Jonathan Sandoval-Castillo is a postdoc at Flinders University. He is a phylogeographer that integrates molecular and ecological data to study the evolution of elasmobranchs. Jonathan shares his recent work on the cryptic lineages and speciation of guitarfish.
Jonathan sampling elasmobranch tissue from artisanal fisheries. Jonathan visited over 30 artisanal fishery camps around the Gulf of California and the Baja California Peninsula during his PhD fieldwork.
Institute. Flinders University
Academic life stage. Postdoc
Major research themes. I am attracted to the biogeography, phylogeny, and evolution of marine organisms, especially the speciation process in elasmobranchs (sharks and rays). I am interested in the integration of molecular and ecological approaches to elucidate evolutionary histories in aquatic ecosystems.
I study guitarfish. These fish show an intermediate body shape between sharks and rays, and are a diverse group of elasmobranchs with several species living in sympatry. They are highly abundant and, being predators, they are an important component of coastal benthic ecosystems. In addition, some species have high value meat and are the main component of several artisanal fisheries in developing countries. However, guitarfish are also one of the vertebrate groups most vulnerable to overexploitation. Two main factors constrain their effective management and conservation: a lack of basic biological information and numerous difficulties surrounding proper identification of the species.
Current study system. I study guitarfish. These fish show an intermediate body shape between sharks and rays, and are a diverse group of elasmobranchs with several species living in sympatry. They are highly abundant and, being predators, they are an important component of coastal benthic ecosystems. In addition, some species have high value meat and are the main component of several artisanal fisheries in developing countries. However, guitarfish are also one of the vertebrate groups most vulnerable to overexploitation. Two main factors constrain their effective management and conservation: a lack of basic biological information and numerous difficulties surrounding proper identification of the species.
Recent paper in JBI. Sandoval-Castillo J, Beheregaray LB (2020) Oceanographic heterogeneity influences an ecological radiation in elasmobranchs. Journal of Biogeography 47:1599–1611. https://rdcu.be/b4fuY
Motivation for this paper. Speciation is one of the most important and least understood processes in nature. Most biologists agree that species are fundamental biological units for several ecological and evolutionary processes. However, contention still exists about the definition, delimitation, and origin of species. This challenges the study of processes and mechanisms that create and maintain biodiversity. This is especially true for elasmobranchs. Despite elasmobranchs being a charismatic and highly diverse group of vertebrates, they are underrepresented in the scientific literature and very little has been done to decipher the main mechanisms by which new species of sharks and rays originate. However, because of elasmobranchs’ relatively moderate to high mobility, we expect that ecological isolation plays a major role in their diversification. To test this hypothesis, we selected the guitarfish from the Gulf of California because the group has high diversity in the area. In addition, the Gulf of California has both an active geological history and high oceanographic variability, enabling us to test the relative effect of vicariance events and ecological isolation on diversification of the marine populations inhabiting the region.
A Pseudobatos guitarfish.
Key methodologies. We assessed the role of oceanographic variation in the diversification of guitarfishes (genus Pseudobatos) in the Gulf of California by integrating genetic and environmental datasets. We first used the genetic data (mtDNA sequences and AFLP genotypes) to determine the number of guitarfish lineages present in the Gulf of California and elucidate their phylogenetic relationships. We then combined distribution models and seascape genetic analyses to establish the relative importance of six oceanographic variables that might have affected genetic differentiation between lineages. Finally, we used coalescence models to separate the role of historical geological events from the role of modern oceanographic variation on the diversification of these lineages.
Major results. Our work evidences five distinct lineages of Pseudobatos, with geographic distributions overlapping ecologically discrete bioregions in the studied area. Moreover, genetic differences between lineages are correlated with sharp dissolved oxygen and nutrient concentration gradients between these bioregions. We propose that the bioregions present heterogenous habitat opportunities and a source of divergent selective pressures. These promote metabolic specializations associated with differences in oxygen concentration and diet that together triggered a recent adaptive radiation of Pseudobatos. Our work showcases the role of isolation by environment in generating and maintaining diversity in this group and suggests that mobility might not hinder speciation in sharks and rays. Our study likely represents the first assessment of a recent ecological radiation in elasmobranchs. It also offers a new perspective about the application of integrative approaches to study the effect of divergent selection on biological diversification in the ocean.
The Gulf of California is an excellent system to study biogeography: it has an recent active geological history and high temporal and geographic oceanographic variability (Photos: Israel Sanchez Alcantara).
Unexpected results. We found five distinct lineages of Pseudobatos in the Gulf of California and the Baja California Pacific Coast, including four cryptic lineages. At first, we thought this large number of lineages was a mistake since the 210 samples were identified as just two described species. However, we re-sequenced several samples that validated the presence of these apparently cryptic lineages. Moreover, using museum specimens, Kelsi Rutledge from the University of California recently described subtle but significant morphological differences that discriminate at least two of these lineages, corroborating some of our results and highlighting the need for more exhaustive taxonomic work in the region.
Next steps. I would like to do several genomic analyses on the samples. First, to study the genes involved in this ecological radiation, and second, to perform demographic analysis on a more recent temporal scale and explore the effects of past and current climatic changes. This will help to determine more specific biological and oceanographic factors that promote rapid speciation in these organisms, and in the ocean in general.
If you could study any organism on Earth, what would it be? I would study deep water sharks from the family Etmopteridae, because they are very diverse, can produce bioluminescence, and are poorly studied. There are ~45 species recognized, of which several are considered to have broad geographic distributions, but most likely represent complexes of cryptic species. Some of these complexes would be ideal for studying the speciation process in different stages. The Etmopteridae sharks show several adaptations to deepwater habitats, including bioluminescence. Understanding the evolution of the biochemistry and physiology of these adaptations could be the first step to produce bioluminescence in an ecologically sustainable way. Unfortunately, they are recognized as a group highly susceptible to over-exploitation and human derived climatic change. However, due to the relative inaccessibility of their environment, and the logistical difficulties linked to their maintenance in laboratory, Etmopteridae sharks are poorly studied in general.