Tropical Fish Hobbyist - October 2010

liveb e arers

unlimited
Breeding Livebearers
This month, for the traditional “Trick or Treat” switch that Ted Coletti and I do for our October columns (see “The Planted Tank” this month
for Ted’s advice on overwintering your
pond plants), I thought I’d talk a little bit
about the main trait that truly sets the
livebearers apart—their ability to deliver
live young—and provide some tips about
their breeding in the home aquarium.
There are several ways that fish can give
birth to live young, but the fish we most
often keep in the aquarium are divided
into two main groups that accomplish this
in two somewhat different ways.

Rhonda Wilson has a lifelong interest in all things aquatic and started keeping aquariums at age six. She has a fishroom with about 80 planted tanks. Rhonda has read TFH since the mid 70s, and she co-authored the T.F.H. book The Simple Guide to Planted Aquariums. Active in local and national aquarium groups for over 16 years, including as the past chairman of the American Livebearer Association, she now maintains a forum on her website at http://naturalaquariums.com.

photographs by the author
rhonda wilson

Poeciliids

As with most fishkeepers, some of my first fish were guppies, which are poeciliids. I remember a little black male guppy that my mom really liked and eventually became a family favorite, and since then I’ve had a lifelong fondness for guppies—especially black-tailed or black-bodied ones. I usually don’t keep many other fancy livebearers, but I have been keeping Giessen guppies for 15 years.

Poeciliids are the best known of the livebearers. There are many kinds of domesticated poeciliids, and the group includes not only guppies, but also mollies, platies, and swordtails. Breeders have developed strains of these fish in a huge variety of beautiful shapes and colors.

In addition to the well-known aquarium varieties, there are many other livebearing poeciliids that still retain their natural characteristics and can be found in the aquarium hobby. These wild-type fish may not be regularly available at many fish stores but can sometimes be found in some specialty shops, through aquarium clubs, and from individual breeders. Sources can be found locally, through mail order, or online.

Poeciliids are usually ovoviviparous, which means the eggs are stored in the body until they hatch and are released as free-swimming fry. There are also some poeciliids that lay eggs, but most of the American poeciliids are livebearers. Even though the eggs are kept in the mother’s body after fertilization, the parent fish provides little or no more nutrients than what is originally available in the egg’s yolk.

Determining the sex of many domestic poeciliids is straightforward: The males may be more colorful, or have larger finnage or tails, but determining the sex of all livebearing poeciliids is fairly easy because the males and females have different-looking anal fins. The anal fins of the females and fry are rounded while the fin rays in those of the males are longer and form a tube-like structure. This structure, the gonopodium, is used to transfer sperm to the female. The end features microscopic hooks, which help hold the gonopodium in place while sperm is transferred from the male to the female fish. A female may retain sperm to fertilize later batches of fry, so she can have more than one litter even if she is no longer with a male.

In domestic poeciliids that have extremely elongated fins, the gonopodium is no longer functional, as in the Giessen guppies and some of the lyretail swords or mollies.

This is why, when breeding these fish, long-finned females are bred to normal-finned males that have a shorter, functional gonopodium. The resulting fry will have a mix of finnage types.