Negative allometric growth during ontogeny in the large pelagic filter-feeding basking shark

Abstract

Many pelagic shark species change body and fin shape isometrically or by positive allometry during ontogeny. But some large apex predators such as the white shark Carcharodon carcharias or the tiger shark Galeocerdo cuvier show distinct negative allometry, especially in traits related to feeding (head) or propulsion (caudal fin). In particular, changes in propulsion are attributed to a shift in swimming mode. The more heterocercal caudal fin of younger individuals with its large caudal fin span seemingly aids in hunting small, agile prey. In contrast, the less heterocercal caudal fin with a larger fin area in larger individuals aids a long-distance slow swimming mode. We were interested if negative allometric effects can be observed in a planktivorous shark, the basking shark Cetorhinus maximus, a large species adapted to long-distance slow swimming. To address this question, we compared three size classes, specifically < 260 cm (juveniles), 299-490 cm (subadults), and from adults > 541 cm total length. Comparing literature data, we found negative allometric growth of the head and of the caudal fin, but a more rapid decrease of relative caudal fin size than of relative head length. Hereby, we provide the first evidence for early negative allometric growth of the caudal fin in a large pelagic filter-feeding shark. Our study further demonstrates that ecomorphological approaches may add valuable insight into the life history of animals that are challenging to study in their natural habitat, including large roving sharks such as the basking shark.

Length–weight relationships for six elasmobranch species from the Adriatic Sea

“…The observed growth type seems to confirm that small‐bodied sharks allegedly undergo isometric morphological growth and don't change their body shape during their life, as already observed in previous studies (Ahnelt et al, 2020; Fu et al, 2016; Irschick et al, 2017; Irschick & Hammerschlag, 2015). Other studies provided different b values for the species considered, sometimes suggesting an allometric growth type.…”

Using pictographs as traits to explore morphological diversity in sharks

“…These discrepancies arise most frequently in the measurements from rostrum tip to second dorsal fin origin, to the pelvic origin, and to the anal fin origin and infrequently in the rostrum tip to first dorsal fin origin as well as in the fin length measurements (Figure 1). Nonetheless, these residuals, on average −0.31% across all species and measurements, are well within the range of proportional measurements from actual specimens, on average 1.61% across all species and measurements (Ahnelt et al, 2020; Cooper et al, 2020; Yano et al, 2007; Data S1). This validates the collated pictographs as representative of actual specimens and bolsters the use of pictographs as morphological trait metrics.…”

Assessing the impact of captivity on the growth and development of pyjama sharks (Poroderma africanum)

“…The overall trend for growth in pyjama sharks within this study suggested isometric growth for females and negative allometry for males regardless of being captive or wild-caught. Previous studies have suggested that small-bodied sharks that exhibit little movement typically follow an isometric growth pattern (Irschick and Hammerschlag 2015; Ahnelt et al 2020). Allometry in sharks is likely driven by habitat change (Gayford et al 2023), therefore sexual segregation across habitat could be considered as the primary driver for negative allometry in males.…”