The Structure and Growth Processes of Caudal Thorns

Abstract

Caudal thorns from bathyrajids, for which surface band patterns have previously been correlated with an annual cycle of vertebral band deposition, were examined to ascertain the underlying growth band structure with a view to understanding their growth processes. Caudal thorn samples from Bathyraja brachyurops, a commercial ray species from the Falkland Islands were sectioned transversely using both resin embedding and decalcification/wax embedding procedures. Sectioning revealed that internal bands formed as a series of cones underlying each other, and that broad surface broad bands formed annually during periods of rapid somatic growth. Surface ridges representing a near stasis in somatic growth formed at the periphery of each cone. This proposed growth process was further verified by the incorporation of tetracycline at the periphery of caudal thorns from a tag-recaptured bathyrajid. The benefits of using sectioning to enhance band resolution of larger specimens of slower growing species, particularly in the case of worn and tightly spaced surface band patterns are also discussed. Introduction In the majority of ray species caudal thorns are found securely embedded in the caudal tissue overlying the spinal column. It is probable that they have a defensive function, and may also assist in the emergence from the egg case. The surface bands of caudal thorns from bathyrajids have previously been correlated with an annual cycle of vertebral band deposition (Gallagher and Nolan, 1999). For a structure to be used as a valid ageing tool it is important to have an understanding of its growth and band formation processes (Cailliet, et al., 1983; Casselman, 1987). To date, however it is unclear how growth bands form in caudal thorns, and how the surface sculpture relates to the underlying morphology. Furthermore bathyrajids are typically slow growing species (Gallagher and Nolan, 1999) and therefore difficulties have been encountered in resolving both worn bands near the thorn tip and tightly spaced bands towards the periphery (Gallagher, 2000). Sectioning of growth structures has previously been recommended for both vertebral centra and dorsal fin spines to help improve band resolution, particularly in slow growing elasmobranchs (Cailliet, 1990; Branstetter and Musick, 1994). This study investigates the structure and growth processes of caudal thorns from Bathyraja brachyurops, a commercially important bathyrajid from the Falkland Islands, which an ageing study using whole caudal thorns has been carried out previously (Gallagher and Nolan, 1999). * Current address; Irish Sea Fisheries Board (BIM), Crofton Road, Dun Laoghaire, Co. Dublin, Ireland.