The Facts of Amazonia Fish Life
by Paul Sikkel
In the cacophony of swimming fishes in the Zoo's Amazonia exhibit, it's difficult to decide what to focus on. For most of us, fish behavior is unfamiliar and, asked what some fish is doing, we're apt to answer "swimming," and then move on. But if you know what to look for, you may catch a glimpse of the fascinating and diverse reproductive behaviors displayed by these freshwater fishes native to the Amazon Basin.
Take the lemon tetra, which is named for its bright yellow fins. This small fish is a relative of familiar aquarium fishes such as neon tetras and silver tetras, which you also can see in Amazonia, as well as the infamous sharp-toothed piranhas and the bizarre blind cave fish of Mexico and Brazil (look for the former in the Bat Cave). Lemon tetras live in schools of from several dozen to several hundred and can be found near cover.
Female lemon tetras spawn about every four days, so chances are good you will see this behavior-if you get up early: A female releases about 150 eggs in about 20 spawning episodes during the first four hours of daylight. Scientists call the act of releasing eggs and sperm a "spawning quiver" because the bodies of the breeding fish appear to vibrate or quiver during gamete release. The female quivers her eggs into the water column, usually near aquatic plants, and they are fertilized simultaneously by one or more males. The falling fertilized eggs then either stick to a plant or to the substrate, where they hatch in about one day.
If you don't see any quivering female lemon tetras, look for courting males, which are very conspicuous. Excited males, usually silver with a peppering of dark pigment, turn darker, then employ one of two strategies to increase their spawning success. Some males establish a small territory near some vegetation and perform elaborate courtship dances to attract females. In laboratories, where much of the reproductive behavior of tetras and most other Amazon fishes have been chronicled, these males stake out an area covering several feet over a suitable spawning substrate. A dancing male swims erratically, zigzagging, dipping, and swimming in circles. Other lemon tetra males may simply chase gravid females until they release eggs.
Studying lemon tetras, McGill University biologists Ken Nakatsuru and Donald Kramer shattered a long-held assumption about the mating patterns of animals. The prevailing view of behavioral and evolutionary biologists was that males in general had virtually no limit on how many females they could successfully mate with because sperm are so much smaller than eggs and "cheap" to produce. But this isn't the case in male lemon tetras. Nakatsuru and Kramer found that the proportion of eggs a male fertilized in a spawning episode declined with the number of episodes he'd participated in that day. Further, by his thirtieth spawning episode in one day, the percentage of eggs he fertilized was virtually zero. In other words, male sperm supplies are rapidly depleted so that the first female a male spawns with in the morning will have most of her eggs fertilized, while the thirtieth may have none. The biologists also found that, perhaps not surprisingly, females prefer to spawn with "fresh" males, thus ensuring a higher fertilization rate for their eggs. However, Nakatsura and Kramer could find no obvious visual cues to tell the males apart.
Aquarium devotees may also recognize the "corys" in Amazonia. Members of the genus Corydoras, these small armored catfishes possess heavy, plate-like scales, the feature for which they are named. Mating is also a morning affair in the corys. Look for a female--which is larger and more robust than the male, with a rounded rather than pointed dorsal fin--swimming frantically about, followed by one or more males. While she swims, the female stops and cleans prospective laying spots, sometimes assisted by an accompanying male. Eventually the male starts nudging the female behind her head, until she places her head in contact with the male's genital papilla, so the two partners form a "T." In other words, the female's head is near the male's vent with her body below and perpendicular to that of the male. The partners then quiver as they release eggs and sperm for ten to 20 seconds.
What happens next has long been a mystery. Exactly how do the sperm reach the eggs with the pair in the T position? In an ingenious study, Masanori Kohda and coworkers at Osaka City University in Japan solved the puzzle. To follow its course, they dyed the sperm of males and found that the female swallows the sperm while she is in the T position. It then passes through her digestive tract and exits her vent near her newly laid eggs four or five seconds later. Unlike many fishes, females do not dump their eggs into the water. Rather, they release them into a ventral fin pouch. After they are fertilized by sperm that have passed through her digestive tract, she carries the eggs to a suitably cleaned spot and deposits them there. Depending on the species, a cory female may deposit all her eggs at one site, or deposit one or a few at many different sites. In the latter, a female may spawn more than 15 times, with a different male partner for each spawning incident. Again depending on the species, a female may lay anywhere from 20 to 800 eggs in a spawning series, which can last from a half hour to several hours.
You can look for similar mating behavior in the suckermouth armored catfish, members of the genus Plecostomus (now referred to by many as Hypostomus), in which male and female also form the T position. Courting males stand out, however, as they develop bony bristles on their heads that are used to touch the female's body in movements that may act as a form of tactile courtship. (A variety of other fish have such structures.) Males may also be observed caring for their eggs--fending off intruders, cleaning the sheltered area around the eggs--until they hatch one to two weeks after mating.
Finally, look for the curious earth-eater cichlid, Geophagus jurupari. This species gets its name from its habit of sucking in, then spitting out mouthfuls of soil while searching for small invertebrates to eat. You might see earth-eater cichlids defending shelter and breeding territories in small caves or crevices. The female lays her eggs on stones nearby and, after they are fertilized, covers them with a layer of sand. Once the eggs hatch, the female carries the fry in her mouth for about 14 days to guard them.
Armed with this quick glance into the private lives of a few fishes of Amazonia, you may surprise the next person who asks, what is that fish doing?
Paul Sikkel studies reef fishes at the Bellairs Research Institute of McGill University and is an adjunct professor of biology at Murray State University.
(ZooGoer 276) 1998. Copyright 1998 Friends of the National Zoo. All rights reserved.)
