Sea plankton may be shrinking in size, which could have a ripple effect on our oceans' largest mammals, such as these humpback whales.

Amos Nachoum/Corbis

cientists have long observed that cold-blooded marine animals grow up to be slightly smaller adults when the water they live in is unusually warm. But the reason for this phenomenon, known as the "temperature-size rule," has long eluded scientists — until a recent study offered new insights. A concise guide to the findings:

How was this study conducted?
Scientists at Queen Mary, University of London, studied marine planktonic copepods — the ocean's primary form of plankton and a source of food for many marine animals. Using more than 40 years of existing data, the team measured two things: The animals' growth rates ("how fast mass is accumulated") and development rates ("how fast the animal passes through life stages").

What did they find?
Warmer water temperatures cause a "decoupling" of growth and development rates in the tiny shrimp-like creatures. Researcher Andrew Hirst explains: In warm water, "a species grows faster but matures even faster still, resulting in them achieving a smaller adult size."

Why should we care about these tiny plankton? 
These creatures matter plenty, at least to the oceans' ecosystem. According to Tim Wall of Discover, "Since copepods are food for marine animals, from fish to whales, what happens developmentally to the shrimp-y crustaceans could affect the entire ocean's food web." Smaller plankton means less food to go around at the base of the ocean's food chain.

And what does this have to do with global warming?
Global warming is causing our planet's oceans to get a little hotter every year. And because this "temperature-size rule" affects all cold-blooded marine animals, "as the planet heats up, many animals' sizes may go down," Wall says.

Sources: Discover, International Business Times, United Press International