Those unidentified pink crustaceans I found on a beach last month are gifts from the sea that just keep on giving … e-mail. A reader wrote last week, "I am surprised that someone from the University cannot put a species ID on them!"

But I’m not surprised. The number of marine animals swimming or drifting in the Pacific Ocean is astronomical. And if identifying each species isn’t tough enough, a lot of those creatures are shape-shifters.

A Smithsonian scientist wrote that my stranded pinkies with 10 legs and big black eyes were crustaceans in a developmental stage called a megalops. They are "probably a crab," he wrote, "but maybe a lobster."

The species is unknown because at this early stage a lot of them look alike. The term megalops simply means big eyes. Only in later molts do some crustaceans begin to resemble their adult forms.

The Smithsonian researcher added, "There are multiple instances where the juveniles were thought to be a separate species … until they were reared in the laboratory and ‘turned into’ a species people had known all along."

One famous ID mix-up involved not juveniles, but sperm packets. In 1829 the renowned French naturalist George Cuvier (1769-1832) discovered a parasitic worm in female offshore octopuses known as paper nautiluses.

This "worm," it was later learned, was the male paper nautilus’ detachable appendage containing sperm. In other octopus species, the male drops off his precious payload in the female and then withdraws his arm. In paper nautiluses, however, the delivery arm breaks off and squirms.

Cuvier named this "parasite" Hectocotylus octopodis, "hecto" meaning hundred, "cotylus" meaning cup, referring to the numerous suckers on the wiggler. Subsequent researchers kept the apt name hectocotylus but redefined it to mean the special sperm-transfer arm in all octopus species.

Cuvier, an outstanding scientist, apparently had a sense of humor about the budding field of biology. A story goes that for a dictionary the French Academy defined crab as "a small red fish which walks backwards." When asked for his approval of this definition, Cuvier wrote, "Your definition, gentlemen, would be perfect, only for three exceptions. The crab is not a fish, it is not red and it does not walk backwards."

Crustaceans might be difficult to identify at various stages of life, but some reef fish can also be challenges to name.

Chief among the tricksters are the wrasses, well known for having bright colors and bold patterns that change dramatically with age and sex. In addition, as adults, some wrasse species change from female to male, and the resulting so-called supermales are different colors from the ones that start life as males. In numerous cases over the years, one wrasse’s juvenile, female, male and supermale forms were named as separate species.

With museums, aquariums and other organizations offering photos and up-to-date information about plants and animals, the Internet is making it easier to standardize names and identify organisms. A site called the Tree of Life (www.tolweb.org) is a worldwide effort of biologists from 35 countries to create a page about every known species.

That number, though, keeps growing. Through deep sea exploration, DNA sequencing and international collaboration, researchers continue to find new species.

And calling the ones we already know can be hard. As I worked on this column, I received from the Smithsonian biologist this one-line postscript: "2nd thought, maybe that crab is a shrimp … hard to tell."

Susan Scott can be reached at www.susanscott.net.