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FACTSHEETS: December 2024 - no. 342

***Ageneiosus* inermis (Linnaeus, 1766)**

e carry on from our November factsheet and stay with the Auchenipteridae family to end the year of 2024 with our December factsheet and a look at a species with the common name of the "Bottlenose catfish", namely Ageneiosus inermis.

Ageneiosus inermis

Ageneiosus inermis

Ageneisosus inermis was first described as far back as 1766 by Linnaeus as Silurus inermis and then it has went on to have another incredible thirteen synonyms The reason for this is the juvenile markings on this species which can change rapidy from a marbled colour pattern (see image below which was captioned as A. marmoratus) through colour and pattern changes untill adult where it has more of a silver colouration.

A. inermis is widely distributed in South America, east of the Andes.

Ageneiosus inermis - juvenile

Ageneiosus inermis - juvenile

This genus was previously placed in its own family of Ageneiosidae and you can still find some publications that still follow this line. This species prefers rivers in overgrown backwaters where the current is not too strong.

South America = Surinam

Distrbution: Widely distributed in South America, east of the Andes. Type locality: Surinam.

Ageneiosus inermis inhabits the deepest parts of the river channel and lentic stretches, with flooded marginal vegetation (Melo et al. 2007) and in flooded areas during the dry season. It is piscivorous (Oliveira et al. 2005), and it is short-distance migratory species (Garavello et al. 2010). The species may have sexual dimorphism and have courtship behaviour during the reproductive season, internal fecundation, relatively low fertility, medium-sized oocytes, external development. The flooded areas are natural nurseries for the larvae, which are carried downstream (Bailly et al. 2008).

Common Name

Bottlenose catfish

Synonyms

Silurus inermis, Ageneiosus armatus, Ageniosus brevifilis, Ageneiosus brevifilis, Hypothalmus dawalla, Ageneiosus dawalla, Davalla schomburgkii, Ageneiosus axillaris, Ageneiosus sebae, Ageneiosus therezinae, Ageneiosus therezine, Ageneiosus ogilviei, Ageneiosus gabardinii, Ageneiosus marmoratus.

Family

Auchenipteridae

Subfamily

Auchenipterinae

Distribution

South America: Widely distributed in South America, east of the Andes. Type locality: Surínam.

Size

50.0cm. (20ins)

Temp.

22-24°c (71-75°f.)

p.H.

6.5-7.5.

Characteristics

Anal soft rays: 34 - 40; Vertebrae: 46 - 51. Body naked and elongated. Head is pointed and depressed at the top. Mouth is big and wide. The lateral eyes are situated along the level of the mouth, ensuring a vision above and below. Exhibits sexual dimorphism, like A. dentatus. The first ray of the dorsal fin is very long and has pointed spines.

Colouration

Colouration and markings change from juveniles with large marbled blotches on to many guises with lateral stripes and smaller spots to adult where it takes on a more dull silver colour to the body with a darker colouration to the dorsum. The caudal fin can have a tinge of red with a black band to the outer rim of the fin.

Aquarium Care & Compatibility

As per this genera they will be fine as juveniles, but as they mature they become predatory so would need to be kept with fish that are not too small. A large tank would be needed with plenty of swimming space but it is not really recomended for home aquaria, but public aqauria, unless you have a vast experience of keeping large predatery catfish and its needs.

Reproduction

During reproduction, the mandibular barbels and dorsal spine of the male change, and a copulatory organ appears at the anterior level of the anal fin. Fertilisation is internal, the female being capable of keeping the spermatozoids inserted in the epithelium of her genital tract.

Sexual differences

At spawning time, the male develops an enormous dorsal fin spine and a penis-like copulatory organ. With the dorsal-fin spine, the male clamps the female; mating with internal fertilization follows, allowing the female to lay fertilized eggs later. The dorsal fin spine regresses after spawning.

Diet

Ageneiosus inermis are predatory fish that catch fish with their huge mouths, which contain countless, small teeth. Feeds on fish and crustaceans. Its flesh is esteemed for its very fine flavour. Is reported to be nocturnal. Frequently captured with nets, the males are difficult to release from the nets because of its strong dorsal spine bordered with denticles.

Glossary of Terms

Anal fin: The median, unpaired, ventrally located fin that lies behind the anus, usually on the posterior half of the fish.
Caudal fin: The tail.
Dorsal fin: The primary rayed fin(s) on top of the body.
Mandibular: Pertaining to the lower jaw. (mandibular barbels).
Piscivorous: Fish eating.

Etymology

Ageneiosus: beardless, pertaining to the very small maxillary barbels.
inermis: “pinnis inermibus,” i.e., unarmed fins, an apparent misnomer since this catfish possesses dorsal- and pectoral-fin spines.

References

Boujard, T., M. Pascal, F.J. Meunier and P.-Y. Le Bail 1997 Poissons de Guyane. Guide écologique de l'Approuague et de la réserve des Nouragues. Institut National de la Recherche Agronomique, Paris, 219 p.
Ferraris, C.J. Jr., 2007. Checklist of catfishes, recent and fossil (Osteichthyes: Siluriformes), and catalogue of siluriform primary types. Zootaxa 1418:1-628.
Frederico, R.G. 2023. Ageneiosus inermis. The IUCN Red List of Threatened Species 2023.
Froese, R. and D. Pauly. Editors. 2009. FishBase. World Wide Web electronic publication. www.fishbase.org, version (11/2011).
Le Bail, P.-Y., P. Keith and P. Planquette 2000 Atlas des poissons d'eau douce de Guyane (tome 2, fascicule II). Publications scientifiques du Muséum national d'Histoire naturelle, Paris:307 p.
Max Pedley. Pers comm. 2020.
Ribeiro, F. R. V., L. H. Rapp Py-Daniel and S. J. Walsh 2017 (9 Feb.) Taxonomic revision of the South American catfish genus Ageneiosus (Siluriformes: Auchenipteridae) with the description of four new species. Journal of Fish Biology.
The ETYFish Project Fish Name Etymology Database © Christopher Scharpf.

IUCN Red List

Ageneiosus inermis has most recently been assessed for The IUCN Red List of Threatened Species in 2020 and is listed as Least Concern. The species is recorded in some protected areas in Brazil. However, there is no information on conservation actions in place or recommended for this species (IUCN 2020).

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