This article is from
Creation 41(4):55, October 2019

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Fish giving birth?

by Philip Bell

Excited comments about ‘fish pregnancy’ and wild claims of ‘evolutionary leaps’ accompanied the capture by scientists in February 2019 of a female stickleback fish from waters of the Outer Hebrides.¹ She was unexpectedly found to be carrying fertilized, developing fish eggs within her body. Why the fuss?

We know that most mammals give birth to live young; while birds, reptiles, amphibians, and fish lay eggs, right? Well, there are some exceptions to the rule. For instance, whereas most species of lizards are oviparous (they produce eggs), a few are truly viviparous, meaning they undergo live birth—and yes, those lizards nourish their unborn babies through a type of placenta, as do mammals.²

Fish ‘pregnancy’ like this has been seen before on very rare occasions, but this is the “the first time that [such] embryos were birthed and developed into healthy adults.”³

Male seahorses do appear to give birth to miniature versions of themselves but it is the female who produces the eggs—she lays hundreds of eggs inside the male’s brood pouch where they are fertilised, then nourished as they develop into young seahorses, then squirted out into the ‘outside world’.⁴

The recent scientific report that captured attention and spiked imaginations concerns a single three-spined stickleback fish (Gasterosteus aculeatus), a species that is 3–4 cm (1.2–1.6 in) long at maturity. Nicknamed ‘Mary’, one news outlet exclaimed that she “appears to have leapt across a major reproductive divide on the evolutionary tree of life.”³ This, however, is overblown nonsense, as we shall see.

Many wildlife films have depicted fish reproduction: as soon as the female lays her eggs, the male fertilizes them. It’s the same story with many thousands of fish species. In Mary’s case, the researchers were both intrigued and puzzled to discover that she was carrying developing eggs inside her body.

Was this a case of a virgin birth (technically known as parthenogenesis), as occurs in creatures as diverse as aphids⁵ and komodo dragons?⁶ For a fish’s unfertilized eggs to develop inside her is unusual but certainly not unprecedented.⁷

Apparently, however, genetic tests ruled out this possibility.⁸ Instead, researchers concluded that male stickleback sperm had somehow migrated into Mary’s body, likely through the tube that serves to permit passage of the eggs to the outside, and had fertilized her yet-to-be-released eggs.

Contrary to journalistic hype, this case provides no insights into the supposed ‘evolutionary leap’ between egg-laying and live birth.³ In fact, Mary’s pregnancy was on the verge of killing her. She became so bloated that she needed a caesarean section to ‘deliver’ her tiny fish embryos—56 of these microscopic fish fry survived to become healthy adult sticklebacks, but only by being carefully reared in the lab. Sadly, Mary didn’t survive the ordeal.