The spotfin killifish is the smallest member of its genus, rarely exceeding 50 mm total length or 40 mm standard length.
Larval fish transition to juvenile stage at around 10 mm standard length.
Sexual maturity is attained at approximately 24–27 mm TL (males) and 28–30 mm TL (females).
Body shape is elongated and less stocky than Fundulus heteroclitus, with an upturned mouth, flattened head and rounded caudal fin characteristic of the genus.
Adults are sexually dimorphic, although both sexes are darker on top with a lighter belly.
Juveniles and adult females are similar in appearance, with a body that is grayish-green to olive green in color, lacking a dorsal ocellus, and generally without vertical bars, although females may sometimes have sidebars (usually faint).
Mature adult females may also have a visible sheath of tissue along the front of the anal fin, which is believed to function as an oviduct.
Adult males have 10–14 dark vertical bars and the eponymous dorsal ocellus.
During spawning season males develop dramatic coloration, consisting of vivid yellow-orange pigment extending over the belly, pelvic and anal fins, caudal peduncle, and caudal fin.
Breeding males may also develop contact organs–small fleshy protuberances–on the head, sides of the body, and medial fins.
Dorsal fin rays number 8–9 (usually 8) which is different from the mummichog, which has 11–12 dorsal fin rays.
The anal fin has 10 rays.
The spotfin killifish's dorsal fin originates posteriorly to the anal fin origin; this is another useful character that differs from other fundulids, including F. heteroclitus.
While adults are distinct, larval fundulids can be tricky to ID to species.
Longitudinal scale rows number 34–36.
The gill opening is restricted superiorly.
The spotfin killifish is the smallest member of its genus, rarely exceeding 50 mm total length or 40 mm standard length. Larval fish transition to juvenile stage at around 10 mm standard length. Sexual maturity is attained at approximately 24–27 mm TL (males) and 28–30 mm TL (females). Body shape is elongated and less stocky than Fundulus heteroclitus, with an upturned mouth, flattened head and rounded caudal fin characteristic of the genus. Adults are sexually dimorphic, although both sexes are darker on top with a lighter belly. Juveniles and adult females are similar in appearance, with a body that is grayish-green to olive green in color, lacking a dorsal ocellus, and generally without vertical bars, although females may sometimes have sidebars (usually faint). Mature adult females may also have a visible sheath of tissue along the front of the anal fin, which is believed to function as an oviduct. Adult males have 10–14 dark vertical bars and the eponymous dorsal ocellus. During spawning season males develop dramatic coloration, consisting of vivid yellow-orange pigment extending over the belly, pelvic and anal fins, caudal peduncle, and caudal fin. Breeding males may also develop contact organs–small fleshy protuberances–on the head, sides of the body, and medial fins. Dorsal fin rays number 8–9 (usually 8) which is different from the mummichog, which has 11–12 dorsal fin rays. The anal fin has 10 rays. The spotfin killifish's dorsal fin originates posteriorly to the anal fin origin; this is another useful character that differs from other fundulids, including F. heteroclitus. While adults are distinct, larval fundulids can be tricky to ID to species. Longitudinal scale rows number 34–36. The gill opening is restricted superiorly.
Fundulus luciae