October Fish of the Month: Green Swordtail Xiphophorus helleri Fish of the Month

Dr. Geoffrey Smith
NWFSC Biology Instructor

The Green Swordtail, which is native Central America, has been introduced in to Florida’s fresh and brackish waters via the aquarium trade and hobby. They have also established populations in several other states. Green Swordtails have escaped from fish farms during flood events and have been released by aquarists. Being a semi-tropical fish, Green Swordtails are most commonly found in south Florida, but there are several populations in the Tampa Bay area (particularly around Ruskin, FL where there are a large number of fish farms). However, the reason I’m talking about them, is that there is also a wild population of this species in our own backyard in several water bodies in Mary Esther and Fort Walton Beach. This population was first documented by Hurlburt Field staff and researchers at the University of West Alabama (UWA) and is thought to have originated prior to 2007. It took some digging to find this documentation when we first encountered this species about two years ago and thought we had found a new undocumented populations. The Hurlburt and UWA staff documented this species in a few drainage ditches and creeks on base property as well as in the small creek at Liza Jackson Park, which is where we found them in a study documenting fish use of several local tidal creeks (Fig. 1). We also saw them in another creek in FWB and have heard of them being seen or captured in several other local water bodies. It is likely that this species is fairly widespread in the FWB area and just has not been well documented due to a lack of fish monitoring in area. At some point, we hope to better establish how widespread this species is in our area. It is suspected that this population originated from an aquarium release, possibly originating in Hurlburt Field.

Now we’ll actually get into some of the features of this species. The Green Swordtail’s common names is in reference to the iridescent blue-green coloration on the sides of this fish, especially males, and the elongated lower caudal (tail) fin lobe of males. The genus name is derived from the Greek words xiphos and pherein, which means sword bearer, in reference to this elongated caudal fin. And the species names is in honor of an Austrian naturalist, Karl Bartholomäus Heller. The background coloration of this species is typically gray to white, with several stripes running down the side of the body that are shades of red to brown and iridescent blue-green. The male’s “sword” is typically also iridescent blue-green with a black border (Fig. 2). In the aquarium hobby there are also a variety of color variants that have been produced through selective breeding and hybridization with several closely related species (Fig. 3). The swordtails belong to the family Poeciliidae, which are commonly referred to as the livebearers. The males of this family have a modified anal fin that serves as a copulatory (reproductive) organ to internally fertilize females. The females retain the fertilized eggs while they continue to grow prior to giving birth. In most cases, the young are growing only from the yolk in the eggs and not via a placenta-like connection to the mother. This type of egg development is known as ovoviparity or ovoviviparity, which loosely translates bearing live young from an egg, compared to oviparity where eggs are laid and viviparity where live young are born after having a direct connection to the mother for development. Swordtail brood sizes can range from 10 to over 100 young, and the females are capable storing sperm from a mating event for up to a year. These combined features allow them to very rapidly reproduce and spread in an area, which is part of the reason they have been able to establish non-native populations with relatively few individuals. Green Swordtails are relatively small, not overly aggressive, and eat algae and a variety of small invertebrates. Because of this, they appear to have had relatively minimal impacts in their introduced ranges. There appear to be some food competition with native species in isolated hot springs in the western US and possibly links to declines of native aquatic insects in Hawaii.

Fig. 3: Some examples of selectively bred and/or hybridized Green Swordtail varieties (www.emeraldking-aquatics.com).