Acrochordonichthys is a genus of catfishes (order Siluriformes) of the family Akysidae. It includes ten species.
Distribution and habitat
Acrochordonichthys species are generally found at the bottoms of rivers throughout Southeast Asia. Many of the species are only known from Borneo. A. guttatus is known only from the Barito River drainage in southern Borneo. A. mahakamensis is known only from the Mahakam River drainage in eastern Borneo it is named for. A. chamaeleon and A. strigosus are known only from the Kapuas River drainage in western Borneo. A. falcifer is known only from the Kinabatangan and Segama River drainages, and possibly from the Kayan River drainage, in north-eastern Borneo. A. pachyderma is known only from the Kapuas, Mahakam, and Kinabatangan River drainages in western, eastern, and north-eastern Borneo, respectively.
A. septentrionalis is known only from the Mae Klong River drainage in Thailand and the Pahang River drainage in Peninsular Malaysia; it may be found in the Bernam River drainage in Peninsular Malaysia. A. rugosus is known from the Solo, Ciliwung, and Citarum River drainages in Java; the Barito, Kapuas, Mahakam, and Rajang River drainages in Borneo; the Bernam, Terengganu, Mae Nam Sungai Kolok, and Pattani River drainages in Peninsular Malaysia and southern Thailand; and the Musi and Tulangbawan River drainages in Sumatra. A. ischnosoma is known from the Citarum River drainage in western Java and the Musi River drainage in southern Sumatra. A. gyrinus, from the Yom River of the Chao Phraya basin in Thailand, represents the northernmost distribution of this genus.
Description
Acrochordonichthys is characterized by a highly rugose skin with tubercles arranged in longitudinal rows along the side of the body, the presence of a long, low adipose fin, and a truncate caudal fin. The tubercles on Acrochordonichthys may become either greatly hypertrophied or greatly reduced at different stages of the moulting cycle; moulting is known to occur in the related genus Breitensteinia. When they are most developed, the tubercles appear more rounded and tightly packed, but are more squamous (flattened) and further apart when least developed.
The head is broad and depressed, while the body is moderately compressed. The dorsal profile rises evenly but not steeply from tip of snout to the origin of the dorsal fin, then slopes gently ventrally from there to end of caudal peduncle. The ventral profile is horizontal to origin of the anal fin, then slopes dorsally to end of the caudal peduncle. The head is covered with small tubercles with poorly demarcated and indistinct margins, and the body with such tubercles arranged in 5–6 longitudinal rows on each side. The dorsal fin origin is nearer the tip of the snout than caudal flexure. The pectoral spine is stout, with or without serrations on the posterior edge. The caudal fin is weakly emarginate.
Sexual dimorphism has been reported in Acrochordonichthys. Males have the anus situated immediately in front of a genital papilla, which is located posterior to the pelvic fin base. The genital opening is situated at the tip of the papilla, covered by a fleshy flap. In females, the anus is situated more posteriorly and the genital opening is located at the tip of a short genital appendage. In A. ischnosoma, males have a long genital papilla located immediately posterior to anus, while females have a conical genital papilla located immediately posterior to anus.
Species groups
Based on external morphology, two groups of species can be distinguished easily, both of which may be artificial. The first group, the A. ischnosoma species group, includes A. ischnosoma, A. guttatus, A. gyrinus, A. mahakamensis, A. septentrionalis, and A. strigosus; these species have a narrower head, a more slender caudal peduncle, and 39–41 vertebrae. The second group, the A. rugosus species group, includes A. chameleon, A. falcifer, A. pachyderma, and A. rugosus, in which the fish have a deeper caudal peduncle, a broader head, and 35–37 vertebrae.
Acrochordonichthys species are cryptically colored. The colouration is extremely variable, particularly in the A. rugosus group; colouration is less variable in the A. ischnosoma species group. Most have a light-brown saddle extending for the length of the adipose fin and some have spots on the side of the body in place of the saddle. Variation in colouration may be due to moulting of the skin. However, even though the colouration is highly variable, some general patterns are evident that allows colour to be a useful diagnostic character when variation is taken into account.