Studies On The Adaptive Evolution Of Visual Organ Of Oxudercine Gobies

Natural selection plays a key role in the adaptive evolution and attracts wide attetion on theoretical research in biology. To adapt their different inhabiting environments, fish species have formed corresponding unique sensing, motion and respiration system through the long process of evolution. These morphological or physiological specializations are obviously outstanding especially for fishes which live in special habitats. Oxudercine Gobies, which live at intertidal area, are described as a unique transitional biota showing specialized tissues and organs to adapt to their amphibious life. In the past studies, researchers focused on the taxonomy, breeding, gill morphology and respiration efficiency rather than the field of vision.There are totally 10 genera and 41 species of Oxudercinae live in the intertidal mangrove or muddy beach area of tropical, subtropical and temperate waters. This study observed the SEM (scanning electron microscope) of the cornea's morphology and microstructure (include the corneal epithelial cell density, cell type, type of microstructure, microridge and microridge separation width) of'amphibious'fishes. Then the image information was collected for statistical analysis to study the adaptive morphology characteristics matching amphibious environment. By comparing the corneal surface micro structure morphology and structural differences, as well as the association to the gobbies'habitat, the results are shown as below:Microridge would be found on all the 6 species of Oxudercinae. The larva Boleophthalmus pectinirostris and larva Periophthalmus magnuspinnatus both possessed the four types of microstructure. The corneal cell density, microridge width and microridge separation width of both species larvae were all greater than the value of adults. Reticular-type cell and mix type cell can be observed on adult B. pectinirostris whereas only reticular-type cell were on adult P. magnuspinnatus. The microvilli appeared on the epithelial cell of Oxuderces dentatus, Scartelaos histophorus, B. pectinirostris and P. magnuspinnatus. The microstructure microholes existed on the epithelial cell of Pseudapocryptes elongates, O. dentatus, B. pectinirostris and P. magnuspinnatus. Microridge-type of cells appear on Apocryptodon punctatus, P. elongates, S. histophorus and B. pectinirostris. The corneal epithelial cell density of the 6 species of Oxudercinae is ranked as:B. pectinirostris< S. histophorus< A. punctatus< P. elongates< O. dentatus< P. magnuspinnatus; The microridge width ranked as:S. histophorus< P. magnuspinnatus < O. dentatus< A. punctatus< P. elongates< B. pectinirostris; The microridge seperation width ranked as:P. magnuspinnatus< A. punctatus< O. dentatus< S. histophorus< P. elongates< B. pectinirostris. This study also showed that the cornea characteristics among 6 species of Oxudercinae and 6 other species (from aquatic to terrestrial), in terms of epithelial cell micro structure and cell type, the order of A. punctatus, P. elongates, O. dentatus, S. histophorus, B. pectinirostris and P. magnuspinnatus is correct. In addition, this study revealed that most Oxudercinae owned the structure of microridge. Microplicae, microholes and microvilli mere more frequently found on the species live at intertidal, mudflats and cave environments (more amphibious); Reticular-type cell and mix-type cell only existed in the species that live at mudflats and cave waters. There was no microvilli on aquatic Oxudercinae. In addition, corneal and habitat data correlation analysis showed a strong correlation between the microridge width and habitat, with a correlation coefficient of 0.8, and corneal cell microridge width appears gradually narrowing trend from aquatic transition to the terrestrial environment.