A STUDY OF CHEMOSENSORY SYSTEMS IN FRESHWATER AND MARINE FISHES OF VIETNAM

Abstract

The sensory systems of tropical fish remain poorly studied despite the richness and diversity of the tropical ichthyofauna. Studies of the olfactory and gustatory systems in fishes of Vietnam have not been previously carried out. The article for the first time provides information about the morphology of the olfactory organ in several species of freshwater and marine fish of families Anabantidae, Pomacentridae, Labridae, Mullidae. The morphological peculiarities of the olfactory cavity, olfactory rosette and lamellae, and ventilation olfactory sacs are discussed. Structures previously unknown for the olfactory organ of fish are described for the first time. The mechanisms of ventilation of the olfactory cavity are considered. Data on the effectiveness of natural taste deterrents and their significance in providing chemical protection for marine animals and plants against fish are presented. The relationships between hosts and symbionts in the example of echinoderms and other invertebrates of coral reefs of Vietnam are discussed. How the animal chemical defense and their coloration are related is analyzed. Data on taste preferences in cultivated fish are obtained.

1. INTRODUCTION

Chemosensory systems are involved in regulation of the most important behavioral patterns in fish. Olfaction provides fish with information about presence of predators and partners in the group, allows fish to search for food, home territory and migration routes, to distinguish conspecifics and their population, hierarchical position, sex and readiness of specimens to spawn [1]. Based on taste system fish evaluate the food organisms and feed with appropriate ones [2]. Most of the information about chemosensory systems concerns fish inhabited water bodies of the temperate zone. Tropical fish, especially coral reef fish, are much less studied despite the richness of the ichthyofauna in low latitudes. Systematic studies of the chemosensory systems of Vietnamese fish have not previously been carried out. The aim of the work that has been carried out in recent years in the Coastal branch of Joint Vietnam-Russia Tropical Science and Technology Research Centre within the framework of the project Ecolan 3.2 - task 2 was to study the morphology of the olfactory organ in fish, the most common in freshwater and marine waters of Vietnam. The aim of the work also included the study of taste reception and its role in the regulation of trophic relationships between fish and invertebrates of coral reefs.

2. MATEARIALS AND METHODS

The study of the morphology of the main structures of the olfactory organ was analyzed in fish caught with a netting gear in the coral reefs of the Nha Trang Bay (the East Sea), afterward fixed in 10% formalin and transferred to 70% ethanol. The examination was conducted using a binocular MBS-1 microscope. An ocular micrometer was applied for the measurements, and Levenhuk M500 Base digital camera was used to take photographs. The taste perception of fish was studied using a method based on the behavioral response of fish to pellets made from agar-agar gel (2%) containing chemicals or water extract of various marine animals and plants (300 mg/ml). Extracts and solutions were prepared on the basis of seawater. The fish were placed in aquariums combined into a closed system with water circulation (29 ± 1°C) through a biofilter. The consumption of grasped pellets, the total number of grasps in a trial, and the pellet retention time in fish mouth were recorded.

3. RESULTS AND DISCUSSION

The olfactory organ in Abudefduf sexfasciatus and A. vaigiensis, like in many damselfish (Pomacentridae), has one nostril. In A. sexfasciatus, the nostril is slightly elongated and slightly widened in the caudal part but in A. vaigiensis in widened in the rostral part. The arrow-shaped olfactory rosette in both species is located on the medial side of the olfactory cavity. In both species, the rosette is oval, elongated in the rostrocaudal direction, it has a similar number of olfactory lamellae, symmetrically attached to the septum, regularly increased in size and changed in shape towards the caudal edge of the rosette. The maximum number of olfactory lamellae in A. sexfasciatus is 15 (in specimens, 13-14.5 cm body length), in A. vaigiensis, 19 (in specimens, 12-15 cm). There is no secondary folding on the lamellae. The absolute dimensions of the olfactory rosette—the length and width, and the length of the olfactory lamellae slightly differ in both species having similar body length. Both species have two ventilation sacs, with the lacrimal sac much larger than the ethmoidal one. The similarity of the general structure of the olfactory organ in these two closely related fish species confirms the existing data on the structural conservatism of the olfactory system in fish (Fig. 1, 2) [3, 4]. There are also obvious differences in the structure of the olfactory organ. In A. vaigiensis, the olfactory rosette occupies a smaller part of the olfactory cavity; in A. sexfasciatus, the rosette fills almost the entire cavity. The close arrangement of the rosette in A. sexfasciatus is achieved mainly due to its smaller size compared to A. vaigiensis. This is typical for fish with a well-developed sense of smell and is considered an adaptation that increases the sensory capabilities of the organ. The process of water exchange in the olfactory organ in Abudefduf differs due to the ratio of the size of olfactory cavity and rosette and different role of olfactory lamellae in this process. In A. vaigiensis, the shape of the olfactory lamellae is extremely peculiar which not found in other fish. The distal part of the largest lamellae is designed so that under the action of water entering the organ, the lamellae move apart, and when water comes out, lamellae fold to the center of the rosette. This streamlines the flows and favors water exchange in the olfactory cavity, but requires additional space. In A. sexfasciatus, the olfactory folds are flat (Fig. 3) [3, 4].