A note on Aaptosyax grypus, a very rare, unique cyprinid fish

Abstract

Rainboth (1991) described a new cyprinid fish, Aaptosyax grypus, from the Mekong River in Thailand, establishing a new monotypic genus. I was very interested in his figures of the species, which was similar to Chanos chanos in general appearance, and visited the fishermen’s base at Khong Chiam, eastern Thailand, several times from April 1998 to January 1999. Then I obtained a total of ten specimens of A. grypus: two from Khong Chiam and eight from the lower reaches near Khone Falls, Laos. All specimens were adult and larger than 700 mm in standard length (SL). One specimen was obtained in August (Khong Chiam), while the others were all obtained from the end of November to December, which may be the spawning season of the species, when the temperature of the Mekong decreases to 20 C, the coolest period (see ‘‘Ecological notes’’). Since Rainboth (1991) first described the superficial characters of A. grypus in detail, little biological information has accumulated for this species (e.g., it is categorized as ‘‘Data Deficient’’ in the IUCN Red List; Kottelat 1996). Here, I report some of the anatomical characteristics and other biological aspects of this very rare fish. Although the specimen for anatomical observation was not kept, another specimens were deposited in Lake Biwa Museum, Shiga, Japan [LBM 1210014953 (740 mm SL), 1210014954 (916 mm SL); tissue for DNA analysis, 1210015335; pharyngeal teeth, 1210026911, 1210047434; all from the Mekong River, Laos]. Mouth parts. In most free-swimming cyprinid fishes, the upper jaw can protrude through the action of the kinethmoid (Alexander 1966). In A. grypus, however, the upper jaw curves strongly and the postero-dorsal end of the premaxillary is supported by a large, curved, arched plate (probably corresponding to the kinethmoid) posteriorly, limiting the antero-posterior movement of the premaxillary. This structure seems to be associated with the biting jaw movement of this species, instead of protrusion (Fig. 1). Weberian ossicles. The claustrum is tightly connected to the supraneural by ligamentous tissue, in contrast to its free state in other cyprinid fishes (Fig. 1). There are also differences in the base of the scaphium and intercalarium, which are not pointed but wide. The connection of the tripus to the air bladder is remarkable. There is no flexible elongation of the transformer process in the end of the tripus; the latter is located free in a thick ligamentous mass that fills the gap between the air bladder and the os suspensorium anteriorly and the vertebral column dorsally. This thick mass is covered with an overhanging curtain from the centrum. There is neither a sagittally running slit in the antero-dorsal corner of the tunica externa nor a thinned area of the tunica interna coinciding with the slit in the tunica externa of the air bladder, which are widely seen in cyprinid fishes (Alexander 1962); these imply that pressure transmission is less effective. All of these characters suggest that the Weberian ossicles of A. grypus are incomplete in communicating sound from the air bladder to the inner ear. Filamentous bones. In lower teleosts, such as Sardinops and other clupeoid fishes, a dense growth of filamentous bones called the osseous brush and the filamentiferous rod (Phillips 1942) develops in the dorso-anterior region of the vertebral column. These bones are not generally found in cyprinid fishes. However, A. grypus possesses well-developed dense filamentous bones similar to those of clupeoids, which are attached to the posterior region of the neurocranium and extend into the trunk muscles (Fig. 1). Y. Tomoda (&) 415-76 Hamabun, Imazu, Takashima, Shiga 520-1604, Japan