Hagfishes (Myxinoidea)

Abstract

Hagfishes (Myxinoidea, myxinoids), sometimes also referred to as slime eels, are benthic marine organisms that occur in temperate or cold water at depths varying from approximately 10 to 1000 m. The following features are characteristic of myxinoids (Fig. 11.1): the body is cylindric and elongated; the adults of most hagfish species achieve a body length of 40–50 cm. Paired fins are lacking, but the tail displays an unpaired sagittal fin fold that is stabilised by cartilaginous rays dorsally. However, in contrast to lampreys (Petromyzontoidea) and jawed vertebrates (Gnathostomata), there are no muscles associated with these fin rays (Janvier 1981). The skeleton is entirely cartilaginous, the notochord persists throughout life, and neural and hemal arches as well as vertebrae are lacking. In contrast to lampreys, hagfishes are direct developers; the eggs are polylecithalic and large. Fertilisation and embryonic development occur externally (Dean 1899). The nasal opening is unpaired and located at the tip of the snout. It is surrounded by two pairs of barbels which contain mechanoand chemoreceptors (Georgieva et al. 1979; Andres and von During 1993a; Fig. 11.2a, b). An unpaired nasal duct connects the nasal opening with the nasal cavity, and, unlike in lampreys, there is an unpaired naso-pharyngeal duct which connects the nasal cavity with the pharynx (Fig. 11.2c). In some species of hagfishes (genus Eptatretus) the number of gill pairs is higher than in any other craniate (up to 15) and variable, even within one species (Worthington 1905a).