Brain Size and Brain Organization of the Whale Shark, <i>Rhincodon typus</i>, Using Magnetic Resonance Imaging

Abstract

Very little is known about the brain organization of the suction filter feeder, Rhincodon typus, and how it compares to other orectolobiforms in light of its specialization as a plankton-feeder. Brain size and overall brain organization was assessed in two specimens of R. typus in relation to both phylogeny and ecology, using magnetic resonance imaging (MRI). In comparison to over 60 other chondrichthyan species, R. typus demonstrated a relatively small brain for its body size (expressed in terms of encephalization quotients and residuals), similar to the lamniforms Carcharodon carcharias, Cetorhinus maximus, and Carcharias taurus. R. typus possessed a relatively small telencephalon with some development of the dorsal pallium, which was suggestive of moderate social behavior, in addition to a relatively large diencephalon and a relatively reduced mesencephalon. The most notable characteristic of the brain of Rhincodon was a large and highly foliated cerebellum, one of the largest cerebellums within the chondrichthyan clade. Early development of the brain was qualitatively assessed using an in situ MRI scan of the brain and chondrocranium of a neonate specimen of R. typus. There was evidence that folding of the cerebellar corpus appeared in early development, although the depth and number of folds might vary ontogenetically in this species. Hierarchical cluster analysis and multidimensional scaling ordinations showed evidence of convergent evolution with the basking shark, Cetorhinus maximus, another large-bodied filter feeding elasmobranch, supporting the claim that organization of the brain is more similar in species with analogous but independently evolved lifestyles than those that share taxonomic classification.

Does more maternal investment mean a larger brain? Evolutionary relationships between reproductive mode and brain size in chondrichthyans

“…Data on brain weight and bodyweight for 85 chondrichthyan species were obtained from previously published studies (Northcutt 1977(Northcutt , 1978Kruska 1988;Ito et al 1999;Yopak et al 2007;Lisney et al 2008;Yopak and Montgomery 2008;Yopak and Frank 2009). Data on reproductive mode were collected from published literature (Dulvy and Reynolds 1997;Last and Stevens 2009) and each species was categorised into one of two reproductive modes, namely lecithotrophic or matrotrophic ( Fig.…”

“…Lecithotrophic species exhibit a wide range of fecundities, from 2 to 300 pups per litter (Cortés 2000), and species with the largest litter sizes tended to have the lowest relative brain sizes (i.e. Rhincodon typus, Notorhynchus cepedianus) (Yopak et al 2007;Yopak and Frank 2009). Indeed, in the construction of the pGLSs to explain apparent differences in the relative brain size, the best-fit models according to AIC criterion included both gestation length and maximum litter size (C. Mull, unpubl.…”

“…Relative brain size and the relative development of major brain areas have been correlated with ecology (i.e. habitat type, feeding mode), and these patterns do not necessarily follow phylogenetic groupings (Yopak et al 2007;Lisney et al 2008;Yopak and Montgomery 2008;Yopak and Frank 2009), similar to patterns seen in other vertebrates, such as teleosts, birds and mammals (Huber et al 1997;Kotrschal et al 1998;de Winter and Oxnard 2001). There is some evidence that reproductive mode may be correlated with variation in brain size and organisation; the sharks with placental viviparity, such as the Carcharhinidae and Sphyrnidae, have among the largest relative brain sizes within chondrichthyans (Yopak et al 2007), although this has not as yet been statistically tested.…”

A conserved pattern of brain scaling from sharks to primates

“…As a group, these species sit closer to the first divergence of vertebrates, where multiple solutions to fundamental adaptive problems might have emerged and been stabilized, with mammals representing only one branch of this early tree. Chondrichthyans occupy a wide range of aquatic niches (14), have an extremely wide range of body size (15), and exhibit substantial variations in brain size and brain organization (16)(17)(18)(19)(20)(21). Because neurons are generated widely within the fish brain throughout life (22,23), engagement of neurogenesis in some aspect of later life history, which is not a major factor in mammals, also might alter numerical relationships established in early development (24).…”

Comparative Brain Collections Are an Indispensable Resource for Evolutionary Neurobiology

“…All modern studies of cetacean brains depend on salvage operations to obtain material [Butti et al, 2009] and many other studies have similarly taken advantage of salvaging material [Kalisinska, 2005;Yopak et al, 2007;Yopak and Frank, 2009]. Given the current funding and permitting restrictions, it is going to become increasingly difficult to procure many species, making it even more important to collate datasets, take advantage of existing brain collections and salvage specimens whenever possible.…”