Taxonomy & naming
The species was described by López S. in 1983, originally in the genus Cichlasoma (later treated under Amphilophus and Astatheros), from the Atlantic-slope Sixaola drainage of southeastern Costa Rica. The epithet rhytisma derives from the Greek rhytisma, a patch or blotch, referring to the dark markings on the flank. The species was transferred to the new genus Cribroheros when Říčan and colleagues (2016) restricted Astatheros to its single Pacific type species and erected Cribroheros for the Atlantic-slope substrate-sifting heroines. Cribroheros belongs to the heroine cichlids (tribe Heroini), the genus name referring to the sieve-like sifting feeding of the group. It is closely allied to, and shares its Sixaola-basin range with, Cribroheros bussingi, from which it is separated on coloration and meristics.
Morphology
Cribroheros rhytisma is a small cichlid, FishBase reporting a maximum of about 5.5 in SL. It has the moderately deep, robust body of the small southern Cribroheros, with a silvery-grey to olive ground bearing the dark flank blotch or patch that names the species, a broken midlateral line of markings, a dark caudal-base spot, and barring that strengthens with mood and breeding condition; fine spangling and reddish fin margins appear in good colour. The mouth is sub-terminal, suited to sifting soft substrates. It is distinguished from the sympatric Cribroheros bussingi by details of its coloration (notably the flank patch), body proportions and meristics. Sexual dimorphism follows the heroine pattern: mature males grow somewhat larger with more produced, pointed dorsal and anal fin tips, while ripe females are rounder-bodied and may show more contrasting markings when breeding.
Habitat
Cribroheros rhytisma is restricted to the Atlantic slope of southeastern Costa Rica, in the Sixaola River drainage on the Panama border. It inhabits warm lowland and foothill rivers and their pools and slower reaches, over substrates of sand, mud and gravel where it can sift for food, among rock and submerged wood. FishBase cites a relatively cool temperature range of roughly 75–79 °F, consistent with the higher-gradient Caribbean-foothill streams of this region. Its narrow range overlaps that of Cribroheros bussingi within the same drainage; together they are the characteristic small sifting cichlids of the Sixaola system.
Feeding
Cribroheros rhytisma is a benthic omnivore and substrate-sifter with a low trophic level — FishBase places it near 2.5, toward the detritivore- and herbivore-leaning end of the genus. Its recorded diet is aquatic insects and detritus, taken by sifting the soft bottom; like its congeners it takes mouthfuls of sediment and sorts out the edible fraction in the winnowing manner that gives the genus its name. The sub-terminal mouth suits this bottom-oriented diet. In its river communities it functions as a small generalist forager of the substrate, working the soft bottoms of the Caribbean-slope foothill streams alongside its close relative Cribroheros bussingi.
Mating
As a heroine cichlid, Cribroheros rhytisma is a monogamous, biparental species that pairs and defends a joint breeding territory. Pair formation follows the standard cichlid courtship — lateral display, fin-flaring, jaw contact and cooperative cleaning of a spawning surface — with the pair intensifying in colour and becoming territorial as spawning nears. As a small species it is only moderately aggressive, but a bonded pair defends its patch firmly during the breeding cycle. Pairs select and clean a hard surface in advance of spawning.
Breeding
Cribroheros rhytisma is a biparental open substrate spawner. The female lays a clutch of adhesive eggs on a cleaned hard surface, and both parents share extended care: the female tends and fans the eggs while the male guards the territory, the roles converging as the brood develops; once free-swimming the fry are guarded as a school by both parents, who lead them over the soft bottom where they begin to forage. Clutch sizes in so small a fish are modest by genus standards. Wild breeding behaviour is not exhaustively documented, but it follows the well-established biparental-substrate-spawner pattern of the group; the species is rarely kept, and its captive reproduction is correspondingly little recorded.
In the aquarium
Cribroheros rhytisma is very rarely seen in the aquarium trade and is kept, when at all, by specialists in southern Central American cichlids. Its small adult size means modest space requirements — a tank of around 40–65 US gal would suit a pair. Reflecting its cooler foothill-stream origins it should be kept at the lower end of the tropical range (about 75–79 °F) in neutral to moderately alkaline, well-oxygenated water with good filtration and regular water changes. As a substrate-sifter it should be given a bed of fine sand, which lets it feed naturally and spares its gills the abrasion of gravel; furnish with rock and driftwood forming broken sight-lines and a flat stone as a spawning site, with hardy anchored plants. As a small, moderately aggressive species it mixes reasonably with other robust fish of similar size and water needs, with breeding pairs given space at spawning time. Feeding should reflect its detritus- and insect-based natural diet — a quality sinking pellet plus frozen and live invertebrate foods and some vegetable matter. The main husbandry note is to provide sand and respect its preference for cooler, well-oxygenated water.
Conservation
The IUCN Red List now assesses Cribroheros rhytisma as Endangered, a marked increase in concern from the Least Concern rating given in the 2007 assessment. The species remains restricted to the Sixaola drainage on the Costa Rica–Panama border, a lowland and foothill river system exposed to deforestation and conversion to banana and other plantation agriculture, with the associated runoff, sedimentation and agro-chemical pollution, together with introduced species — pressures that weigh heavily on a distribution confined to a single border drainage. It carries no CITES listing. As with all of the genus, the health of its river system is the key determinant of its long-term status.