Morphological and physiological responses of seagrasses (Alismatales) to grazers (Testudines: Cheloniidae) and the role of these responses as grazing patch abandonment cues

Abstract

Green sea turtles, Chelonia mydas, are grazers influencing the distribution of seagrass within shallow coastal ecosystems, yet the drivers behind C. mydas patch use within seagrass beds are largely unknown. Current theories center on food quality (nutrient content) as the plant responds to grazing disturbances; however, no study has monitored these parameters in a natural setting without grazer manipulation. To determine the morphological and physiological responses potentially influencing seagrass recovery from grazing disturbances, seagrasses were monitored for one year under three different grazing scenarios (turtle grazed, fish grazed and ungrazed) in a tropical ecosystem in Akumal Bay, Quintana Roo, Mexico. Significantly less soluble carbohydrates and increased nitrogen and phosphorus content in Thalassia testudinum were indicative of the stresses placed on seagrasses during herbivory. To determine if these physiological responses were the drivers of the heterogeneous grazing behavior by C. mydas recorded in Akumal Bay, patches were mapped and monitored over a six-month interval. The abandoned patches had the lowest standing crop rather than leaf nutrient or rhizome soluble carbohydrate content. This suggests a modified Giving Up Density (GUD) behavior: the critical threshold where cost of continued grazing does not provide minimum nutrients, therefore, new patches must be utilized, explains resource abandonment and mechanism behind C. mydas grazing. This study is the first to apply GUD theory, often applied in terrestrial literature, to explain marine herbivore grazing behavior.

ncing. About 737 (527 unigenes) clones from the forward library and 757 (483 unigenes) clones from the reversed library were informative. Sequence BlastX analysis showed that there were more transporters and adenosylhomocysteinase-like proteins in E. crassipes cultured in nitrogen deficient medium; while, those cultured in nitrogen replete medium had more proteins such as UBR4-like e3 ubiquitin-protein ligase and fasciclin-like arabinogalactan protein 8-like, as well as more cytoskeletal proteins, including actin and tubulin. Cluster of Orthologous Group (COG) analysis also demonstrated that in the forward library, the most ESTs were involved in coenzyme transportation and metabolism. In the reversed library, cytoskeletal ESTs were the most abundant. Gene Ontology (GO) analysis categories demonstrated that unigenes involved in binding, cellular process and electron carrier were the most differentially expressed unigenes between the forward and reversed libraries. All these results suggest that E. crassipescan respond to different nitrogen status by efficiently regulating and controlling some transporter gene expressions, certain metabolism processes, specific signal transduction pathways and cytoskeletal construction.