So, a legless crab walks into a wildlife sanctuary.

If I were a comedian, I might have a snappy punch line, but as a naturalist, I only have a stimulating story. The crab in question was all body. Lacking legs, it sauntered in with a Felix Neck visitor. Its small body, called a carapace, was nestled a jewelry box. As it was a signature blue Claudia’s box, I had hoped perhaps it was a holiday present.

It was in fact a gift, left on the deck of the Katama resident who presented it to me for identification. Likely, a bird dropped it off, since in its current state it couldn’t have strolled in on its own.

True crabs are decapod crustaceans, referring to their five pairs of legs, which can be a diagnostic tool. The pair of legs nearest to the eyes are their pinchers, used for fighting, eating and displaying to their friends and foes. The other four pairs of legs are generally used for locomotion and can be flat like a paddle for crabs that swim or pointed for those that walk (usually sideways).

The lack of legs presents a challenge for the identifier. Without them, it can’t be determined which of those two categories the crab belongs to — the walking or swimming variety. Local species of swimming crabs include blue crabs and lady or calico crabs, both of which hail from the family Portunidae. To add to the confusion is the invasive green crab which is also from the family Portunidae, but has walking legs. Walking crab varieties include fiddler crabs, mud crabs, spider crabs and rock crabs, to name a few.

Though this crab was small enough to fit in a jewelry box, size wasn’t entirely helpful, since crabs can take time to grow and fully mature and can be found in their smaller, juvenile stages. In fact, some crabs cease all growing in winter. In this case, size really didn’t matter. Nor did its color, since crabs can come in a variety of colors and patterns, even amongst the same species.

The body was all I had, and was enough to identify the crab. The shape of the body is useful. It makes a difference if the body is longer than it is wide, or whether it is rounder or more square. Ultimately, in this case, the final say came from the creature’s teeth. No special tools were needed to examine these teeth, nor did the crab need to open wide — its mouth was not even part of the identification equation. A count of its marginal teeth, which are located on the edge of its shell on the outer margins of the eyes, would do the trick. The number of marginal teeth can often aid in differentiating crab species.

It appeared that there were four or maybe five marginal teeth on this carapace. It was hard to distinguish as the tooth nearest the eye seemed to have a half ridge or hump. In reviewing my crab guides, the only four-toothed crabs were the mud crabs. However, there were a few five-toothed crabs, including green and calico crabs. The mud crab, however, was described as having an extra ridge on the first tooth. That clue gave me a leg up to make identification. Additionally, the lack of pattern ruled out calico, and the area between the crab’s eyes differentiates green crabs from mud crabs.

My best guess, then, after dabbling in crab dentition, is that our limbless creature is a mud crab.

A puzzle in a box is always a welcome gift, but in this case if it weren’t for some especially detailed pointers to help identify the creature, neither of us would have had a leg to stand on.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown, and author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.