Urea (ka)! The role of urea and oxygen in the relationship between length at first maturity and maximum length in ureotelic and non-ureotelic cartilaginous fishes

The Gill Oxygen Limitation Theory (GOLT) is a single unifying theory which uses the geometric constraints of fish gills to explain their growth, reproduction, and ecology. A major aspect of the GOLT describes that with growth, relative oxygen supply declines, leading to a critical hypercapnic/acidic threshold. This threshold is assumed to trigger maturation and explains why the relationship between maximum length (Lmaxᴰ) and length at maturity (Lmᴰ) is conserved across evolutionary distinct fishes (‘D’ represents the growth of the gills). While well documented in bony fishes, this relationship has only been minimally explored in cartilaginous species. This research addresses this knowledge gap through investigation of chimaera, ray, and shark species in which at least one Lmax/Lm data pair could be obtained from the literature. Using multivariate regression modeling, it was shown that Lmaxᴰ/Lmᴰ is similarly conserved across marine Chondrichthyes (Lmaxᴰ/Lmᴰ≈1.14). However, this relationship is lower than seen in bony fish (Lmaxᴰ/Lmᴰ≈1.35), suggesting that cartilaginous species mature at a greater fraction of their maximum size. Following up on this anomaly, it was hypothesized that the high concentrations of urea retained by marine Chondrichthyes (used for osmoregulation) is the likely reason for this delay. This hypothesis was tested through comparison of marine, euryhaline, and freshwater rays which all retain differing concentrations of urea. Modeling among these groups indicated that decreasing urea retention is correlated to an increase in Lmaxᴰ/Lmᴰ. Notably, freshwater rays (no retention of urea) display an Lmaxᴰ/Lmᴰ ratio (≈1.30) almost identical to bony fishes. It is suggested that the carbon dioxide required for urea production allows ureotelic species to offset the critical maturation threshold, through prolonged maintenance of a neutral internal pH, compared to non-ureotelic species. Furthermore, the increased oxygen demands associated with viviparity (live-bearing) is also correlated to a higher Lmaxᴰ/Lmᴰ ratio, independent of phylogeny or habitat. However, analogous to non-viviparous species is a similar relationship between Lmaxᴰ/Lmᴰ and urea retention, implicating the role of the gills and urea in maturation of these species. Overall, this thesis lends strong support to the existence of a critical maturation threshold while also suggesting a role for urea that was previously unknown.Science, Faculty ofOceans and Fisheries, Institute for theGraduat