Allometric Scaling of the Optic Tectum in Cartilaginous Fishes

Abstract

In cartilaginous fishes (Chondrichthyes; sharks, skates and rays (batoids), and holocephalans), the midbrain or mesencephalon can be divided into two parts, the dorsal tectum mesencephali or optic tectum (analogous to the superior colliculus of mammals) and the ventral tegmentum mesencephali. Very little is known about interspecific variation in the relative size and organization of the components of the mesencephalon in these fishes. This study examined the relative development of the optic tectum and the tegmentum in 75 chondrichthyan species representing 32 families. This study also provided a critical assessment of attempts to quantify the size of the optic tectum in these fishes volumetrically using an idealized half-ellipsoid approach (method E), by comparing this method to measurements of the tectum from coronal cross sections (method S). Using species as independent data points and phylogenetically independent contrasts, relationships between the two midbrain structures and both brain and mesencephalon volume were assessed and the relative volume of each brain area (expressed as phylogenetically corrected residuals) was compared among species with different ecological niches (as defined by primary habitat and lifestyle). The relatively largest tecta and tegmenta were found in pelagic coastal/oceanic and oceanic sharks, benthopelagic reef sharks, and benthopelagic coastal sharks. The smallest tecta were found in all benthic sharks and batoids and the majority of bathyal (deep-sea) species. These results were consistent regardless of which method of estimating tectum volume was used. We found a highly significant correlation between optic tectum volume estimates calculated using method E and method S. Taxon-specific variation in the difference between tectum volumes calculated using the two methods appears to reflect variation in both the shape of the optic tectum relative to an idealized half-ellipsoid and the volume of the ventricular cavity. Because the optic tectum is the principal termination site for retinofugal fibers arising from the retinal ganglion cells, the relative size of this brain region has been associated with an increased reliance on vision in other vertebrate groups, including bony fishes. The neuroecological relationships between the relative size of the optic tectum and primary habitat and lifestyle we present here for cartilaginous fishes mirror those established for bony fishes; we speculate that the relative size of the optic tectum and tegmentum similarly reflects the importance of vision and sensory processing in cartilaginous fishes.

Ontogenetic Shifts in Brain Organization in the Bluespotted Stingray <b><i>Neotrygon kuhlii</i></b> (Chondrichthyes: Dasyatidae)

“…The mesencephalon was subdivided into the optic tectum and the tegmentum ( Fig. 1 ) using the criteria of Smeets et al [1983], Smeets [1998], and Yopak and Lisney [2012]. The caudal boundary of the tectum was set at the velum medullare anterius and the rostrodorsal boundary was set at the commissura posterior.…”

“…Given that relationships between brain organization and behavior are well-established in teleosts [e.g., Kotrschal et al, 1998] (and to an increasing extent in chondrichthyans [e.g., Yopak et al, 2007Yopak et al, , 2010Yopak et al, , 2015Lisney et al, 2008;Yopak and Lisney, 2012]), it is tempting to surmise that the ontogenetic changes in brain organization we have reported here for N. kuhlii also re- Sex and maturity status (fixed factors) and the interaction term between them (sex*maturity) were considered a priori to potentially have a significant influence on the scaling of overall brain size, and so were also included in the GLM. Statistically significant results (i.e., p < 0.05) are shown in bold.…”

Quantitative Classification of Cerebellar Foliation in Cartilaginous Fishes (Class: Chondrichthyes) Using Three-Dimensional Shape Analysis and Its Implications for Evolutionary Biology

“…In the future, when applied across a broad multi-taxon dataset, the SWD will allow for a better-resolved, phylogenetically-informed comparative analysis of cerebellar foliation and the allometric and ecological parameters that may be driving brain structure variation [e.g. Yopak, 2012a; Yopak and Lisney, 2012; Yopak et al, 2015]. Further, as the true degree of intraspecific variation in foliation is currently unknown [Puzdrowski and Gruber, 1992; Yopak and Frank, 2009; Ari, 2011], the SWD can provide a powerful means to assess variability within a single species, for quantifying behavioral and/or cognitive differences between individuals throughout ontogeny or between populations, to better assess the links between cerebellar foliation and behavioral specializations.…”

PSA‐NCAM expression in the teleost optic tectum is related to ecological niche and use of vision in finding food

“…The optic tectum and retina grow continuously throughout a fish's life (Marcus et al ., ) and relative size of the adult tectum is correlated with an increased reliance on vision, characteristics of primary habitat and lifestyle in general (Yopak & Lisney, ). The teleost's optic tectum is a neatly laminated structure (Ebbesson & Vanegas, ) consisting of paired lobes that form the roof of the mesencephalon (Butler & Hodos, ).…”

“…: +38 931 505615; email: mheffer@mefos.hr 474 I . L A BA K E T A L. reliance on vision, characteristics of primary habitat and lifestyle in general (Yopak & Lisney, 2012). The teleost's optic tectum is a neatly laminated structure (Ebbesson & Vanegas, 1976) consisting of paired lobes that form the roof of the mesencephalon (Butler & Hodos, 2005).…”