Dorsal-fin spine growth of Heterodontus portusjacksoni: a general model that applies to dorsal-fin spines of chondrichthyans?

Abstract

A prospective model of dorsal-fin spine growth in chondrichthyans is devised by studying the growth of spines from captive and wild Heterodontus portusjacksoni injected with several fluorochromes. Evidence was found for only two dentine layers in the spine trunk of H. portusjacksoni, contrasting with conjectures about the presence of a third middle dentine layer in some squalids. The spines have three simultaneous growth zones. The first growth zone is along the internal edge of the inner trunk dentine layer, where growth bands are deposited towards the centre (centripetally), increasing spine length and width by pressure against the cartilage rod. The second growth zone is along the external edge of the outer trunk dentine layer at the spine base, where growth bands are deposited in an opposite direction to the centre (centrifugally), increasing spine width. A third growth zone at the base of the cap enables downward expansion over the trunk. The model of concentric cones describes correctly the inner dentine layer growth pattern, whereas the outer dentine layer growth increments appear to be related to the external bands on the surface of the trunk. Growth increments from the three growth zones of the spine are all potentially useful for age estimation.

The spine-brush complex in symmoriiform sharks (Chondrichthyes; Symmoriiformes), with comments on dorsal fin modularity

“…In a large temporal scale, major growth marks are produced in modern sharks by periodical (seasonal) changes in three main physical and biological factors: water temperature, food resources and breeding behavior (Cailliet and Radtke 1987;Soler-Gijón 1999;Ramos 2007;Tovar-Avila et al 2008). The presence of major growth marks in immature specimens of O. platypternus (and the juveniles of other xenacanth species; see Soler-Gijón 1999) indicates that the growth cyclicity as recorded in the hard tissues was related to water temperature and food availability rather than reproduction.…”

“…The xenacanth dorsal spine was a non-replaced structure, in similar fashion to the dorsal-fin spines of extinct and modern chondrichthyans (Maisey 1978(Maisey , 1979Soler-Gijón 1999;Clarke et al 2002;Ramos 2007;Tovar-Avila et al 2008;Barnett et al 2009). As such, dorsal spines have been the subject of histological and skeletochronological studies (Dick 1981;Soler-Gijón 1999;Soler-Gijón and Siebert 2001;Turner and Burrow 2011).…”

Morphology and histology of acanthodian fin spines from the late Silurian Ramsåsa E locality, Skåne, Swede

“…Developmental data of the fin spines from extant Callorhinchus, Heterodontus, and Squalus show that the central cavities house a supporting cartilage that extends to the very tip in mature spines (Maisey, 1979;Tovar-Ávila et al, 2008;Jerve et al, 2014). Oval to rounded central cavities are present in morphotypes D-G (Figures 5 and 6) and decrease in size apically (Figures 5 and 6.3), suggesting the presence of a supporting cartilage that extends to the tip.…”

“…The morphology and histology are investigated (where possible) from synchrotronmicrotomography data collected from the European Synchrotron Radiation Facility in Grenoble, France. These data are used to compare the Ramsåsa fin spines to other fossil taxa and the current growth models for extant sharks described by Maisey (1979), Tovar-Ávila et al (2008), and Jerve et al (2014).…”

Comparative rates of growth of the Port Jackson shark throughout its southern Australian range

“…Annual increment periodicity has been partially verified in the vertebrae of the Port Jackson shark (Ramos 2007); however, annual increment formation has been validated for the dorsal fin spines of the species (Tovar-Á vila et al 2008). Given that corresponding sectioned fin spines and sectioned vertebral centra possess equivalent counts of growth increments (Tovar-Á vila et al 2009a), it was assumed that counts of sectioned vertebrae accurately estimated the ages of Port Jackson sharks.…”