Winter’s effect on animals isn’t always noticeable. Often, it’s because they’ve flown south, are hibernating, or are hunkered down, having reduced their activity to cope with cold and snow. Under the ice on ponds and lakes, however, hordes of planktonic animals, including tiny a crustacean, daphnia, are almost as lively in winter as in summer.
Pinhead-sized and with shrimp-like features, daphnia are active, busy producing new generations all winter long. They produce more frequently as spring arrives with rising temperatures and longer daylight hours.
Daphnia, perhaps the most common crustacean in fresh water, play an important role in the ecology of lakes and ponds. Because they appear in vast numbers, they are an important source of food for certain species of fish and other predators. As filter feeders, they consume bacteria, protozoa, rotifers and single-celled algae, some of which, if left unchecked, can create toxic conditions in the water.
Daphnia are short-lived, each lasting only a matter of days. Their summertime reproduction is through an asexual process known as parthenogenesis, in which females clone themselves to produce more females. In autumn, however, with reproductive trickery, males are produced that mate with existing females, their offspring providing genetic variation that precedes the next round of parthenogenesis. By spring daphnia clones are again reproducing at very short intervals as they head toward summertime abundance.
You might guess that these clones would be identical, but they’re not. They change in appearance over the course of a year, and scientists are unsure why.
What do daphnia look like?
Examined under a microscope, daphnia in a side view usually appears almost oval, with a rounded domed head and a short spine. Seen head-on, it appears thin from side to side. Its flattened, disk-like shape offers a large surface area compared to body mass, and this allows it to remain suspended almost effortlessly in water. Daphnia has a nickname, water flea, and it’s because, like a flea, it swims in jerky bursts, using oar-like antennae.
Daphnia undergo shape changes that correspond with the seasons. While the rear spine’s length varies in successive generations, it’s the head that changes most in cyclical fashion throughout the year. Fully developed, the head resembles an absurd, high-pointed helmet. Helmeted daphnia with their greater surface area are common in summer, while helmet-less round-headed daphnia are the rule in winter.
As a result of these seasonal differences, early 20th-century biologists who studied plankton thought generational changes in daphnia’s surface area related directly to—or were triggered by—differing water densities.
Here’s how their thinking went:
Environmental temperature determines to what extent water molecules carom off one another: The warmer it gets, the more they speed up. The increased volume caused by warm water’s excited molecules offers less support to a floating object than winter’s dense, cold water. So an enlarged body surface is especially helpful when water is the least dense. This means daphnia’s increased surface area in summer helps it maintain its position in a pond or lake. But winter generations, supported by cold water’s increased density, economize by not building enlarged body parts.
But are daphnia shape changes really affected by water density changes? Such a theory of cause and effect lasted through my undergraduate days and decades beyond, but questions eventually arose, and so many other explanations became plausible that water density could no longer be considered a primary cause—if it were a cause at all.
Science is self-correcting. When a theory doesn’t stand up under scrutiny or the test of time, it is replaced by another (or others!) closer to a solution.
Have scientists settled on an answer? Do we know why daphnia change shape?
Many theories now exist. Heated arguments can develop in the world of science and, odd as it may seem, some of them revolve around the cause of daphnia’s changing helmet size.
One theory is that daphnia’s enlarged shape in summer makes it tougher to be gulped down by its small predators, whose numbers are also up during the warm season. Other theories suggest that shape relates to the need for altered swimming behavior or to the abundance or absence of certain food.
Whatever the cause for change, it must be important. Daphnia’s different generations would not be expending the energy and materials necessary for summertime bodybuilding without some kind of environmental or physiological pressure.
It remains possible that none of the theories in vogue hold water, or that several factors interact with one another to cause the change.
No one can claim the workings of nature are simple. Daphnia’s unexplained change from winter to summer is proof.