Taxonomy & naming
Crenicichla tendybaguassu was described in 1992 by Carlos A. S. de Lucena and Sven O. Kullander in their landmark revision of the pike cichlids of the Uruguay (Uruguai) River drainage in Brazil, published in Ichthyological Exploration of Freshwaters (vol. 3, pp. 97-160). The holotype (MCP 10876) is a specimen from the rio Piratinim at Fazenda dos Hinz, Coimbra, municipality of Santo Ângelo, Rio Grande do Sul, southern Brazil, with paratypes drawn from several upper-Uruguay tributaries across Rio Grande do Sul and Santa Catarina. No junior synonyms have been proposed; the name has been treated as valid by every subsequent worker. The genus Crenicichla Heckel, 1840 is the most species-rich of all cichlid genera, with on the order of 90 valid species, and Crenicichla tendybaguassu sits within its southern 'lacustris group' — specifically the small Uruguay River species flock (the so-called missioneira complex) alongside Crenicichla missioneira, Crenicichla minuano, Crenicichla celidochilus and Crenicichla hadrostigma. A 2023 integrative revision by Varella, Kullander, Menezes, Oliveira and López-Fernández (Zoological Journal of the Linnean Society) reorganized the pike cichlids: it kept Crenicichla tendybaguassu in the genus Crenicichla but placed the lacustris-group lineage in the subgenus Lacustria, while moving many other former Crenicichla into separate genera (Saxatilia, Lugubria, Wallaciia and others). Because that subgeneric arrangement is still settling in and the trade has not adopted it, this profile keeps the long-standing combination Crenicichla tendybaguassu.
Morphology
This is a medium-sized pike cichlid, reaching about 7.5 in total length and a published weight near 80 g; most adults seen are somewhat shorter. The body is the classic Crenicichla 'pike' silhouette — moderately elongate and torpedo-shaped, with the dorsal and anal fins set well back, a rounded caudal fin, and long, isognathous (evenly meeting), sharply pointed jaws. Body depth runs roughly 19-23% of standard length. The single feature that instantly identifies the species is its lips: they are hypertrophied and fleshy, drawn out into long medial lobes, a development described by Lucena and Kullander as unique within the entire genus. Other diagnostic traits include a smooth-margined (non-serrated) preopercle and a suborbital stripe reduced to a few scattered spots rather than a continuous bar. Live coloration is a greenish-gray ground broken by irregular black blotches above and below the upper lateral line that reach up to the dorsal-fin base; the iris is red, dark preorbital and postorbital stripes run across the face, and the dorsal fin is red at the base and middle, black distally, and carries a black subcircular spot that in some fish is ringed with white. The caudal fin bears a black subcircular spot just above its midline, and orange pigment flushes the flank behind the pectoral fin. As in most pike cichlids the sexes differ: breeding females, first documented in life from Uruguayan specimens, develop the most intense red-and-black dorsal markings and orange flank flush, while males tend to grow larger and show the most exaggerated lips.
Habitat
Crenicichla tendybaguassu is endemic to the Uruguay River drainage of subtropical South America, recorded from the upper Uruguay basin in the Brazilian states of Rio Grande do Sul and Santa Catarina, from the Province of Misiones in Argentina (the Soberbio River, per the original description), and — since 2011 — from the middle Uruguay in northern Uruguay, where Serra and colleagues collected it in the upper Cuareim (Quaraí) River on the Brazil-Uruguay border, extending the known range roughly 249 mi south. It is a benthopelagic, strictly freshwater fish of running water. The habitat that defines it is shallow, clear, fast-flowing rapids floored with rock and broad rock slabs: at the Cuareim River sites the water is clear and shallow with abundant rocks, the very structure that makes crevice-feeding, periphyton-grazing and molluscivory all viable for the resident pike-cichlid assemblage. There it is genuinely sympatric with several congeners — at one stretch it co-occurs with Crenicichla minuano, Crenicichla missioneira, Crenicichla celidochilus, Crenicichla scottii and Crenicichla lepidota, all caught from a single reach of shoreline. These are subtropical waters rather than tropical Amazon lowlands, so they are warm in summer but cooler and more seasonal than equatorial rivers; published aquarium guidance places the species in the roughly 64–79 °F range with near-neutral, soft to moderately hard water. Because it is a shallow-rapids specialist, its depth band is narrow — effectively the rocky shallows, on the order of the first metre or two of the water column.
Feeding
Crenicichla tendybaguassu is the crevice-feeding specialist of the Uruguay River species flock, and its whole skull is built for the job. The hypertrophied lips function as a compliant gasket: pressed against the uneven faces of rocks and rock slabs, they let the fish form a seal and suck out the rock-associated invertebrates — insect larvae (caddisflies, midges and the like) and crustaceans — that shelter between, beneath and along the stones. This diet is supplemented with small fractions of snails, generally swallowed whole. Matching the prey, the oral jaws are long and robust and the pharyngeal jaws are intermediately built, bearing simple conical teeth rather than the molars of the flock's mollusc-crushers. FishBase estimates a trophic level near 3.3, consistent with a carnivore taking hard- and soft-bodied invertebrates rather than a dedicated piscivore. The hypertrophied 'big-lip' phenotype is an evolutionary convergence that has appeared independently across cichlids worldwide — in Lakes Tanganyika, Malawi and Victoria, in Nicaraguan crater lakes, and in river fishes — wherever rocky, crevice-rich habitats reward it. Within all of Crenicichla it has arisen only twice, in parallel: in Crenicichla tendybaguassu of the Uruguay flock and in Crenicichla tuca of the Paraná flock (with some Uruguay populations of Crenicichla missioneira reported to develop thickened lips, possibly through polymorphism or hybridization with Crenicichla tendybaguassu).
