title: Embryonic Development of the Grass Shrimp, Palaemonetes pugio, and the Influence of Salinity on Cardiac Physiology
Creators: name: Romney, Amie Lynn
Publisher: name: University of Nevada, Las Vegas
Year: 2011

description: The grass shrimp, Palaemonetes pugio, inhabit the brackish waters along the Atlantic coast of North America and are a key component of the estuarine ecosystem. These decapod shrimp can tolerate large fluctuations in environmental parameters including daily and seasonal fluctuations in salinity between 0 to 55 parts per thousand (seawater at 30-32 ppt). Any observed distribution patterns of adult P. pugio in relation to salinity may be accounted for by their relative ability to tolerate a range in salinity and their ability to maintain internal water volumes and cardiac performance in the earliest life stage, the embryonic period. This thesis describes embryonic development and cardiac physiology of P. pugio embryos at standard conditions (20˚ C, 30-32 ppt seawater) and under the influence of hypersalinity (20˚ C, 45 ppt seawater). By investigating developmental physiology, fundamental mechanisms of cardiac compensatory responses can be evaluated in face of osmotic stress induced by variable salinity.
A concise description and understanding of embryonic development is necessary to provide the foundation for the further investigation of physiological development. Here, we establish for the first time, a staging scheme for the embryonic period of development for the grass shrimp using an 8-stage sequence. Under constant conditions, (20˚ C, 30-32 ppt ), embryos were described by observable morphological changes using photo- and video-microscopy. The mean time for embryonic development from fertilization to eclosion was 13 days and mean clutch size was 190 (±5) embryos per female. The heart begins to contract at stage VIb. Using standardized nomenclature for decapod development to describe the ontogenetic sequences and cardiac maturation of P. pugio, patterns of development and cardiac physiology can be more accurately addressed.
The cardiovascular system is the earliest system to become functional during development and so it can be utilized to examine developmental strategies of basic physiological processes. Specifically, cardiac function should be linked to embryonic maturation through the establishment of internal convective processes to meet increasing metabolic demands. Cardiac physiological parameters (heart rate, stroke volume, and cardiac output) have been characterized here in embryos of the grass shrimp under standard conditions (20˚ C, 30-32 ppt). Furthermore, the relationship between the initiation of cardiac function and embryonic growth was examined by measuring dimensional growth of the egg, water content and metabolizing mass. It was found that the initiation of cardiac contraction occurs during a time when the embryo outgrows the diffusive capabilities of the chorion. The transition to internal convection is a likely attempt to sustain metabolic homeostasis for further development and growth of the embryo.
The effects of osmotically driven water movements between animal and environment are known to influence cardiovascular function in many species. However, the embryos of the grass shrimp are exposed to similar fluctuations in salinity as are the adults yet lack the ability to osmoregulate. In order to examine the ability of grass shrimp embryos to adjust cardiac parameters in response to osmotically driven water exchanges, cardiac parameters (heart rate, stroke volume, and cardiac output) were measured in embryos of P. pugio when transferred from control conditions (20˚ C, 30-32 ppt) to hypersaline conditions (20˚ C, 45 ppt). In higher salinity seawater, embryos showed a significant decrease in embryonic water content, egg volume, and surface area that likely suggest a decrease in the volume of hemolymph within the cardiovascular system. In response, embryos showed observable compensations for hemolymph volume loss by increasing heart rate and decreasing stroke volume to maintain optimal levels of cardiac output that are necessary for convective transport. Demonstrations of a homeostatic role in cardiac responses to environmental stress may imply early development of cardiac regulation necessary for P. pugio survival.
Invertebrate cardiovascular physiology holds fundamental clues to evolutionary conserved mechanisms of cardiac regulation that exist in many advanced animal forms. Early developmental patterns in physiological ontogeny also supply a basic understanding of homeostatic function in adults. By investigating the effects of fluctuating salinities on cardiovascular development and physiology in the grass shrimp, P. pugio, we can better understand their environmental tolerances and survivability. This can lead to accurate predictions of the effects of future global climate change on decapod populations.