Electric organ of skates (cartilaginous fishes related to sharks) and the use of the pelvic fins of skates for locomotion

David M. Koester, Ph.D.
Department of Anatomy
College of Osteopathic Medicine
207-283-0171, extension 2207

My research interests are in vertebrate functional morphology. Functional morphology is a discipline that relates the structure of an organ to its function.

koester1Skates, (basically flat sharks), have been my research animal of choice because of their abundance along coastal waters and their adaptability to captive conditions. In addition, skates possess several relatively unique organ systems that have received little attention in the scientific community.

Electric Organ
One of these systems is the electric organ. The electric organ of skates is a paired structure located in the tail. The cells that comprise the electric are called electrocytes and are modified skeletal muscle fibers. The electric organ emits a weak electric signal or discharge, but the precise role of the electric organ of skates is not known. However, it is likely involved in some type of communication function since skates also possess a sensory system that is able to detect the electric organ discharge.

Punting
Another unique feature of skates is that, in addition to swimming, they also possess a pedestrian type of locomotion called punting. Skates make use of a modified portion of their pelvic fins (called crura; meaning leg-like) to generate thrust for locomotion by pushing off the ocean floor. These two structures, the electric organ and the pelvic fins, have been the primary focus of my research at UNE. The significance of my research is based upon the fact that skates are a dominant and important component of the benthic ecosystem and that some species are currently being over-fished either incidentally as by-catch or as a targeted commercial fishery. Much remains to be learned about many aspects of their behavior and life history, both of which have significant impact on other ground-fish species.

Recently I investigated how skates use their pelvic fins for locomotion along the bottom. Swimming has always been considered to be the dominant form of locomotion in skates. However, anecdotal reports in the literature and personal observations of skates in captivity indicate that skates may use their pelvic fins to “walk” along the bottom. Therefore, along with Carl Spirito, Ph.D. (COM Physiology Department), I investigated the use of the pelvic fins for locomotion along the bottom in the little skate, Leucoraja erinacea, by recording and analyzing locomotor behavior both in the both in the field and in captivity. We also examined the structure of the pelvic fin, specifically the crural portion, using various anatomical preparations.

koester2The skeletal elements and musculature of the crura are highly modified compared to the rest of the pelvic fin and comprise a functionally distinct appendage that possesses three flexible joints. Locomotion of skates along the bottom is almost always due to the exclusive activity of the crura of the pelvic fins pushing off the substrate synchronously to generate thrust. The skate then glides a short distance as the crura are repositioned for the next thrust phase. This type of thrust and glide locomotion is called punting. From our study we conclude that punting is a significant form of locomotion suited to the benthic lifestyle of skates. If you are interested in seeing some of the video of skates punting in the field and in captivity please go to my website.

Presently, I am engaged in a detailed analysis of the musculoskeletal and peripheral nervous organization within the pelvic fins to understand further the mechanics of the crura of the pelvic fin during punting.

Locomotion is an activity that is vital to many other behaviors such as feeding, migration, reproduction and escaping predators. Therefore, an understanding of skate locomotor activity will provide additional insight on the life history of skates.

Furthermore, the crura of the pelvic fins of skates closely resemble the tetrapod limb. Since the tetrapod limb evolved from the fins of fish-like ancestors, information on the biomechanics of the skate pelvic fin will provide information on how the ancestral tetrapod limb might have been used in the aquatic environment and the physical and mechanical constraints placed on its evolution in an aquatic environment.

Central Motor Pathways
My research efforts on the electric organ are aimed at identifying (using dissection and neuroantomical tracing techniques) the central motor pathways involved in the control of the electric organ discharge. Motor pathway components identified thus far include: (1) the location of spinal cord motoneurons (electromotoneurons) that innervate the electric organ, and (2) the location of the medullary neurons that form a descending spinal pathway to the electromotoneurons. The next step is to identify input from higher brain centers to the medullary neurons that form the descending spinal pathway.

Current research activities focus on the electric organ of skates (cartilaginous fishes related to sharks) and the use of the pelvic fin of skates for locomotion.

I have also started to investigate the development of the electric organ to better understand the mechanisms whereby mature skeletal muscle fibers are transformed into the electrocytes of the electric organ. The initial phases involve identifying the stages of development of the electrocytes in embryos and neonates.

Recent Publications

Koester, D.M. Punting: An Unusual Mode of Locomotion in the Little Skate Leucoraja erinacea (Chondrichthyes: Rajidae), Copeia 2003(3):553-561.

Koester, D.M. Anatomy and Motor Pathways of the Electric Organ of Skates, The Anatomical Record 273A:648-662 (2003).
    
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