Surface architecture of the mouth cavity Surface architecture of the mouth cavity in a carnivorous fish Scomber japonicus (Houttuyn, 1782) (Scombridae)

Chub mackerel Scomber japonicus (Scombridae) is a pelagic species that inhibits warm and temperate coastal waters of the Atlantic, Indian and Pacific Oceans. The topological characteristics of the mouth cavity of the carnivorous fish S . japonicas was studied by using light microscope. The results revealed that all the teeth of S . japonicas are pointed and canine-like, papilliform teeth present on the premaxillae and the anterior regions of the dentaries are associated with seizing, grasping and holding of prey. The morphology and distribution of different taste buds of S . Japonicas were predominantly localized at tongue and the anterior regions of the mouth cavity. Two types of taste buds are elevated from the epithelium at different levels, which may be useful for ensuring full utilization of the gustatory ability of the fish, detection and analyzing of taste substances, as well as for assessing the quality and palatability of food, during its retention in the mouth cavity. Observations of the surface architecture of the mouth cavity of S . japonicas have been discussed within the context of feeding and habitat preferences, as well as ecomorphological adaptation of the species.


Introduction
Chub mackerel Scomber japonicus Houttuyn (1782), is middle size pelagic species, with a very wide distribution over the continental shelf of the tropical and subtropical regions of the Atlantic, Indian, Pacific oceans and adjacent seas, it is primarily coastal species, found from the surface down to 300m depth [5; 8& 24]. Along its distribution, the species is found in isolated population with complex intraspecific structure [19]. Scomber japonicus is a key component in the marine food web of many tropical and subtropical areas of the Atlantic and Pacific oceans. In the Pacific Ocean, they are eaten by albacore Thunnus alalunga [27&4]. Scombrid fishes (Family scombridae) are considered as one of the important fishery resource in the Gulf of Aden. They are caught by the pursesein fishery. Their average landing has been estimated to be around 2202 tons, contributing about 12٪ of the total purse sein catch. Because of the great importance of Scomber japonicus to the economy of Yemeni fisheries, (Gulf of Aden and adjacent area), it was studied by [6,7, 16 and 23].The mouth shapes and buccal cavities of fishes are a good and important component of the digestive tract. It may be involved in the seizure, the selection of food, rejection of undesirable items ingested by fish and the predigesting preparation of food. Among species, buccal or oropharyngeal cavity structure shows great plasticity and structural adaptability for the exploitation of different food items [17,15,31,13 and 1]. The mouth cavity has been described using light microscopy in Oryzias latipes [12],Oncorhynchus rhodurus [20], Tribolodon hakonensis [21] and Barbus bynni, Chrysichthys auratus, Mormyrus kannume and Synodontis schall [1].In any vertebrate species, gustation contributes to the acceptance or rejection of potential foods for survival, since taste buds primarily function in the feeding behavior to detect chemicals from preys. Carnivorous fishes are endowed with taste buds, not only in the oral cavity including gill regions, but also on the lips, barbells, and external skin surface [11,25,10 and 30].Palatability and taste are two terms often used interchangeably which are determined mostly by the chemical characteristics of the food, although its physical properties can also affect the acceptability and final ingestion or rejection of the food [29]. Fish have been often employed as a model for taste research, as they show a higher sensitivity (estimated thresholds for the most potent substances are less than10 -9 M), to tastants than mammals [18].Taste buds are secondary sense organs of the gustatory chemosensory system, it may be useful in assessing the palatability of the food and decide whether to swallow or spit it out [31]. So, the aim of the present work is to examine and provide a better understanding of surface architecture of the mouth cavity of a carnivorous S. japonicas using light microscope.

Materials and Methods
Chub mackerel Scomber japonicus were collected from the fishermen at Mukalla sea (Gulf of Aden) (Fig. 1). These were brought to the laboratory of Biology, Faculty of Science, Hadhramout University. The shape, direction, position of mouth and the distribution of teeth in the different regions of buccal cavity were observed and recorded and, for the measurement of head, 10 specimens for each species were examined and the following measurements were taken

Morphometrical Study
In the present work, the mouth is forward, spacious and elongated with M.O.H. equal to 3.22±0.80cm and M.O.W. was 1.88±0.41cm, having one row of numerous small teeth and palatine teeth, TL ratio was 0.84±0.31. Different measurements were taken, measured and listed in Table  (1) and (Fig. 2).The buccal cavity of S. japonicusis spacious and opens anteriorly through a wide transverse mouth, which is bordered by the upper and the lower lips. The mouth cavity is divided into two regionsthe dorsal roof and the ventral floor. The roof and the floor of the mouth cavity comprised antero-posteriorly, an upper jaw with premaxillary teeth and vomerine teeth, respectively. In the lower jaw, there are dentary teethobserved. Superior and inferior pharyngeal teeth are observed, the last one was found near the gill arch (Fig.3).

Histological Studies
Optical microscope micrographs showed that taste buds are abundant on upper and lower lips of S. japonicus. These taste buds are elevated and located on epithelial protrusion. And they bears pointed canine-like and around between most of the surface of epithelial of the mouth cavity (Figs. [4][5][6][7][8][9].

Discussion
In Scomber japonicus, the position and direction of mouth is suitable for capturing and obtaining food; its terminal and forward direction facilitate the process of predation and indicate the predatory nature of fish under investigation. This result agrees well with that recorded by [9&14].A diversity exists in the form and size of fish teeth. The character of dentition is a clue to the fish's feeding habits and the kind of food it consumes [22].Carnivorous S. japonicas has pointed canine-like teeth, this type of teeth does a good job of grasping and piercing.Our findings agree with studying the teeth of Atherion elymus [28], studying the teeth of carnivorous in Rita rita; [31],studying the teeth of carnivorous Bagrus docmak and the omnivorous Claris gariepinus; [13]studying the teeth of herbivorous Barbus bynni, carnivorous Chrysichthys auratus and Mormyrus kannume and omnivorous Synodontis schall [14]and studying the teeth of carnivorous Epinepheluschlorostigma [14]. Differences described for dentition and distribution of taste buds on the epithelial cells at different regions of the roof and floor of the mouth cavity of fishes could be considered as adaptation to various food preferences and feeding behavior of fish [31, 13, 1, 10, 3, 30 and 31].The present study describes the morphology and distribution of different taste buds that located on the epithelium surface of the mouth cavity of S. japonicus. The taste buds may be useful in assessing the palatability of the food and decide whether to swallow or spit it out.