Akysis varius NG & KOTTELAT, 1998

Variable Stream Catfish

Etymology

Akysis: from the Greek á- (Gr. ἀ), meaning ‘not, without’ and kýstis (Gr. κύστις), meaning ‘bladder’, in reference to genus members lacking a swim bladder.

varius: Latin term meaning ‘variable’, in allusion to the highly variable number of serrae on the posterior edge of this species’ pectoral-fin spine.

Classification

Order: Siluriformes Family: Akysidae

Distribution

Described from the Xe Bangfai river system with type locality around 3 km upstream of Ban Pakphanang village in Khammouane/Khammouan province, central western Laos.

It’s also known from the Mun River basin in Ubon Ratchathani and Nakhon Ratchasima provinces, central eastern Thailand as well as the Siem Reap river, which flows through the province of the same name, in northwestern Cambodia. All known localities form part of the much larger Mekong drainage.

Habitat

Members of this genus are near-exclusive inhabitants of swiftly-flowing streams and rivers with substrates generally consisting of any combination of sand, gravel and rocks. Water is normally transparent to slightly turbid, though overall volume, flow, and thus the amount of suspended material tend to increase significantly during rainy periods.

Aquatic plants are not normally present though there may be patches of leaf litter or underwater structures such as fallen branches or roots protruding from banks.

The Mun River is highly diverse and other species recorded there include Yasuhikotakia modesta, Y. lecontei, Y. morleti, Syncrossus helodes, Acantopsis choirorhynchos, Lepidocephalichthys hasselti, Barbonymus altus, B. gonionotus, Cyclocheilichthys apogon, C. repasson, Discherodontus ashmeadi, Epalzeorhynchos frenatum, Esomus metallicus, Hampala dispar, H. macrolepidota , Mystacoleucus marginatus, Opsarius koratensis, Osteochilus hasseltii, Raiamas guttatus, Rasbora borapetensis, R. dusonensis, R. rubrodorsalis, R. trilineata, ‘Puntius‘ orphoides, ‘P.‘ partipentazona, Gyrinocheilus aymonieri, Trichopodus pectoralis, T. trichopterus, Trichopsis pumila, T. vittata, Pseudomystus siamensis, Mystus singaringan, Kryptopterus cryptopterus and Mastacembelus favus.

Maximum Standard Length

35 – 40 mm.

Aquarium SizeTop ↑

A tank with base dimensions of 45 ∗ 30 cm is more than sufficient, even for a group.

Maintenance

Not difficult to keep but should really be provided with a soft, sandy substrate since some of its time is spent completely buried, or with only eyes protruding. It also appears to enjoy squeezing into crevices between rockwork and other solid surfaces so additional décor could include some water-worn rocks of various sizes, plus perhaps some driftwood roots and branches.

Although aquatic plants are unlikely to be a feature of its natural habitat some can be added if you wish with those that can be grown attached to rocks and wood, such as Microsorum, Anubias or various moss species particularly useful since they will not be disturbed by the fishes’ burrowing activity. Leaf litter can also be used with dried beech, oak or Ketapang leaves all suitable.

Like many fishes that naturally inhabit running water it’s intolerant to accumulation of organic pollutants and requires spotless water in order to thrive. It also does best if there is a relatively high proportion of dissolved oxygen and decent water movement in the tank. Weekly water changes of 30-50% tank volume should be considered routine.

Water Conditions

Temperature: 16.5 – 24 °C

pH: 6.0 – 7.5

Hardness: 36 – 215 ppm

Diet

Akysis species are primarily nocturnal micropredators feeding on small aquatic crustaceans, worms, insect larvae and other zooplankton. They can be a little picky in the aquarium and may not accept dried foods although in some cases they will learn to take them over time.

At any rate they should always be offered regular meals of small live or frozen fare such as Artemia, Daphnia, bloodworm or glassworm, ideally after dark in order that they can demonstrate natural behaviour, though once settled most specimens will emerge at feeding times. If the specimens are very small you may need to chop the food accordingly.

