Morphological specializations in heterocongrinae (Anguilliformes: Congridae) related to burrowing and feeding

The remarkable lifestyle of heterocongrines has drawn the attention of many authors in the past, though no or little attention has been paid to the morphology of the tail and the head of these species. In order to examine the true nature of possible morphological specializations of the head and tail and their relation to their tail-first burrowing habit and/or feeding mode, a detailed myological and osteological study of Heteroconger hassi and Heteroconger longissimus was performed. The osteological similarities of the cranial skeleton between H. hassi and H. longissimus are striking. Most of the cranial muscles show no variation in presence, insertion or origin between these two species except for the adductor mandibulae complex, the adductor hyomandibulae and the intermandibularis. The adductor mandibulae complex is small, compared to that of other anguilliform species, and is probably related to their suction-dominated feeding mode and a diet, comprising mainly small, soft prey items. Heterocongrinae have undergone several morphological specializations in the tail for their tail-first burrowing lifestyle. The skeleton and musculature of the tail of H. hassi and H. longissimus are similar. In both species the caudal skeleton is highly reduced and fortified, forming a firm, pointed burrowing tool. Intrinsic caudal musculature is reduced and some muscles (interradials, supracarinalis) are even absent.