What Could Chemistry Tell Us About the Ecology of
Sharks and Rays?
Sharks and rays are a group of fishes that are referred to as elasmobranchs. This designation refers to the multiple gill slits (5-7) they have, an arrangement which is different from the gill openings of other fishes. Another major difference between sharks and rays and other fishes is that the skeleton of elasmobranchs is made up of cartilage rather than bone. Sharks and bony fishes alike are exposed to wide range of naturally occurring elements that are dissolved in the water that surrounds them. These elements may enter a fish and become incorporated into their bodies’ as a byproduct of respiration and feeding. Some elements will be excluded from the body and the concentration of others will be carefully regulated. However, the concentration of some elements that are taken up by fishes can reflect the concentrations that they were exposed to in seawater. As a fish grows and time passes, distinctive concentrations of elements that are incorporated into the hard body parts of fishes – such as the ear bones or otoliths of bony fishes or vertebrae of sharks and rays – can provide an indication of the type of environment in which an individual lived. Trophic level, temperature, depth, salinity, latitude, and habitat are some factors that influence the chemical composition and concentration within bony fishes, producing differing “elemental signatures”. Elemental signatures, including trace metal and stable isotopes, have proven to be powerful markers in ecological studies, providing insight into movement patterns, population structure, habitat use, connectivity, and natal origins.
Many types of fishes rely on nursery areas, distinct locations in which the early life stages grow and live which is separate from adult habitat. The use of nursery areas is widespread among sharks and rays. Small and large species alike typically give birth in shallow estuaries, lagoons, or coastal areas that are not occupied by adults. This segregation of juvenile and adult habitat creates a strong potential for distinctive elemental compositions to be incorporated into growing individuals as a result of the differing physical and chemical concentrations of these nursery areas. If these natural markers are incorporated into the vertebrae of sharks and rays as they grow, these signatures could provide not only a new way to distinguish natal origins but also provide critical insight into habitat use, dispersal, and extent of population mixing. The possibility of tracking and identifying groups of fishes based on elemental signatures may offer considerable time- and cost-saving advantages over traditional methods of assessing population dynamics that are essential for the development of conservation and management strategies. However, these tools have never been applied to investigations of shark and ray populations.
Sharks and rays lack the calcified structures, known as otoliths, that have been successfully used for studies of movement patterns and natal origins in bony fishes. Otoliths are metabolically inactive, grow throughout the lives of fish, and are deposited in distinctive alternating annual bands from which ages can be determined. The calcified cartilaginous vertebrae of sharks and rays also continue to grow throughout their lives and develop alternating annual bands from which ages can be determined, but little is known of the mineralization of vertebrae. The potential for metabolic activity within their cartilaginous cellular matrix has raised questions about the ability of these structures to preserve chemical signatures. Resorption or physiological reworking of this cellular matrix may alter vertebral chemical composition during the course of a lifetime and limit their usefulness as a record of past environments in which an individual has lived.
Project Objectives
Could the vertebrae of sharks and rays reveal a chemical record of past locations and environments that an individual lived in? If so, distinct, traceable signatures that distinguish the location or region of birth has enormous potential for management, as it could be used to better understand movement patterns, identify nursery grounds, and identify sites which could function as highly effective marine reserves. The purpose of this study is to determine if elemental and stable isotopic signatures incorporated into the vertebrae of young-of-the-year sharks and rays provide distinct, site-specific markers. We are conducting both field and captive laboratory investigations to evaluate spatial and temporal variability of elemental signatures within vertebrae and the relationships between water and vertebral chemical composition.
This study has been developed and is being conducted by: Wade D. Smith, Fernando Márquez-Farías, Jessica A. Miller, and Selina S. Heppell
Funding for this project has been provided, in part, by: the Mamie Markham Research Award (HMSC),
National Science Foundation Small Grants for Exploratory Research, and OSU Aquaculture Collaborative Research Support Fellowship
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