Hydrocyon falcirostris Cuvier, 1819
Falcirostris: From the Latin falx, meaning ‘sickle, scythe’, and rostrum, meaning ‘snout’.
- Order: Characiformes
- Family: Acestrorhynchidae
Although the type locality of this species is recorded as simply “Brazil,” it is actually found throughout numerous northern Amazonian drainages, including those in Peru, Ecuador, and Brazil. It can also be found in the Río Orinoco in Venezuela, as well as in various coastal river systems in Guyana, Suriname, and French Guiana. Therefore, despite being originally identified in Brazil, this species has a much wider distribution across the Amazonian region and neighboring areas.
This species is typically found inhabiting major river channels and tributaries, but it is known to move into flooded areas during the wet season. While it has been recorded in both clear and black water environments, it appears to prefer these types of water over turbid “white” waters.
The Río Orinoco also hosts a variety of other fish species that are commonly traded, including Corydoras delphax, Platydoras costatus, Baryancistrus beggini, Hypancistrus inspector, Panqolus maccus, Panaque nigrolineatus, Hemigrammus rhodostomus, H. stictus, Hyphessobrycon sweglesi, Paracheirodon axelrodi, Pristella maxillaris, Copella nattereri, Biotodoma wavrini, Heros severus, Mesonauta insignis, Satanoperca daemon, and Uaru fernandezyepezi.
Maximum Standard Length
320 – 350 mm SL.
To provide optimal conditions for this species, it is important to keep in mind that it is predominantly found in open water and excess cover can actually cause stress. Therefore, the majority of the tank should be free of decorations, with ample open space. If a natural effect is desired, a sandy substrate with a few handfuls of leaf litter and some driftwood branches or roots can be added. Plants that can grow in sand, such as Microsorum pteropus, Taxiphyllum barbieri, or Anubias spp., as well as those that prefer to be attached to solid surfaces, can also be included. Lighting can be tailored to the specific needs of the plants being used.
For deeper tanks, emergent branches or plants can be added to create a visually appealing effect. However, it is important to use a tightly-fitting cover, as Acestrorhynchids are powerful jumpers.
Efficient filtration is crucial when keeping predatory species due to the amount of waste produced. It is recommended to install one or more external canister filters and/or a sump system, with the return organized in such a way as to create some surface movement.
Regular water changes are also essential, with weekly changes of 30-50% considered mandatory. This species can be sensitive to organic pollutants and fluctuations in water chemistry, and for this reason, it should never be introduced to biologically immature setups.
- Temperature: 22 – 28 °C
- pH: 6.0 – 7.5
- Hardness: 18 – 215 ppm
This species is an obligate piscivore which is capable of consuming prey that is relatively large compared to its own body size.
Upon importation, individuals may only accept live prey, but they can often be transitioned to consuming dead alternatives once they recognize them as food. Some individuals may even learn to accept dried foods.
It is important to note that like most predatory fish, this species should not be fed mammalian or avian meat such as beef heart or chicken. These types of food contain lipids that cannot be properly metabolized by the fish and may lead to excessive fat deposits and organ degeneration.
Additionally, the use of “feeder” fish such as livebearers or small goldfish is not recommended as they can introduce parasites or diseases and do not have a high nutritional value unless properly conditioned beforehand.
Behaviour and Compatibility
This species is generally peaceful with tankmates that are too large to swallow, and can be kept in a community aquarium if other fish are chosen carefully.
Avoid highly territorial or very competitive species, and instead opt for placid fish such as Geophagus spp., Acarichthys heckelii, medium-sized doradid or loricariid catfishes, and characids from genera like Ctenolucius, Mylossoma, or Myloplus.
While not aggressive towards conspecifics, juveniles tend to exhibit a marked schooling instinct. Older individuals tend to be more solitary, but still group together from time-to-time, and it’s best to maintain them in groups of four or more.
It’s important to note that acestrorhynchids are cannibalistic, given the opportunity. Therefore, if purchasing a group or adding to an existing school, ensure that all individuals are of similar size to avoid any aggression or predation.
Sexually mature females tend to grow a little larger and be deeper-bodied than males.
It has been reported that courtship and spawning have been observed in aquariums, but no fry have been successfully raised. According to reports, spawning takes place in midwater with the female stationary and the male swimming in a figure-eight pattern around her. The eggs are scattered in large numbers, and there is no parental care provided for the offspring.
This species is a part of the closely-related A. nasutus group within the genus, along with A. nasutus, A. maculipinna, and A. isalineae. These species are distinguished by possessing two dark, longitudinal stripes: one running from the tip of the snout to the caudal-fin base, and the other from the posterior edge of the lower maxilla to the underside of the caudal peduncle. In juvenile specimens of A. falcirostris, this patterning is present, but absent in adult specimens. Additionally, A. falcirostris can be distinguished from the other three species by its larger adult size and the absence of the body stripes in adults, with the other species tending to grow no larger than 100 mm SL. Menezes’ (1969) diagnostic characteristics for A. falcirostris include 2 spinous and 9 soft dorsal-fin rays, 5 spinous and 18-24 soft anal-fin rays, 14-19 pectoral-fin rays, 8 ventral-fin rays, 140-175 pored lateral-line scales, 30-37 scale rows between lateral line and dorsal-fin origin, 17-22 scale rows between lateral line and anal-fin origin, 30-39 rakers on lower part of first gill arch, opercle with two dark patches separated by a narrow pale strip, and an almost round black blotch on the caudal-fin base. Additionally, López-Fernández and Winemiller (2003) note that A. falcirostris possesses a black fringe on the posterior edge of the operculum, giving it a “collared” appearance.
The remaining members of the genus are currently composed as follows: A. microlepis group: A. britskii, A. grandoculis, A. microlepis, A. minimus; A. nasutus group: A. falcirostris, A. nasutus, A. isalineae, possibly A. maculipinnis; Not assignable to any group: A. heterolepis. The family Acestrorhynchidae is most closely related to the Cynodontidae family, which includes the genera Cynodon, Hydrolycus, and Rhaphiodon.
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