biology zoology blog sex environment animals

Sex and the Environment

There is a connection between sex and the Environment

Did you know that the majority of the eels found inland in freshwater lakes and rivers are (or become) females, while those which do not migrate upstream but remain in the estuary are males (or become males)? It is still under investigation as to whether eels stay in these habitats because of their gender or their sex is, in fact, determined by the environment in which they mature. That the latter is entirely possible has been shown for a large number of fishes and reptiles. In fact, way back in 1961 as a school boy I must have been one of the first to take note of environmental sex determination in fishes through temperature, but I never published my observations, then for as the school boy that I was at that time I had no idea of scientific reports and publications and how to prepare them.

What did I observe? All my Heterandia formosa baby fish, all 15 of them, had turned into males when the aquarium temperature was 30°C, but at 18°C I obtained only females from the offspring of these live-bearing approximately 3 cm long fish. The school boy’s findings were confirmed in 1986 when Sullivan and Schultz reported almost identical results in the scientific journal “Evolution”. Other environmental factors, which were later found to influence sex determination in some species of fish included the pH of the water (the more acidic, lower pH led to an increased number of males), salinity, photoperiod, and nutrition.

In turtles, too (but I did not, of course, observe that myself), the environment, and in particular incubation temperature, can have a profound influence on the sex of the young. In species in which the female is the larger individual of the sexes, warmer temperatures lead to predominantly females. When, however, the males are bigger, as, in crocodiles, the situation is reversed. In the various invertebrate animal groups examples of environmentally induced sex determinations are even more numerous and population density, temperature, humidity and quantity as well as type of nutrition are most frequently involved.

One of the earliest known and probably the most famous example of environmental rather than chromosomal / genetic determination comes from the marine spoonworm Bonellia viridis. If the larvae in this worm early in their life settle on a female individual they will turn into tiny male sex partners, but if such a rendez-vous does not take place, the solitary larva gives up the search and settles into turning into a female and then starts to grow and grow and grow until it is about 1,000 times the size of a male.

Thus is the benefit for the individual that postpones sex till later – in the worm Bonellia viridis, but not in those deep-water angler fishes in which dwarf males are the rule. For males of these deep-sea species it is vital they find a big female body that they can literally attach to and give up their independence, for if they don’t they are doomed and have to die. To locate a host female, the desperate little males use their keen sense of smell and vision, but once they’ve reached their goal it is the female “environment” (possibly signals delivered from the female via the fused blood system between the attached male and the host female) that causes an almost complete degeneration of the male’s inner organs including sensory ones and muscles, with the exception of the sperm producing testes: they are important and the only reason why female anglerfishes tolerate these parasitic suckers.

© Dr V.B. Meyer-Rochow and, 2018.
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