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.
Highlights
Recent investigations have shown size-related spatial and trophic guild differences within and between shark species (Irschick and Hammerschlag 2015; Fu et al 2016; Irschick et al 2017)
Using morphological data from three size classes extracted from the extant literature, we examined if negative allometric changes as described for large predatory sharks adapted to a long-distance slow swimming mode occur in the large filter feeding basking shark which exhibits a similar swimming mode (Lingham-Soliar 2005; Sims 1999, 2008; Irschick and Hammerschlag 2015)
We provide the most extensive compilation of morphometric measurements and features of three size classes of the basking shark, C. maximus, representing three ontogenetic stages to date
Summary
Recent investigations have shown size-related spatial and trophic guild differences within and between shark species (Irschick and Hammerschlag 2015; Fu et al 2016; Irschick et al 2017) These studies revealed that small-bodied sharks seemingly undergo isometric morphological changes during ontogeny, i.e., they maintain a similar body shape throughout their life as opposed to many large shark species. The distinctly larger dorsal lobe in juveniles is an indicator of ontogenetic change in the kinematics of swimming during ontogeny (Lingham-Soliar 2005; Irschick and Hammerschlag 2015; Fu et al 2016) Such allometric changes during ontogeny are closely linked to ecological demands
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