Taxonomy & naming
Cynodonichthys fuscolineatus was described in 1980 by William A. Bussing, originally as Rivulus fuscolineatus, in his comprehensive treatment of the freshwater fishes of Costa Rica — a foundational work that described or catalogued much of the country's rivuline killifish diversity. The specific epithet fuscolineatus (Latin, "dark-lined") refers to a dark lateral stripe or line pattern noted in the original description. Following Costa's 2011 phylogenetic revision of the old, broadly circumscribed genus Rivulus, the Central American lineage to which this species belongs was transferred to Cynodonichthys. Eschmeyer's Catalog of Fishes lists the valid combination as Cynodonichthys fuscolineatus (Bussing, 1980), within the family Rivulidae.
Cynodonichthys is one of several genera — alongside Anablepsoides, Laimosemion, Atlantirivulus and Melanorivulus — into which the old, unwieldy Rivulus was split once molecular phylogenetics clarified that the genus as originally conceived was not a natural group. Cynodonichthys specifically covers the Central American and northern South American rivuline lineage.
Morphology
As with other Cynodonichthys, C. fuscolineatus is expected to show the characteristic rivuline body plan: an elongate, cylindrical body, a flattened head and dorsally placed eyes suited to an ambush-feeding lifestyle near the surface of shaded forest-stream pools. The dark lateral stripe referenced in the specific epithet is presumably a key field character, though a full modern description of colour pattern and precise measurements was not confidently available from the sources consulted here — data sparse.
Sexual dimorphism in Cynodonichthys generally follows the rivuline pattern of more vividly marked, often more elongate-finned males against plainer, rounder-bodied females, but species-specific detail for C. fuscolineatus is not confirmed here.
Habitat
Cynodonichthys fuscolineatus occurs in forest-stream habitat in Costa Rica, consistent with the genus's general preference for small, shaded, slow-flowing watercourses with leaf-litter substrate and dense marginal vegetation. Unlike the seasonal-pool annual killifish of South America, Central American Rivulidae such as Cynodonichthys generally occupy permanent water bodies and do not depend on an annual drying-and-refilling cycle.
Precise drainage-level locality data for this species were not confidently available from the sources consulted here — data sparse — though Costa Rica's Pacific- and Caribbean-slope forest streams both support a rich rivuline fauna of this general type.
Feeding
No dedicated dietary study of Cynodonichthys fuscolineatus was located. Rivulidae generally are carnivorous ambush predators, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and fallen insects — from just below the surface of forest-stream pools, a feeding mode consistent with the flattened, upward-oriented head typical of the family.
Mating
Courtship behaviour specific to Cynodonichthys fuscolineatus has not been documented in the sources consulted. Non-annual Rivulidae generally are egg-scattering spawners: pairs deposit adhesive eggs among leaf litter, fine roots or plant matter over an extended period rather than in a single event, without elaborate courtship display or nest construction.
Breeding
Published captive-breeding accounts specific to this species are data sparse. Non-annual Central American Rivulidae of similar size generally breed without particular difficulty in soft, slightly acidic to neutral water with fine-leaved plants or a leaf-litter substrate for egg deposition, with eggs hatching within roughly two to three weeks. Because these species do not require the diapause and dry-storage cycle of true annual killifish, they are generally considered more approachable for continuous-cycle breeding than their South American annual relatives — but specific husbandry parameters for C. fuscolineatus were not confirmed here.
In the aquarium
Cynodonichthys fuscolineatus is not established in the mainstream aquarium trade and is known mainly to specialist killifish keepers with an interest in Central American Rivulidae. General husbandry for the genus — soft, slightly acidic water, subdued lighting, dense planting or leaf litter, and a tightly fitting lid, as rivulines are capable jumpers — reflects the natural forest-stream habitat this genus is adapted to, though species-specific care notes for this species were not confirmed.
Conservation
Cynodonichthys fuscolineatus is listed by the CARES Preservation Program at the Endangered (CEN) priority level. Costa Rican and wider Central American forest-stream killifish face pressure from deforestation, agricultural conversion of catchment land, and the resulting sedimentation and warming of the small streams they depend on. Given the species' evidently restricted range within Costa Rica, ongoing habitat conversion pressure is sufficient grounds for the conservation concern reflected in its CARES listing, even without a detailed, published population assessment specific to this species.