SCALE SENSILLAE OF THE FILE (SERPENTES: ACROCHORDIDAE) AND SOME OTHER AQUATIC AND BURROWING

by

DAVID POVEL and JEROEN VAN DER KOOIJ

(Section Dynamic Morphology,Institute of Evolutionaryand Ecological Sciences, Leiden University,P.O. Box 9516, 2300 RA Leiden, The Netherlands)

ABSTRACT

The acrochordid snakes are aquatic, living in environmentswith often a poor visibility. It therefore was investigatedhow these detect their prey. Two earlier studies of their scales revealed a rather complex scale organ, composedof hairlike protrusions and plate-like structures. However, no satisfactory explanation was given for the structures found, e.g., an undefined sensilla or a gland. Skin samples from various sites of the body of granulatus and A. javanicus were studied. Scanning electron microscopic pictures revealed that each scale of the head contains up to seven sensillae, and each of the keeled scales of the rest of the body has one. Also a modified Allochrome staining procedure on tissue samples was performed to detect glycogen, which is known to occur in discoidal nerve endings of tactile sense organs of . Light microscopicslides revealedglycogen particles in a small pillow-shaped area just below the hairlike protrusions of an organ. Moreover, small nerves were recognized near the same location. No indications were found for the scale organs to have a glandular function. Because of the reported reactions of a snake when it is touched by a fish, these scale sensilla are proposed to be very sensitivemechanoreceptors. Comparisons were made with the scale organs of snakes from various habitats, viz. the seasnake Lapemis hardwicki, and burrowing snakes such as unicolor and Cylindrophisrufus. Their skin sensillae revealed to be of a much simpler outer morphology and only concentrated on specific parts of the head and neck region.

KEYWORDS: Acrochordus, aquatic snake, mechanoreceptor,scale sensillae.

INTRODUCTION

Acrochordid snakes are aquatic and can be found in coastal areas, from West to the Indo-Australian archipclago in the East, up to the North coast of Australia as a Southern boundary. The snakes are mainly nocturnal (DOWLING, 1960; LILLYWHITE, 1991), and prefer mangrove areas of which the water transparency is often very poor (HOUSTON & SHINE, 1993; SHINE et al., 1995). The snakes are feeding on fish. However, at night vision is negligible and during daytime murky waters rendcr it ineffective, how docs the snake detect its prey? 444

There are three living known of this group, viz. which prefers mainly salt water conditions, A. javanicus found in both salt and fresh water conditions, and A. arafurae favouring fresh wa- ter habitats. Acrochordid snakes have small eyes, which are located almost on top of their head in which respect they resemble the Anaconda. Instead of a good eyesight in a horizontal plane, such an eye position is optimal to inspect the water surface for detecting and following the movements of a prey above, the outline of which is contrasted by its brighter environ- ment. Acrochordid snakes feed mainly on fish and crustaceans (HOUSTON & SHINE, 1993; LILLYWHITE, 1991; SHINE, 1986; VORIS & GLODEK, 1980). The conditions mentioned above are especially good for an ambush type of hunting style. DowLING (1960) noticed that a specimen of A. javanicus laid still until touched by a fish browsing along the bottom. The snake quickly grasped the fish by throwing a body coil over and around it. After this the prey is seized by the head and swallowed. When prey is in the vicinity of the head or neck region and visibility clear it can also, in a swift, sidewards directed strike, catch the prey (DOWLING, 1960). The snake could even grasp and hold a fish with two different body coils holding another one in its jaws (DOWLING, 1960). This behaviour suggests a mechano-sensitivity of the snake for its prey, which is most probably to be found in its skin receptors. Although the scales and scale organs of many reptiles have been examined, e.g., PROSKE (1969a, b), JACKSON (1977), JACKSON & DOETSCH (1977a, b), HILLER (1978), and SHERBROKE & NAGLE (1996), those of the acrochordid snakes have not yet been investigated in depth. The skin of acrochordid snakes has scales with keels, by which these snakes are also known as the file or wart snake. SCHMIDT (1918) performed a light microscopic study to examine the scales and the scale organs in A. javanicus and A. granulatus. He found scale organs which occurred all over the body. The organs consisted of a tuft of hairlike projections and a flat plate-like structure. Schmidt consid- ered the scale organs as sensillae, however, a tactile function was excluded by him. First, because the sensilla lay in shallow depressions of the keel by which he assumed that the sensillar hairs could not be stimulated; second, similar sensillae as found on the back occurred contra-intuitively also on the ventral side of the body and therefore could not have a mechanorecep- tive function. However, he considered the plate-like structures of the scale organs as possible tactile sensors. On the other hand, PRICE (1982) studied the scales' outer morphology by means of a scanning electron microscope. He noticed gland-like openings at the top of the keels and therefore interpreted the peculiar shape of the scale organs as having a glandular function. Price did not cite SCHMIDT