Behaviour and CompatibilityTop ↑

Not especially competitive and should not be kept with much larger or more aggressive fishes. Miniature schooling cyprinids such as Boraras, Microdevario or smaller Danio/Rasbora species make good tankmates and if geography isn’t an issue many similarly-sized characins and livebearers should also work.

Other similarly-sized akysid catfishes plus smaller sisorids such as Erithistes, Erethistoides, Hara, Pseudolaguvia and many Glyptothorax spp. are recommended for the lower levels.

In addition, many balitorid loaches from genera such as Gastromyzon, Pseudogastromyzon and Homaloptera are also suitable. Some members of the families Botiidae, Cobitidae and Nemacheilidae are ok but proper research is essential as some can be excessively aggressive/territorial or simply grow too large, and it’s also proven to co-exist peacefully with freshwater shrimp.

It’s nonaggressive towards conspecifics and can be kept singly or in a group as preferred.

Sexual Dimorphism

Unreported, but sexual dimorphism has been reported in A. alfredi (Ng and Kottelat, 1998), A. fuscus (Ng, 1996) and A. microps (Ng and Tan, 1999). In the former two species the anus is located anterior to a genital papilla in males, this structure having a genital opening covered by a fleshy flap at its tip and situated immediately posterior to the base of the pelkvic fins.

In females the anus is located more posteriorly and the genital papilla is formed by a short appendage with an opening at the tip, this structure being situated approximately level with the poterior end of the inner pelvic fin ray.

In A. alfredi the distance between the origin of the first pelvic fin ray and the first anal fin ray is less than in females (13.8-17.5% of standard length (SL) vs. 17.7-18.4%) but in A. fuscus the opposite is true. The pelvic fins in males are also shorter than in females (12.9-14.2% SL vs. 14.1-15.5%), closer together (distance between pelvic fins 4.0-4.9% SL vs. 4.8-5.5%), usually overlapping posteriorly (vs. spread laterally) nand often curved to forma bowl-like shape (vs. straight).

In A. microps the genital papilla in males has a bulbous base with the genital opening at the tip not covered by a fleshy flap, while in females the genital opening is only partially covered by an appendage and there are no obvious differences in pelvic fin morphology between the two genders.

According to Vishwanath et al. (2007) male specimens of A. prashadi possess a short, conical genital papilla immediately posterior to the anus while females have a flattened papilla with radial grooves at the pore.

Reproduction

Unreported, though a short report was published in the description of A. vespa (Ng and Kottelat, 2004), in which a successful spawning event in that species was apparently triggered by a significant decrease in temperature of around 44.6°F/7°C.

Several males were observed in pursuit of a gravid female, and the successful individual was allowed to wrap its body around that of the female at which point eggs and milt were released. The eggs hatched 4 days post-spawn with the yolk sac fully-absorbed after 10 days. Adult patterning was fully-developed after just 23 days.

NotesTop ↑

This species is among the smaller members of the family. It can be differentiated from all other Akysis species by the following combination of characters: caudal peduncle length 16.1-19.8% of standard length (SL); caudal peduncle depth 6.5-9.3% SL; length of adipose–fin base 25.6-29.5% SL; interorbital distance 42.0-46.0% of head length (HL), eye diameter 13.0-20.0% HL; length of nasal barbel 44.0-66.0% HL; lateral margins of the head appear straight when viewed from above; possession of 0-5 serrae on posterior margin of the pectoral spine; caudal fin truncate.

This species exhibits a variable number of serrations on the posterior margin of the pectoral fin spine, with two of the larger specimens examined during its description lacking them completely, while smaller individuals had 4 or 5.

The authors noted that this character is in contrast to that seen in other Akysis species with serrated pectoral spines in which the larger specimens tended to possess a greater number of serrations than the smaller. It remains unconfirmed whether this relates to sexual dimorphism or not.

Members of the family Akysidae are often referred to collectively as ‘Asian stream catfishes’ since the majority are exclusive inhabitants of such environments. All have four pairs of barbels, and within this larger grouping Akysis species are diagnosable by possession of tuberculate skin and cryptic colour patterning typically comprising yellowish bands or blotches on a brownish background.

