Circadian Rhythms in Locomotor Activity of the Hagfish, Eptatretus burgeri VI. The Effects of Cutting the Spinal Cord

Abstract

Due to their curious phylogenetic position and anatomy, hagfishes have attracted the interest of zoologists, especially in the context of vertebrate evolution. Embryological information on these animals is now also needed in the field of evolutionary developmental biology (Evo-Devo), as it is expected to provide hints about the origin of vertebrate traits, whether the hagfishes are an in-or outgroup of vertebrates. This review summarizes the importance of hagfish embryology from a phylogenetic perspective, and the history of attempts to obtain hagfish eggs and embryos. Clearly, the main difficulty associated with these animals is their deep-sea habitat. To circumvent this problem, this review also discusses the future prospects for obtaining embryological material, both from the wild and in the laboratory.Key words: hagfish, embryo, evolution, phylogeny, development Significance of hagfish embryology in the vertebrate Evo-Devo studyHagfishes have been, and still are, recognized as among the most important animals for understanding vertebrate evolutionary history ( Fig. 1; reviewed by Janvier, 1996;Hall, 1998Hall, , 1999. One reason is that their ontogenetic development is poorly known, and the other is that their phylogenetic position remains enigmatic. A number of authors have investigated hagfishes in various ways in various biological fields, including physiology, endocrinology, immunology, and molecular biology (Brodal and Fänge, 1963;Gorbman, 1997;Jørgensen et al ., 1998;Kubota et al ., 2001;Pancer et al ., 2005). Recent studies involving large-scale sequencing analyses have also elucidated intriguing characteristics of this group (Suzuki et al ., 2004Pancer et al ., 2005). Thus, it appeared to us that it would be useful to summarize and review the few embryological studies and the previous attempts made to collect embryos of this animal group, especially with the aim of obtaining embryological information for future projects.As to the question of phylogeny, there are two major hypotheses, the cyclostome theory and the craniate theory ( Fig.