Mating
Detailed wild courtship has not been published for this species specifically, but pike cichlids share a consistent reproductive playbook and Crenicichla tendybaguassu fits it. They form monogamous pairs and become strongly territorial as spawning approaches. In the genus, pairing is preceded by mutual displays and, often, considerable initial aggression that has to be worked through before two fish settle into a bonded pair; a hard-won pair then jointly holds a territory centered on a chosen spawning cavity. Sexual signalling is colour-based — the female's intensified red-and-black dorsal fin and orange flank flush, documented in breeding-condition Uruguayan fish, advertise readiness — and the larger, thicker-lipped male takes the perimeter role. Aquarium experience with the closely related members of the flock shows that fully coloured dominant males are intensely intolerant of rival males but generally leave females and subordinate fish alone, so successful pairing turns on giving the fish enough room and structure to negotiate the relationship without lethal conflict.
Breeding
Like all Crenicichla, Crenicichla tendybaguassu is a biparental cave (substrate) spawner. The female lays a clutch of adhesive eggs on a sheltered surface inside a cavity — among these rapids-dwellers, a crevice or hollow under rock — where the eggs cling rather than wash away in the current. Care is divided in the typical pike-cichlid pattern: the female tends, cleans and fans the eggs and guards the wrigglers within the cave, while the male patrols and defends the surrounding territory against intruders. In the genus, eggs hatch in roughly three to four days depending on temperature, with the larvae becoming free-swimming a few days after that; the pair then continues to guard the mobile fry, which retreat to cover when threatened. Wild clutch size has not been published for this species, but it is genus-typical: congeneric pike cichlids of similar size lay on the order of a few hundred adhesive eggs per spawning. The species was, in fact, bred in captivity — most notably by Matthias Kählig in Hannover, Germany, during the brief period (from about 2006) when wild fish reached the European hobby — which is what earned it its reputation as the most interesting of the Uruguayan imports of that era.
In the aquarium
Crenicichla tendybaguassu is a connoisseur's pike cichlid rather than a common aquarium fish. It appeared in the European and North American hobby only as sporadic wild imports from the Río Uruguay drainage beginning around 2006, was bred by a handful of dedicated keepers, and then 'virtually disappeared into oblivion' within a few years as the exports dried up; it is rarely offered today and commands a specialist following when it is. Anyone keeping it should treat it as a subtropical rapids fish, not a warm blackwater one. That means a long tank with a generous footprint — on the order of 47 in / 120 US gal (roughly 100+ US gallons) or larger for a pair, since these are territorial 7–8 in predators — strong, well-oxygenated flow, and clean water with low dissolved waste; large, frequent water changes are the single most important husbandry habit for the genus. A scape of stacked rock and rock slabs with caves and crevices both suits its biology and provides the spawning sites and sight-breaks that defuse aggression. Temperatures in the roughly 64–79 °F band (cooler and more seasonal than Amazon species tolerate) with near-neutral, soft to moderately hard water mirror the wild rapids. Feed a carnivore's menu — live or frozen invertebrates such as bloodworm, mysis, krill and small crustaceans, plus earthworms and cut seafood — rather than a piscivore's fish diet; the big lips are for picking invertebrates from crevices, not for hunting fish. The hardest part is social management: pike cichlids of this group can be lethally aggressive toward conspecifics and unsuitable tankmates, so pairs must be formed carefully in a large, structured tank with escape routes, and tankmates should be robust, similarly sized, open-water species from the same subtropical rivers (for example Gymnogeophagus or Australoheros) rather than anything small enough to be eaten or slow enough to be bullied.
Conservation
The IUCN Red List assesses Crenicichla tendybaguassu as Least Concern (assessed 18 November 2020 by R. G. Frederico, published 2023). It carries no CITES or CMS listing and is considered harmless to humans. The category reflects a species that, while not wide-ranging, is distributed across three countries' share of the Uruguay drainage and is locally common in suitable rocky habitat. It is not a food fish and has only ever moved through the aquarium trade in small numbers, so collection is not a population-level concern. The more meaningful pressures are on the rivers themselves: the Uruguay basin is heavily dammed and continues to attract hydroelectric development, and impounding free-flowing reaches converts exactly the shallow, fast, rocky rapids this crevice-feeder depends on into still reservoir, which is far less suitable for it and for the rest of its specialized species flock. The Cuareim/Quaraí River, where the species was found new to Uruguay, has been highlighted as a high-diversity area whose conservation value is reinforced by hosting Crenicichla tendybaguassu alongside other priority fishes. As a narrow-niche endemic of a specific habitat type, its long-term security depends more on keeping those rapids free-flowing than on any direct threat to the fish.