Isometric growth in the world's largest bony fishes (genus Mola)? Morphological insights from fisheries bycatch data

Abstract

For teleost fishes, the relationship between morphometric traits can provide significant insight into species life history, however gathering such data for noncommercial species can prove challenging. Here, we use data collected opportunistically from fisheries bycatch and stranding events to assess growth scaling over orders of magnitude in the ocean sunfish (genus Mola). Intriguingly, the confidence intervals for the relationship between length and mass suggests that isometric scaling is likely, a growth pattern rarely observed in fishes owing to the scaling of supportive structures. These data also enabled assessment of geometric morphometrics, which indicated that Mola sp shape varies subtly but significantly ontogenetically, with increased fin area comparative to body area as fish increase in size. More practically, total length emerged as an effective predictor for a range of morphological traits, including mass, fin lengths and surface area, which can provide vital baseline data for fisheries modeling and management.

A review of giants: Examining the species identities of the world's heaviest extant bony fishes (ocean sunfishes, family Molidae)

“…Clarity on both data accuracy and metadata is therefore important to avoid future misunderstandings. On that account, our data summary includes information on a small number of inaccuracies found in recent studies in regard to M1, M3, M9, M12 and M13 (McClain et al ., 2015: Data Deposition; Phillips et al ., 2018: Appendix S1, see Table 2 for details).…”

“…Some of the large M. alexandrini specimens reviewed in this study are of great scientific value, and have been used in previous studies, including in analyses of allometric growth in M. mola (McClain et al ., 2015) and Mola spp. (Phillips et al ., 2018). Scaling with growth is an interesting and important question for the large molid species, which are extreme outliers among marine megafauna in terms of growth trajectories ( e.g ., Nyegaard et al ., 2020; Thys et al ., 2020) and have evolved unusual support structures for their heavy bodies ( e.g ., Phillips et al ., 2018).…”

“…(Phillips et al ., 2018). Scaling with growth is an interesting and important question for the large molid species, which are extreme outliers among marine megafauna in terms of growth trajectories ( e.g ., Nyegaard et al ., 2020; Thys et al ., 2020) and have evolved unusual support structures for their heavy bodies ( e.g ., Phillips et al ., 2018). However, while the body form of the large molid species is superficially similar, species‐specific data is presumably necessary to fully understand scaling in each taxon.…”

“…Several length–weight relationships have been proposed for M. mola (Frafjord et al ., 2017; Lee et al ., 2013; McClain et al ., 2015; Nakatsubo et al ., 2007a; Watanabe & Sato, 2008) and Mola spp. (Phillips et al ., 2018). Direct comparisons are difficult, as some of these include some of the very large M. alexandrini specimens reviewed here.…”

“…As for many molid accounts, it is unclear whether these were actually weighed, although the accuracy of some (e.g., 1228.5 kg: Babushkin, 1947;Berg, 1948Berg, , 1962 et al, 2015) and Mola spp. (Phillips et al, 2018). Scaling with growth is an interesting and important question for the large molid species, which are extreme outliers among marine megafauna in terms of growth trajectories (e.g., Nyegaard et al, 2020;Thys et al, 2020) and have evolved unusual support structures for their heavy bodies (e.g., Phillips et al, 2018).…”