September Fish of the Month: Gulf Killifish Fundulus grandis

Dr. Geoffrey Smith

NWFSC Biology Instructor

National Estuaries Week will be occurring later this month from September 21-28, so for this month we’ll be highlighting a fish that is essentially ubiquitous in estuaries of the Gulf coast, the Gulf Killifish. They are also found along the Atlantic Coast of Florida (Figure 1). Near the Florida-Georgia border the Gulf Killifish and its sister species, the Mummichog Fundulus heteroclitus, overlap in the range with the Mummichog replacing the Gulf Killifish in more northern estuaries on the Atlantic coast. The Gulf Killifish is one of the largest members of the topminnow family (Fundulidae), reaching lengths of nearly 8 inches. Most of the killifish species found in coastal estuaries are limited to certain parts of the estuary. For example, Marsh Killifish Fundulus confluentus are found in upper, highly vegetated salt marshes in a wide range of salinities while Longnose Killifish Fundulus similis are typically found near the mouths of tidal creeks and along bay shorelines with higher salinities. However, Gulf Killifish are found throughout estuaries from low to high salinities and from tidal creeks to open bay shorelines. They are an important link in estuarine food webs, they consume a variety of small organisms, mosquito and other aquatic insect larvae, small shrimp, benthic worms, and amphipods, as well as benthic algae. In turn they are an important food source for wading birds, such as Great Blue Heron and Great Egret, recreationally important fish species like Red Drum, Gulf Flounder, and Spotted Seatrout. Gulf Killifish are harvested and aquacultured in moderate quantities and then sold as live bait under the common names Mud Minnow, Bull Minnow, or Cocahoe Minnow. Like most other topminnows and related species, the females are generally a drab coloration while the males, at least when they are breeding, have much brighter colors. Female Gulf Killifish are typically a gray to silver color on the top and side, white below, with darker bars on the sides and a yellow tinge to the fins (Figure 2). The bars and yellow in the fins are more pronounced over darker colored bottom, like mud or seagrass, or in stained (tannic) water. The males have the same base coloration, but with yellow to orange fins and body on the back half with iridescent speckling (Figure 2). This coloration is especially pronounced in stained water and when the males are actively courting females (Figure 3).

Figure 1: Distribution of Gulf Killifish within the Caribbean and Gulf of Mexico (https://biogeodb.stri.si.edu/caribbean).

Figure 2: Typical non-breeding male (left) and female (right) Gulf Killifish coloration. Photo credit: Logan McDonald (Pensacola and Perdido Bays Estuary Program).

Figure 3: Typical breeding male Gulf Killifish coloration. Photo credit: Zachary Randall (Florida Museum of Natural History)