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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Symposium: Functional (secondary) adaptation to an aquatic life in vertebrates 2

Room: Salon A   11:30 am–1:00 pm

Moderator(s): A. Houssaye & F. Fish
AQU2-1  11:30 am  Acoustic fatheads: Parallels in the functional anatomy of underwater sound reception mechanisms in dolphins, seals, turtles, and sea birds . Ketten D/R*, Harvard Medical School/WHOI
Abstract: Cetaceans lack conventional external ears. They have no evident pinnae and it is thought that the occluded residual external auditory canals are likely dysfunctional. Even more remarkable is that the auditory tympano-periotic bullar complex is extra-cranial, residing in a peribullar sinus bordered by the squamosal, occipital, and temporal bones. Pinnipeds, by contrast, have distinct, well-developed sinusoidal, external auditory canals but also valves and tissues that may operate to occlude the canals underwater. Sound conduction mechanisms in diving sea birds and sea turtles are virtually untested and the anatomy minimally investigated. In this study, computerized tomography (CT) and magnetic resonance imaging (MRI) were used to map densities of tissues associated with the outer, middle, and inner ears of five odontocete, two mysticete, two pinniped, three sea turtle, and two sea bird species. Three-dimensional reconstructions of scan data were used to determine species-specific geometry of tissue groups connected to the middle ear or surrounding the ear canal. The analyses show bundles of coherent fatty tissues in contact with the tympanum in all species examined. Densities of these fats are similar across species and are consistent with sound speeds near that of sea water. In seals and birds, these fats sheathed the external auditory canal. In turtles, the fats formed a discrete column communicating with plates on the lateral surface of the head. In odontocetes, the fats formed three distinct bundles: two directed anteriorly along the lower jaw with a third projecting laterally. In mysticetes examined to date, the fats form a single, large, ovoid lateral lobe. These findings suggest that all four marine groups evolved parallel soft tissue mechanisms that act as the primary low impedance channels for underwater sound conveyance to the middle and then inner ear. [Supported by the Mellon Foundation; Seaver Institute; Office of Naval Research; NIH]

AQU2-2  12:00 pm  Hypernatremia in marine snakes: implications for the evolution of a euryhaline physiology. Brischoux F*, Centre d'Etudes Biologiques de Chizé - CNRS
Abstract: Secondary transitions from terrestrial to marine life provide remarkable examples of evolutionary change. Although the maintenance of osmotic balance poses a major challenge to secondarily marine vertebrates, its potential role during evolutionary transitions has not been assessed. However, the widespread relationship between salt excreting structures (e.g., salt glands) and marine life strongly suggests that the ability to regulate salt balance has been crucial during the transition to marine life in tetrapods. In the current presentation, I review the role of oceanic salinity as a proximate physiological challenge for snakes during the phylogenetic transition from the land to the sea. A review of osmoregulatory physiology in species situated along a continuum of habitat use between fresh- and seawater shows that snake species display a concomitant tolerance toward hypernatremia, even in species lacking salt glands. Free-ranging marine snake species usually display hypernatremia despite having functional salt glands. Overall, sea snakes exhibit a marked tolerance to salt load compared to other marine tetrapods and apparently trigger substantial salt excretion only once natremia exceeds a high threshold. Collectively, these data suggest that a physiological tolerance toward hypernatremia has been critical during the evolution of a euryhaline physiology, and may well have preceded the evolution of salt glands.

AQU2-3  12:30 pm  “On the fence” versus “all in”: insights from turtles for functional transitions in the aquatic locomotion of amniotes. Blob RW*, Clemson University; Mayerl CJ, Clemson University; Rivera ARV, Creighton University; Rivera G, Creighton University; Young VKH, Clemson University
Abstract: Though ultimately descended from terrestrial amniotes, turtles have deep roots as an aquatic lineage and are quite diverse in the extent of their aquatic specializations. Many taxa can be viewed as "on the fence" between aquatic and terrestrial realms, whereas others have independently hyperspecialized and moved "all in" to aquatic habitats. Such differences in specialization are reflected strongly in the locomotor system, and we have conducted several studies to evaluate the performance consequences of such variation in design, as well as the mechanisms through which both locomotor specialization and the use of multiple habitats are facilitated in turtles. One path to aquatic hyperspecialization has involved the evolutionary transformation of the forelimbs from rowing, tubular limbs with distal paddles into flapping, flattened flippers, as in sea turtles. Hydrodynamically advantageous for sustained, long-distance swimming, the evolution of such flippers may have been enabled by a reduction in twisting loads on proximal limb bones that accompanied swimming in rowing ancestors, facilitating a shift from tubular to flattened limbs. Moreover, the control of flapping movements appears related primarily to shifts in the activity of a single forelimb muscle, the deltoid. Despite some performance advantages, flapping may entail a locomotor cost in terms of decreased locomotor stability. However, other morphological specializations among rowing species may enhance swimming stability. For example, among highly aquatic pleurodiran turtles, fusion of the pelvis to the shell appears to dramatically reduce motions of the pelvis compared to freshwater cryptodiran species. This could contribute to advantageous increases in aquatic stability among the predominantly aquatic pleurodires. Thus, even within the potential constraints of a body design encased by a shell, turtles exhibit diverse locomotor capacities that have enabled diversification into a wide range of aquatic habitats.

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