July Fish of the Month: Marsh Killifish Fundulus confluentus

Dr. Geoffrey Smith
NWFSC Biology Instructor

The Marsh Killifish is a small fish species with a maximum size of about 3-4 inches. They belong to the family Fundulidae (named after one of the most abundant genera of the group), which are commonly called topminnows due to the tendency to reside just below the surface of the water. The genus name Fundulus is derived from a Latin term meaning “bottom”, and is thought to refer to the tendency of these fish to reside in shallow, muddy bottomed waters where they will often bury in the mud to escape predators. The species name confluentus is derived from a Latin term meaning “flowing together” and may have been in reference to either the schooling behavior of this species or its fusiform (torpedo-shaped) body.

This species exhibits what is known as sexual dimorphism, or a clear, distinguishable difference in the external appearance of males and females. The females are typically drab in their coloration. They are shades of gray to brown with some darker brown to black mottling and bands (vertical bars) and a dark spot towards the rear of the dorsal fin (the fin found on the top of a fish) (Fig. 1). On the other hand, males can be much more vivid in their coloration. Male Marsh Killifish have the same gray to brown background of the females with some dark vertical bands, but their mottling (and sometime bands) is iridescent white, yellow, or orange and reminiscent of fireworks (a belated 4^th of July tie in). The dorsal (top), anal (bottom), and caudal (tail) fins are also yellow to orange in hue, and the males lack the black dorsal fin spot commonly seen in females (Fig. 2). The iridescence and yellow to orange hue of the fins is greatly enhanced when males are actively courting females (Fig. 3).

But beyond their flashy appearance, why do we care about Marsh Killifish? They are an important link in food webs (who eats who and how that food energy moves through the whole system) of estuaries in the southeastern US. They spend their whole lives in estuaries from Texas through the Gulf coast states and into the mid-Atlantic region (Chesapeake Bay area). Here they feed on small aquatic and terrestrial invertebrates (insect larvae, insects that fall into the water, worms, small crustaceans, etc.) and serve as a food source for many juvenile and sportfish (such as Red Drum, Gulf Flounder, and Common Snook) and wading birds (such as Snowy Egret, Great Blue Heron, and Reddish Egret). This links nutrients that came from the land to the wider estuarine ecosystem.

Marsh Killifish are also viewed as what’s known as an indicator species. An indicator species is some type of organism that “indicates” a particular condition. For example, some microscopic aquatic organisms are linked to excess nutrients in a water body while others may indicate more ideal or pristine conditions. Marsh Killifish typically indicate an intact healthy upper estuarine system when they are found in relatively large numbers.

The Marsh Killifish is just one member of a number of estuarine species from the Family Fundulidae (Killifish and Topminnows). Most of the other common estuary members of this family, such as the Gulf Killifish (aka Mud Minnow or Bull Minnow for the anglers out there) and Longnose Killifish (aka Mud Minnow or Tiger Minnow for the anglers), are found more broadly in estuaries, from the backwaters to open shorelines, and often in more disturbed or altered habitats. Marsh Killifish, on the other hand, are found almost entirely in the upper reaches of small tidal creeks, ponds, and lagoons where there is a clear connection to both the land and the wider estuary and abundant emergent (growing out of the water) shoreline vegetation, such as mangroves and salt marsh grasses (Fig. 4).

There is a very limited harvest of Marsh Killifish for live bait (usually as bycatch when harvesting Gulf or Longnose Killifish) and for aquarium display by native fish enthusiasts.