Based on various morphological aspects Ng and Kottelat (1998) split the genus as originally accepted into the A. variegatus and A. pseudobagarius species groups, but all those of the latter group were reassigned to the genus Pseudobagarius by Ferraris (2007) on the basis of the following shared characters: tip of snout extends anterior to margin of lower jaw; mouth subterminal; narial openings (anterior and posterior nostrils) relatively large and closely-set; caudal fin deeply forked.

Pseudobagarius spp. also tend to have a relatively extended body shape compared with Akysis, while in the latter the snout tip extends only slightly beyond the margin of the lower jaw, mouth orientation terminal to slightly subterminal, narial openings relatively small and widely-spaced and caudal fin truncate or emarginate.

In recent phylogenetic analyses the family Akysidae has been grouped into an assemblage informally referred to as ‘Big Asia’ where it occupies a basal position and is most closely related to the families Amblycipitidae, Sisoridae and Erethistidae, these apparently forming a monophyletic group alongside the Bagridae and Horabagridae.

The South American family Aspredinidae has often been affiliated with Akysidae in earlier studies, but is now considered more closely associated with the Doradidae and Auchenipteridae.

All akysids possess venom apparatus at the base of the dorsal and pectoral spines and though not dangerous to a healthy human can inflict a painful sting, so should be handled with care when being removed from the water for any reason or during aquarium maintenance.

References

1. Ng, H. H. and M. Kottelat, 1998 - Journal of Natural History 32(7): 1057-1097
   The catfish genus Akysis Bleeker (Teleostei: Akysidae) in Indochina, with descriptions of six new species.
2. Ferraris, C. J., Jr., 2007 - Zootaxa 1418: 1-628
   Checklist of catfishes, recent and fossil (Osteichthyes: Siluriformes), and catalogue of siluriform primary types.
3. Ng, H. H., 1996 - Raffles Bulletin of Zoology 44(1): 3-10
   Akysis heterurus, a new species of catfish (Teleostei: Akysidae) from eastern Sumatra, with notes on Akysis hendricksoni Alfred.
4. Ng, H. H., 1999 - Raffles Bulletin of Zoology 47(2): 541-548
   The Akysis of Myanmar: a review (Teleostei: Akysidae).
5. Ng, H. H., 2006 - Zootaxa 1150: 19-30
   Akysis longifilis, a new species of catfish (Teleostei: Akysidae) from Myanmar.
6. Ng, H. H., 2007 - Zootaxa 1608: 51-58
   Akysis pulvinatus, a new species of catfish (Siluriformes: Akysidae) from southern Thailand.
7. Ng, H. H., 2008 - Ichthyological Exploration of Freshwaters 91(3): 255-262
   Akysis vespertinus, a new species of catfish from Myanmar (Siluriformes: Akysidae).
8. Ng, H. H., 2009 - Journal of Threatened Taxa 1(1): 2-8
   Akysis portellus sp. nov., a new species of catfish (Teleostei: Akysidae) from the Sittang River drainage, Myanmar.
9. Ng, H. H. and D. J. Siebert, 2004 - Zootaxa 733: 1-8
   A new species of the catfish genus Akysis (Siluriformes: Akysidae) from southern Borneo.
10. Ng, H. H. and W. J. Rainboth, 2005 - Raffles Bulletin of Zoology, Supplement 13: 33-42
    Four new species of Akysis (Teleostei: Siluriformes: Akysidae) from mainland southeast Asia, with comments on A. similis.
11. Page, L. M., R. K. Hadiaty, J. A. López, I. Rachmatika and R. H. Robins, 2007 - Copeia 2007 (2): 292-303
    Two new species of the Akysis variegatus species group (Siluriformes: Akysidae) from southern Sumatra and a redescription of Akysis variegatus Beeker, 1846.
12. Rainboth, W. J., 1996 - FAO, Rome: 1-265
    Fishes of the Cambodian Mekong. FAO Species Identification Field Guide for Fishery Purposes.
13. Sullivan, J. P., J. G. Lundberg and M. Hardman, 2006 - Molecular Phylogenetics and Evolution 41(3): 636–662
    A phylogenetic analysis of the major groups of catfishes (Teleostei: Siluriformes) using rag1 and rag2 nuclear gene sequences.