Tropical Fish Hobbyist - October 2010

The recently imported marble crab Metasesarma obesum is a small crab that makes a good choice for terrariums. Marble crabs are widespread in the Indo-Pacific region. Comparable to hermit crabs, they live on land and hide under leaves, driftwood, or dig small holes in sandy ground. They settle on many islands in the Indo-Pacific on beaches that often have no fresh water.

Morphology

When I bought my first marble crabs and researched them, I found widely varying data regarding their sizes—they ranged anywhere from 3 to 4 cm ( 1¼ to 1½ inches) up to as much as 8 cm ( 3 inches). Ng & Schubart recorded the dimensions of the carapace (body) of the crab to be 14. 3 x 13. 5 cm ( 5½ x 5¼ inches). The specimens I saw up until that point had all been longer than 2 cm (¾ inch).

Marble crabs exhibit wide variations in color and pattern. Some I found were very bright, monochromatic, whitish animals with nearly no pattern on the carapace, but there were also others that exhibited some pink coloration with heavy spotting. Some specimens are almost entirely black. Just a few weeks ago, a new lemon-colored variant was imported. The color of the crab seems to

M. obesum comes in a variety of colors and patterns.

be strongly related to the biotope where it was collected.

In Nature

I found that the marble crab lives on
islands without rivers or other sources
of fresh water on wide gravel and sandy
beaches. They settle in higher regions
that are not influenced by the tide or
areas where water doesn’t reach even
when the island floods, and reside
between stones, leaves, branches, and
broken pieces of coral. The holotype was
collected above the beach between bits
of broken coral. But in contrast to my
findings, the Crustacea database placed
these crabs near rivers and a pond used
for aquaculture. I couldn’t find a picture
of their habitat online.