Hydrolycus tatauaia

Hydrolycus: from the Greek hydro, meaning 'water', and lykos, meaning 'wolf'.

This species is widely-distributed throughout the lower and central Amazon region, the upper Orinoco basin in Venezuela and Colombia plus coastal drainages in Guyana.

Type locality is 'Rio Xingu, Belo Monte, 3°10'S, 51°50'W, Pará, Brazil'.

This species is pelagic and adults tend to be associated with flowing stretches of main river channels and larger tributaries of both white and black water rivers.

Reproduction occurs between November and April and adults perform long upstream migrations in order to do so.

Juveniles presumably occur in smaller tributaries and areas of flooded forest.

Suitable only for public installations or the very largest private aquaria.

Relatively unfussy and often maintained in completely bare set-ups but prefers dim lighting and is said to be nocturnal to an extent.

It does best if there is a high proportion of dissolved oxygen and moderate degree of water movement so external filters, powerheads, airstones, etc., should be employed as necessary.

As stable water conditions are obligatory for its well-being this fish should never be added to biologically-immature aquaria and weekly water changes of 30-50% aquarium volume should be considered mandatory.

A tighly-fitting cover is also essential as Hydrolycus spp. are prodigious jumpers.

An obligate but generalised piscivore capable of consuming surprisingly large prey.

Newly-imported specimens often refuse to accept anything but live fishes but most can be weaned onto dead alternatives once recognised as edible.

Like the vast majority of predatory fishes this species should not be fed mammalian or avian meat like beef heart or chicken, and similarly there is no 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.

Best kept alone, or with similarly-sized, non-aggressive fishes it cannot fit into its mouth since otherwise it's relatively peaceful although it is known to occasionally attack comparably-shaped or coloured species, especially if space is limited.

It can also be maintained in a group in a suitably-sized aquarium but the purchase of at least three specimens is advisable.

We know of only a handful of private aquarists with the facilities required to house this species long-term since its adult size and natural behaviour preclude its suitability as a fish for the home aquarium.

Vernacular names in Brazil include 'Cachorra' or 'Pirandirá', although these are also applied to congeners.

It can be told apart from all congeners by the following combination of characters: head and body silvery with dark dorsal surface; an elongate dark blotch posterior to the opercle; dorsal, caudal and anal-fin rays reddish to orange proximally with some individual variation in intensity and tonality; adipose fin dark, with diffuse black pigmentation.

Hydrolycus can be easily-separated from other genera in the family Cynodontidae by the fact that the dorsal-fin origin is located distinctly anterior to a vertical through the anal-fin origin plus the dorsoventral enlargement of the mesethmoid spine is almost round in shape from a lateral view and enlarged in Hydrolycus scomberoides, H. armatus and H. tatauaia.

It's sometimes included in the putative subfamily Cynodontinae alongside Cynodon and Rhaphiodon, these being separated from other Characiformes by a series of derived features plus their oblique mouth shape and highly-developed dentary canine teeth.

The latter fit into a pair of corresponding openings in the upper jaw which allows the mouth to be closed completely.

Cynodontinae contains two primary monophyletic lineages, one comprising the genus Hydrolycus and the other a clade with Cynodon and Rhaphiodon spp., with members sometimes referred to collectively as 'dogtooth characins'.