Hydrolycus scomberoides

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

scomberoides: from the Greek skombros, meaning 'tuna, mackerel', and the suffix -oides, meaning 'similar to'.

Type locality is given simply as 'Brazil' but this species is currently understood to be widespread in the central and upper Amazon River system in Brazil, Bolivia, Peru and Ecuador, with the rio Tapajós in Brazil appearing to mark the downstream limit of its range.

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.

An aquarium with base dimensions of 210 ∗ 75 cm84 ∗ 30 in should be the absolute minimum considered.

The aquarium should ideally be designed to resemble a flowing stream or river with a substrate of variably-sized rocks, sand, fine gravel, and some larger water-worn boulders.

This can be further furnished with driftwood roots and branches if you wish but be sure to leave plenty of open swimming space.

While the majority of aquatic plants will fail to thrive in such surroundings hardy genera such as Microsorum, Bolbitis, or Anubias spp. can be grown attached to the décor.

Like many fishes that naturally inhabit running waters it’s intolerant to the accumulation of organic wastes and requires spotless water at all times in order to thrive.

It also 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 tightly-fitting cover is essential as this species is a prodigious jumper, and it may also prove beneficial to cover the back and sides of the aquarium in order to reduce the chances of it swimming into the glass since it can be skittish, especially in confined surroundings.

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 since it's actually quite peaceful with those too large to be considered food.

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

Juveniles tend to adopt a presumably cryptic, oblique, 'head-down' position and lurk among the décor whereas adults are fully pelagic.

This species may be referred to using a variety of names including 'scomb', 'sabre tooth tetra', 'sabre tusk barracuda', 'dog tooth characin, 'vampire fish', 'Cachorra' or 'Pirandirá' (the latter two names being used in Brazil where they're also applied to congeners).

It's regularly confused with the payara, H. armatus, though that species grows considerably larger, is popular with sports anglers and unsuitable for all but the very largest home aquaria.

It can be told apart from all congeners by the following combination of characters: serrations present on the exposed field of scales in specimens larger than 100 mm3.9 in SL; pelvic-fin base inserted laterally, distinctly above ventral profile of body; a black spot at the base of the innermost pectoral-fin rays.

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'.