Embryonic staging and external features of development of the Chimaeroid fish, Callorhinchus milii (Holocephali, Callorhinchidae).

The development of Callorhinchus milii, a primitive chondrichthyan fish (Subclass Holocephali) is described in detail based on a complete series of embryos from stage 17 to hatching. The external features of these specimens, in comparison with other chondrichthyan embryos, are used to establish the first staging table for any chimaeroid species. Each stage of C. milii is defined by a suite of morphological characters in addition to total length, including the number of somites, extent of external pigmentation, eye size and shape, head flexure, heart morphology, and size and shape of paired and unpaired fins. Particular attention is given to features of the gill arches and associated structures, including external gill filaments and the opercular flap. Embryos of this species also possess a transient rostral bulb, a feature unique to chimaeroids. Embryological development of Callorhinchus milii is similar to that previously described for sharks and batoids (Subclass Elasmobranchii), including the spiny dogfish, Squalus acanthias, the Japanese bullshark, Heterodontus japonicus, the lesser spotted dogfish, Scyliorhinus canicula, the frill shark, Chlamydoselachus anguineus, the guitarfish, Rhinobatus halavi, and the skate, Raja brachyura. Callorhinchus milii is also similar in overall development to another holocephalan, Hydrolagus colliei. A review of previous staging schemes confirms that early morphological development in all three major chondrichthyan lineages (sharks, batoids, and chimaeras) can be correlated using a common set of stages. A uniform staging system is provided that should prove useful in continuing ontogenetic and phylogenetic studies of this entire clade of fishes. J. Morphol. 236:25-47, 1998. © 1998 Wiley-Liss, Inc.