Research Overview

Biogeography of Neotropical marine fish faunas

Photographs and data for shorefishes on both sides of the Central American isthmus have led to the creation of comprehensive digital field guides and advanced understanding of fish species distribution. The research included the first quantitative analyses of the geographic limits and large-scale subdivisions of the distributions of the shorefishes of the two regions, and assessed how those patterns relate to environmental variation. This has provided a new appreciation of the regions’ biogeography, particularly in the Caribbean, where it revealed previously unsuspected patterns. We have also used that information to predict the actual size of the fauna of the tropical eastern Pacific and are working on a similar prediction for the Greater Caribbean. While the latter region has one of the most well studied shorefish faunas in the world, recent advances in genetic analyses have forced the realization that many presumed widespread species are in fact complexes of multiple, usually allopatric, species and that the biodiversity of this region is much higher than previously thought.

Deep reef fishes of the Caribbean

Due to their accessibility to scuba diving scientists, most of what we know about the Caribbean area reef-fish fauna relates to shallow water species. However, a substantial part of that fauna comprises deep-reef fishes that live well below the limits of scientific divers. While modern rebreather diving allows access to greater depths, it cannot extend to the lower depth limits of deeper living species.

Submarines provide access to the full range of fish species depths, and, most importantly, allow collecting of specimens. I am a participant in DROP (Deep Reef Observation Program), a joint effort by the Smithsonian’s Natural History Museum and STRI to study deep reef fishes using a manned submersible based at Curaçao.

This represents the first submarine-based collecting of deep-reef fishes, and of information on their depth distributions, in the southern Caribbean. It also represents a resumption of U.S. efforts to document tropical deep reef fish faunas along and near its Atlantic shores, a program that terminated decades ago. Our submarine-based collecting at Curaçao has shown how little we know: in an area of only several hectares of reef adjacent to the submarine base, one third of the deep reef fishes we have collected are new to science, and we have encountered new species at every new site around the shores of that small island that we have visited with the submersible. These discoveries provide the impetus for expanding the activities of this submarine to other parts of the Greater Caribbean.

Invasive marine fishes

Only a small handful of exotic marine fishes native to areas outside the Atlantic and eastern Pacific are known to have successfully invaded the Neotropics. My current research includes study of a new invasion by one such Indo-Pacific species, which was discovered in the southwest Gulf of Mexico in 2013. We are working to determine the site of origin of this species, the mode of its introduction, and the full extent of its current geographic range in the Gulf of Mexico. We are also working on estimating its success relative to its native area, the extent of any ecological impact on native reef fishes in the Caribbean area, and the likelihood of its further spread.

The Cobia, a large predatory fish that can be two meters long and weigh up to 78 kilograms, was recently released into the Eastern Pacific through the escape of many thousands of juveniles from a sea cage aquaculture facility in Ecuador. This entirely predictable event occurred a few months after the facility began operation. Cobia does not naturally occur in the Eastern Pacific or across the remaining eastern three quarters — or 13,000 km — of the Pacific. Some of those escaped juveniles travelled at least 1,000 km to Panama in less than three months after their release, indicating a potential for this highly migratory species to spread rapidly. Fish biologists at all Pacific American countries between Peru and the United States and at potential invasion sites in the central Pacific (the Marquesas and Hawaiian Islands) have been alerted to record data about any Cobia captures. This information will allow documentation of the rate and extent of its spread, and estimation of its success in establishing a resident population in the Eastern Pacific. It is not yet possible to determine if such a population will become established and what impact it will have if that occurs.

Mimicry in tropical reef fishes

Describing new cases of supposed mimicry among tropical reef fishes is a popular pastime among reef fish biologists. However, examples involving precise similarity and multifaceted interactions between a model and a true mimic are relatively rare among such fishes. In many (perhaps most) cases, pairs of species probably independently evolved shared similarities in their appearance. These similarities may have led to the development of behavioral associations between such pairs of species through one learning that there are rewards to be gained from doing so. Ongoing research on this topic includes examination of interactions between models and their supposed mimics, and of geographic variation in degrees of resemblance and associations among such pairs.

Feeding biology of parrotfishes

Parrotfishes are common and abundant members of tropical reef fish faunas. The prevailing wisdom concerning their feeding biology is that they are herbivores that consume benthic macroalgae, and, in doing so, help the establishment and growth of living corals. Recent research pioneered by my collaborators in Australia indicates that for some parrotfish species, this view is erroneous and that they actually consume and digest microorganisms that are either scraped from the surfaces of rocky reef substrata or are imbedded in and excavated from below those surfaces. We plan to test this revisionary idea on a complex of species found on Eastern Pacific reefs, which include not only scraping and excavating species, but also a common hybrid between scraper and excavator species. That test would include the first genetic assessment of what these fish are actually consuming and the first stable-isotope determination of what ingested species they are actually digesting.

Diffusion of information through electronic applications on Neotropical shorefish faunas

A long-standing interest in identification guides for tropical reef fishes led me to begin development of a guide book on Tropical Eastern Pacific shorefishes in 1990. Electronic media allow not only the presentation of large numbers of images and other graphical material very economically but also the inclusion and analyses of quantitative data on fish faunas. Shortly after that guide book was released, tools for adaptation of this material to an electronic format became available, which led to the production of a bilingual CD version of the guide, followed by website adaptation and expansion of that guide. This in turn was followed by an equivalent website for the shorefish fauna of the Greater Caribbean, the sister biogeographic region to the Tropical Eastern Pacific on the other side of the Central American isthmus, and most recently, to iOS mobile versions of the websites for both those faunas. The mobile apps are currently being redeveloped to provide both iOS and Android versions. With the completion of these the material in the websites will become available across all major platform types currently in use. Between them, these two app families, which are unique for tropical marine fishes, cover almost 3,000 species, or about 20% of all named tropical shorefishes.

Besides the updating of information in these to app-families to incorporate new taxonomic information and new data on the ranges and ecology of member fishes, ongoing work on these app families is aimed at two aspects: (1) The incorporation of an Image Identification API (one that has been trained using images from these websites), a small, independent program imbedded within the website that will allow users to identify fish in images taken in each region that are presented to the API in that region’s website; and (2) Expansion of the coverage in each fauna to include deepwater and oceanic species.


B.Sc. University of Queensland, Australia, 1966

Ph.D. University of Queensland, Australia, 1974

Selected Publications

Robertson D R (2013). Who Resembles Whom? Mimetic and Coincidental Look-Alikes among Tropical Reef Fishes. PLOS ONE 8: e54939. doi:10.1371/journal.pone.0054939.

Robertson D R & Van Tassell J (2012). Fishes: Greater Caribbean. An Identification Guide to the shore-fish fauna of the Caribbean and adjacent areas. IOS App for iPhone.

Polidoro B, Brooks T, Carpenter K, Edgar G, Henderson S, Sanciangco J, Robertson D R (2012). Patterns of extinction risk and threat for marine vertebrates and habitat-forming species in the Tropical Eastern Pacific. Marine Ecology Progress Series.448: 93-104

Robertson D R, Cramer K (2009). Marine shore-fishes and biogeographic subdivisions of the Tropical Eastern Pacific. Marine Ecology Progress Series 380: 1-17

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