnormal number of tentacles, were discovered. Others exhibited supernumerary organs, or organs of unusual shape and size, e.g., ramified/split ambulacra and longitudinal muscle bands, ramified or supernumerary Polian vesicles, hypertrophied madreporites, inter-vesicle protrusions, and hypertrophied circular haemal vessels. These features are described here for the first time in C. frondosa and have not been reported before in holothuroids. The severity of the morphological changes along with their possible drivers and impacts is discussed.