Ageneiosus inermis

Ageneiosus: from the Greek á- (Gr. ἄ), meaning 'not, without' and géneion (Gr. γένειον), meaning 'chin, beard', probably in reference to the very short maxillary barbels possessed by A. armatus (now a synonym of A. inermis).

inermis: Latin term meaning 'unarmed', an apparently mistaken reference to this species' lack of fin spines (which it does in fact possess).

The type locality is gievn simply as 'Surinami' (Suriname), but the species appears widely-distributed throughout much of South America east of the Andes mountains with records existing from Argentina, Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Suriname, Venezuela and possibly Uruguay. It therefore occurs in a number of major river systems including the Amazon and many of its tributaries (rios Negro, Madeira, Xingu, Tapajós, etc.) plus the ríos Tocantins, Orinoco, Parnaíba, Uruguay, Paraná and de la Plata.

Given its enormous natural range this species is likely to be something of a habitat generalist, but most records we've seen pertain to the lower sections of river channels and larger tributaries. According to reports from sport fishermen it shows a preference for swiftly-flowing stretches among submerged structures or vegetation.

Aquarium base dimensions in excess of 400 ∗ 120 cm157 ∗ 48 in would be required for fully-grown adult specimens, meaning this species is largely unsuitable for private aquaria.

Likely to prefer dim lighting and access to refuges in the form of driftwood, large rocks or lengths of plastic piping. An enormous filtration system and dedicated regime of water changes will also be required and maintenance of high dissolved oxygen levels should be considered mandatory given the species' natural habitat plus the fact that unlike some relatives it cannot respire atmospheric oxygen.

Larger Ageneiosus spp. are obligate carnivores by nature feeding on invertebrates and other fishes, and capable of consuming surprisingly large prey. Newly-imported specimens thus often refuse to accept anything but live fishes though most can be weaned onto thawed frozen whitebait, shellfish or similar once they recognise them as edible. Some individuals even learn to accept dried foods such as pellets designed for carnivorous species.

Like the vast majority of predatory fishes this species should not be fed mammalian/avian meat such as beef heart or chicken, and similarly there is little benefit in the long-term use of 'feeder' fish such as livebearers or small goldfish which carry with them the risk of parasite or disease introduction and at any rate tend not have a high nutritional value unless properly conditioned beforehand.

This species is capable of consuming large prey items and is likely to be safe only alongside similarly-sized catfishes or pelagic characins such as Brycon, Colossoma or Piaractus, a combination only possible in public aquaria in the vast majority of cases.

Adult males of Ageneiosus spp. all display marked, seasonal, sexual modifications, with the area between head and dorsal fin origin (nuchal region) exhibiting a much more acute angle than in non-breeding individuals. The maxillary barbel also becomes ossified and extended inbreeding males with tooth-like odontotes apparent on the dorsal and medial surfaces, while the first branched dorsal fin ray is significantly extended giving the fin an enlarged, curved appearance with sharp odontotes appearing along the anterior margin.

The anterior portion of the anal fin in sexually-active males also exhibits seasonal modifications with the unbranched and first few branched rays becoming thickened, elongate and fused together, forming a structural support for the intromittent organ which itself becomes displaced in such a way that the genital pore is situated at the tip of the modified rays. The interradial membranes and surrounding inclinator/erector muscles also become enlarged. In adult females, the unbranched and first few branched anal-fin rays are much longer than the other rays.

Unrecorded in aquaria although some brief reports of apparent courtship behaviour in the congener A. marmoratus do exist. What is known is that fertilisation occurs internally via the modified genital papillae in males described above, with the ossified and extended maxillary barbels and dorsal fin ray used to court and physically manipulate females.

Since a seasonal spawning strategy is adopted in nature, induced by the onset of the rainy season, simulation of such via a period of warmth followed by several large, cool water changes may prove useful in captivity.

The genus Ageneiosus was at one point classified in the family Ageneiosidae alongside the genus Tetranematichthys, but this was not accepted by all authors. The grouping remains poorly-studied with the last major revision having been conducted by Walsh (1990) in his unpublished dissertation, in which A. inermis was included under the currently synonymous name A. brevifilis. It can be separated from the majority of the genus (except A. marmoratus) by possession of a truncate caudal fin, and from A. marmoratus by a lack of mottled patterning on the body plus its larger adult size.

The genus, meanwhile, is separated from other auchenipterids by a combination of absence of mental barbels in adults, and the presence of enlarged, tooth-like odontodes formed by outgrowths of the maxillae on the dorsal margin of the maxillary barbels inbreeding males. The larger-growing members are often utilised as food fishes in their native countries.

The family Auchenipteridae is itself distinguished from all other catfishes by the following: body lacking bony plates except on dorsal surface between head and dorsal fin origin which is covered by a row of such plates joined together and clearly visible beneath a thin epidermis; no nasal barbel(s); adipose fin small, occasionally absent; eye covered by adipose tissue and lacking a distinct orbital rim; anterior anal fin rays enlarged and thickened in adult males; maxillary barbel fits in a specialised groove below the eye when drawn towards the body; maxillary barbel directed laterally and dorsally when drawn into said groove.