What Does a Sponge Crab Use in a Pinch?

Ordinarily a conforming Sleepy Sponge Crab (Dromia dormia) would be expected to be carrying a sponge on its back. It’s what they do. In Hawai’i that sponge is usually a large, oval piece of yellow sponge, but they will carry other types of sponges too. Like most animals they exhibit individual preferences.

Some of the different types of sponges that have been photographed on sponge crabs (Dromia dormia) in Hawai’i.

Over the years we’ve also seen them carrying other natural items such as a coconut husk or a colonial tunicate. We even saw one carrying a crown-of-thorns sea star that it had customized by trimming off the ends of the arms! Ouch!

This sponge crab (Dromia dormia) is carrying a living crown-of-thorns sea star on its back instead of a sponge. It has modified the sea star to its liking by trimming off the ends of some of its arms. Photo: P. Fiene.

Carrying objects on their back is something that certain species of crabs are, through evolution, adapted to do. In some families the fourth and fifth pairs of legs are permanently oriented upward instead of downward. They are no longer in a position to be used for walking. Instead they are adapted specifically for holding items over the back of the crab’s body with needle-sharp pinchers (see photo below). They do this to cover themselves for camouflage. When the object being carried is toxic, such as a sponge, it becomes yet another line of defense in addition to camouflage.

Evidence that this method is so effective at prolonging the life of a sponge crab is the fact that four of the crab’s ten appendages are dedicated to holding something above its back. The appendages cannot be used for fleeing a predator, for very effective defense or for feeding. It seems to have been a successful evolutionary trade-off though.

A sponge crab (Dromia dormia) carrying a cream-colored sponge. The photo shows two of the four legs and pinchers with which the crab holds the sponge over its back. Photo: P. Fiene.

Sometimes if an acceptable sponge is not available they carry man-made objects! We’ve seen them sporting a baseball cap, a rubber slipper, a piece of burlap (pers. comm. Victoria Martocci), a fabric frisbee – and even men’s underwear. But until this year we had never seen one carrying something as curious as what I’m about to describe.

Sponge crab (Dromia dormia) carrying a man-made sponge on its back. 8/29/2018. Photo: Andy Schwanke.

There is a shrimp boat wreck here on Maui called the St. Anthony. We’ve had many wonderful dives there in the 20-plus years that it has been underwater. A couple months ago two of our divemasters, Warren and AJ, came up from their dive with an entertaining story. They had seen a sponge crab carrying a man-made sponge on its back. Now I’m sure this has happened at some time, somewhere in the world, but we had never seen this.

A couple months went by without another sighting, and then we saw it again on the wreck. Of course we wondered where it had gotten the sponge, but we have seen all kinds of man-made debris in the ocean, so we didn’t think too much of it.

Then one day AJ swam over to me, clearly very excited. On her slate she had written “look in the cabin!” I went over and peered through the window of the cabin, shining my light around. The wreck has been underwater for 22 years, so things are pretty grungy in there. There’s an encrusted toilet, some pieces of metal, the crusty captain’s chair that is fallen over on its side…. nothing remarkable to see. I gave her the “I don’t see anything look” and she wrote “look at the captain’s chair!” When I did, I saw what she was talking about. In the seat of the captain’s chair, a circular piece had been cut out of the cushion! It had the same scored lines in it that were in the sponge that the crab was wearing. We were both blown away at what the sponge crab had done, and couldn’t stop laughing.

The captain’s chair on the St. Anthony wreck showing the seat cushion from which a sponge crab carved a circular piece. Photo: P. Fiene.

Even though the wreck has been underwater for two decades, it has taken time for the chair to deteriorate. The cushion’s cover had to disintegrate before exposing the man-made foam rubber of the seat. And even though it certainly didn’t give off the scent of a sea sponge, it was apparently recognizable enough to the crab to be an acceptable covering.

If you’re wondering how the crab cuts the piece out of a larger sponge, here are two fascinating videos shot just this year by Dr. Katsushi Kagaya from Kyoto University showing how they do it.

We would love to hear of other man-made or non-sponge items that divers have seen sponge crabs carrying in Hawai’i. We’re sure there are many more. Please tell us! 🙂

UPDATE – June 15, 2019 – the next generation

A second crab has found the captain’s chair cushion to be an acceptable sponge! 8 months after the first sponge crab took his piece out of the captain’s chair cushion, Warren saw this second crab. This crab is much younger than the first crab (about the size of a child’s fist). As the small crabs do, he cut out a piece of “sponge” much larger than himself, fitting himself into it almost like a puzzle piece.

A second sponge crab (Dromia dormia) carrying a piece of man-made sponge from the captain’s chair on his back. 6/19/2019. Photo: P. Fiene.

Original sponge crab still carrying his man made sponge from the captain’s chair on the St. Anthony wreck. June 19, 2019. Photo by P. Fiene.

UPDATE – June 19, 2019 – the original crab

We saw the original crab still with his man-made sponge – 10 months after he cut the sponge out from the captain’s chair. He has customized it by squaring off the edges, so it is neater than when he first cut it out. It has also swelled up a bit and is starting to split, but he is still carrying it. It would be interesting to know how it has lasted compared to a natural sponge.

Written by Pauline Fiene

Thanks to Autumn Hill (AJ) for noticing this once-in-a-lifetime (for us) sighting. Thank you Andy Schwanke for getting the only photos we have of the crab’s sponge right after he carved it out; Linda Wright and Patrick Doyle for use of photographs; Victoria Martocci for your observation; and Rachel Domingo for sharing sightings she has had. And thank you, Jim Fiene, for your cute title suggestion :-).