Seminars on Science
Marcelo Carvalho

Dr. Marcelo Carvalho is a researcher in the Department of Biology at the Ribeirão Preto campus of the Universidade de São Paulo in Brazil. He is also a Research Associate of the Department of Ichthyology at the American Museum of Natural History. He studies the anatomy and systematics of sharks and rays, both fossil and recent, in order to better understand which species exist or have existed, their characters and evolutionary relationships, and how they have evolved over time.

Dr. Carvalho with a Potamotrygonidae stingray, which occur only in South American rivers. This specimen was collected on the Rio Tocantins, a tributary to the Amazon.
Dr. Carvalho with a Potamotrygonidae stingray, which occur only in South American rivers. This specimen was collected on the Rio Tocantins, a tributary to the Amazon. ©Dr. M.R. de Carvalho

Marcelo grew up in Rio de Janeiro, Brazil, surrounded by a rich local biodiversity. He fell in love with nature at an early age, with a particular interest in fishes. His curiosity was encouraged by his family through numerous camping and fishing trips. Marcelo studied biology at St. Ursula University in Rio, concentrating on zoology, marine biology, and botany. Brazil's wealth of biodiversity increased Marcelo's interest in anatomy as a way of studying the historical relationships between species and encouraged him to pursue a career in systematics and taxonomy.

While in college, Marcelo met some fishermen from a village near Rio, who caught mako sharks that they sold to local markets. Marcelo accompanied them on their fishing trips. In return for beers, the fishermen would give Marcelo the sharks' heads. Back in the lab, Marcelo dissected the shark heads and studied the anatomy of their skulls and muscles, and the patterns of their nerves and veins.

At about this time, Marcelo began an internship at another university, where he further pursued his studies of shark anatomy, including shark systematics, or the study of the evolutionary relationships between sharks. He also started to focus on stingrays and electric rays at this point in his career. In 1992, he applied for and was awarded a fellowship by the Brazilian government that allowed him to travel to the United States to work on a Ph.D. in the systematics of electric rays, in AMNH's Department of Ichthyology. After successfully completing his Ph.D. in 1999, Marcelo continued to work at the Museum as a postdoctoral research fellow, where he collaborated with curator Dr. John Maisey in the Division of Paleontology. In 2001, he ended this assignment to move into an in-house Research Associate position back in Ichthyology, a relationship he continues today from Brazil.

One of Marcelo's current research interests is the systematic relationships of freshwater stingrays, with a focus on the relationships among different genera of stingrays: their diversity and how the different species are related in evolutionary terms. He received a four-year grant at the Universidade de São Paulo to establish a research laboratory and advise students. The grant also allows him to continue research on the evolution of rays—both fossil and living—with a particular focus on the Potamotrygonidae, a family of freshwater stingrays that occur only in South American rivers. During his fellowship in the Museum's Division of Paleontology, Marcelo studied fossil stingrays from extinct freshwater lakes in Wyoming and extinct coral reefs in Italy. He then started to compare the characters of these extinct fossil species to the characters of stingray species still alive today, such as Potamotrygonidae. By studying the form, or anatomy, of these animals, Marcelo is building a picture of their evolutionary relationships.

The characters in stingrays that most often elucidate relationships between genera are found in their skeletal anatomy, sensory pore pattern, and muscles. A fossil provides a preserved two-dimensional skeletal pattern that contains valuable anatomical information. This is a difficult process because stingrays are anatomically conservative, that is, there is not a great deal of variation in the anatomies of different species. When Marcelo manages to find a character that unites two groups of stingrays, it is an important finding. For example, in one study, Marcelo found that the cartilaginous support for the pectoral fins of two genera of Australian stingrays are derived, or unique. In other words, only those two stingray genera have this particular character in common. In evolutionary terms, they are more closely related to each other than to any other genera.

Marcelo maintains his large collections of potamotrygonid stingrays at the Universidade de São Paulo.
Marcelo maintains his large collections of potamotrygonid stingrays at the Universidade de São Paulo. ©Dr. M.R. de Carvalho

A large part of Marcelo's work, and the topic of his dissertation, focuses on the systematics of electric rays. There are some fifty described species of electric rays all over the world. However, the systematics of electric rays is in a state of chaos, needing much work. Since working with the Museum, Marcelo has discovered some twelve new species of electric rays. In addition, Marcelo is producing a complete study of the fossil rays from Monte Bolca. These rays existed during the Eocene (52 million years ago) in a coral reef environment in what is now northeastern Italy. It is one of the richest localities for fossil rays in the world.

"Most people tend to think of elasmobranchs (sharks and rays) as primitive animals, or a lower form of vertebrates," Marcelo reflects. "There is an unfortunate assumption that these animals are not as advanced as bony fishes or more recent animals, simply because they have existed for somewhere between 420 million and 450 million years. The more we examine their anatomy, however, the more we find that elasmobranchs are actually very specialized animals. For example, the particular calcification of their skeletal cartilage is an evolutionary novelty, rather than a primitive or unevolved character. Their longevity is proof of their evolutionary success."

Little is known about elasmobranchs, not only in terms of taxonomy and biology, but also in terms of their biomedical potential. There are only about a dozen people in the entire world who study the taxonomy and systematics of these animals. Marcelo refutes the common attitude toward sharks that they are terrifying predators of the sea that should be eliminated; instead he is a great advocate for the view that sharks and rays are very important to humans. There remains a wealth of information that we have yet to learn about them.

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