You are looking at an ‘A’ama or a black rock crab.

They are a species of decapods or “true crabs” that are scavengers and herbaviors. Grapsus tenuicrustatus is their genus and species name.

These thin-shelled rock crabs live on wet, black, and rocky shorelines of Hawaii. They are usually shiny and deep purple/black with light colored markings. They blend into the coastline. They have living cells called chromatophores which enable them to camoflauge. Crabs will shed/molt their shells as they grow. However, they are slow growing and do not molt often. This molting tends to happen around a full moon and once they molt, the chromatophores are no longer alive. It is at this time that you can see their true color of the carapace which is bright red or orange.

They are flattened from top to bottom. These crabs first appendages have claws and are called chelipeds. The last pair of appendages are walking legs. They cling onto rocks with their front legs and then back out of their old skeletons. Crabs can grow or shrink during a molt. However, it is less than 10% of their body size in either direction. It will take 1-2 days for their new shell to harden.

In Hawaii, they are eaten as well as used in medicine or ceremonies. Kids love to find them and catch them as you can see from the photo above!

This colorful image was taken off a West Maui Beach by the dedicated staff of Ocean Project Maui. They offer custom, private snorkel tours around the island of Maui, Hawaii. Each #COLORSOFTHEOCEAN image you see in our monthly blog, was captured during a private guided snorkel tour. These amazing images, as well as images above and below the water of the clients/supporters are included in every tour at no additional cost.

Be sure to visit our Blog each and every month for the latest #COLORSOFTHEOCEAN post.

Contact Ocean Project Maui today to build your very own custom Ocean Project Maui snorkel tour.

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