Re‐evaluation of batoid pectoral morphology reveals novel patterns of diversity among major lineages

Abstract

Batoids (Chondrichthyes: Batoidea) are a diverse group of cartilaginous fishes which comprise a monophyletic sister lineage to all neoselachians or modern sharks. All species in this group possess anteroposteriorly expanded-pectoral fins, giving them a unique disc-like body form. Reliance on pectoral fins for propulsion ranges from minimal (sawfish) to almost complete dependence (skates and rays). A recent study on the diversity of planform pectoral fin shape in batoids compared overall patterns of morphological variation within the group. However, inconsistent pectoral homology prevented the study from accurately representing relationships within and among major batoid taxa. With previous work in mind, we undertook an independent investigation of pectoral form in batoids and evaluated the implications of shape diversity on locomotion and lifestyle, particularly in the skates (Rajoidei) and rays (Myliobatoidei). We used geometric morphometrics with sliding semilandmarks to analyze pectoral fin outlines and also calculate fin aspect ratios (AR), a functional trait linked to locomotion. In agreement with previous work, our results indicated that much of the evolution of batoid pectoral shape has occurred along a morphological axis that is closely related to AR. For species where kinematic data were available, both shape and AR were associated with swimming mode. This work further revealed novel patterns of shape variation among batoids, including strong bimodality of shape in rays, an intermediate location of skate species in the morphospace between benthic/demersal and pelagic rays, and approximately parallel shape trajectories in the benthic/demersal rays and skates. Finally, manipulation of landmarks verified the need for a consistent and accurate definition of homology for the outcome and efficacy of analyses of pectoral form and function in batoids.

First Observations of Long‐Distance Migration in a Large Skate Species, the Winter Skate: Implications for Population Connectivity, Ecosystem Dynamics, and Management

“…Given that batoid pectoral shapes are largely indicative of swimming mode and lifestyle (Rosenberger ; Martinez et al. ), the morphological evidence also appears to corroborate an intermediately mobile Winter Skate, as observed in this study. Together, these independent lines of evidence suggest a need for research focused on skate movements and spatial ecology across a broader range of species and locales—especially for data‐deficient species.…”

“…Lastly, data on pectoral morphology (the primary source of propulsion in most batoids) place skates in an intermediate position between benthic stingrays and highly mobile pelagic rays, with the Winter Skate being average within the skates themselves (Martinez et al 2016a). Given that batoid pectoral shapes are largely indicative of swimming mode and lifestyle (Rosenberger 2001;Martinez et al 2016a), the morphological evidence also appears to corroborate an intermediately mobile Winter Skate, as observed in this study. Together, these independent lines of evidence suggest a need for research focused on skate movements and spatial ecology across a broader range of species and locales-especially for data-deficient species.…”

“…Lastly, data on pectoral morphology (the primary source of propulsion in most batoids) place skates in an intermediate position between benthic stingrays and highly mobile pelagic rays, with the Winter Skate being average within the skates themselves (Martinez et al. ). Given that batoid pectoral shapes are largely indicative of swimming mode and lifestyle (Rosenberger ; Martinez et al.…”

“…Recent work has begun to document large-scale movements in other species, such as the Thornback Skate Raja clavata (Hunter et al 2005) and Big Skate R. binoculata (Farrugia Additionally, a recent review of skate movement studies revealed that an inherent lack of biotelemetry approaches compared to conventional tagging has led to significant underestimations of skate dispersal capabilities across many species (Siskey et al 2019). Lastly, data on pectoral morphology (the primary source of propulsion in most batoids) place skates in an intermediate position between benthic stingrays and highly mobile pelagic rays, with the Winter Skate being average within the skates themselves (Martinez et al 2016a). Given that batoid pectoral shapes are largely indicative of swimming mode and lifestyle (Rosenberger 2001;Martinez et al 2016a), the morphological evidence also appears to corroborate an intermediately mobile Winter Skate, as observed in this study.…”

How the Devil Ray Got Its Horns: The Evolution and Development of Cephalic Lobes in Myliobatid Stingrays (Batoidea: Myliobatidae)

“…Manta rays (also known as devil rays) and their relatives of the family Myliobatidae exhibit modified body plan features that arose in association with the evolution of oscillatory swimming and invasion of the pelagic environment (Rosenberger, 2001;Schaefer and Summers, 2005;Mulvany and Motta, 2013;Hall et al, 2018). While myliobatids share classic components of the batoid body plan, including dorsoventral compression and pectoral fins that expand anteriorly and fuse to the head, they also exhibit derived morphological modifications that are unique to the family, such as anterior projections called "cephalic lobes" that are used to locate and manipulate prey during feeding (Sasko et al, 2006;Mulvany and Motta, 2014), loss of anterior-posterior disc symmetry typical of most skates and rays (Hall et al, 2018) and laterally extended pectoral fins that arose in association with oscillatory swimming (Fontanella et al, 2013;Franklin et al, 2014;Martinez et al, 2016). While most batoids use their broad disc or diamond-shaped pectoral fins for both swimming and feeding, the myliobatids exhibit functional separation of swimming and feeding behaviors into the pectoral fins and cephalic lobes, respectively, which have been optimized for those specific tasks (Mulvany and Motta, 2013;Hall et al, 2018).…”