Thousands small crabs on the beach_ Maldives

Asked November 2, 2013, 12:55 AM EDT

Hello everyone.
My name is Tania, I am currently working on a resort in Dhaalu atoll, Republic of Maldives.
A couple of days ago we found thousands of small crabs on the shore. I heard that the night before they were all at the surface, and then of course they were washed up on the shore in the morning. They were still alive when we first saw them on the beach.

I would like to know something more about what happened to these crabs.
I guess there must have been some kind of "massive reproduction" during the night, and then they were all washed up on the beach and die. I have heard of some crabs that reproduce this way.

Do you have any idea of what could have happened to these crabs?
Can anyone please help me to recognize the species?
Or maybe help me to find someone that knows?

SIZE:The carapace it about 4 to 5 mm diameter.
COLOR: brown reddish

Thank you very much
I hope to receive news from you soon.

Best regards,

Tania Militello

Outside United States science marine fisheries

1 Response

Hi Tania-
I am not sure of the exact species, but based on my knowledge of crab biology I have several suggestions for you on why they are there. First, I believe these are immature crabs. They still seem to have immature features, and I would guess they were born within the last year. I would guess that many of them are molts and not live crabs. Crabs must shed their old shell as they grow. It's common for this to happen on lunar cycles so that you see many at one time. In the Unites States we often find younger crabs moving to shallow water to escape predators to molt, or shed their too-small shell. The old shell then often washes up on the beach. Additionally, molting is very stressful on a crab, and a common time when crabs die. Some in the pictures probably are crabs that did not survive the molt. If these are molts, the inside of the shell is actually mostly empty. However, crabs do shed their gills too. From the pictures, this would be my explanation.

Additionally, if they are not molts, there are several other reasons. If there was a quick change in the temperature or in freshwater input, these reasons have also been known to cause large die-offs.

I am guessing from the pictures that these are mangrove crabs, but I am not 100% sure. Even land crabs often hatch their eggs out in the open water. The eggs and baby crabs develop for weeks to months then when reach juvenile size molt again and move onto land.
I hope that helps.