Construction Crews – who digs those holes in the sand?


Gliders, Mantis Shrimps and Gorgeous Prawns… Mouth filled with sand, the Pennant Glider emerged from his hole and expelled the sand onto the growing pile at the edge of the hole. His partner darted into the hole and emerged with another mouthful of sand that he too spat onto the pile. Their burrow was formed in a crevice in the sandy rubble bottom and they had simply taken up residence in it and kept it clear. But the varying currents kept filling it, shells rolled into it, and they maintained it by suction and jet expulsion. This simple method of clearing a hole is used by many fish species as a sort of undersea vacuum cleaning system.

Not all such undersea residences are natural however. Some of the most impressive burrows on the reef are made by the Mantis Shrimp. They have immensely powerful front claws, and some species are able to break a camera lens with a blow. They can easily dig out a substantial hole in the sand on the reef, and Mantis shrimp are largely responsible for the more impressive holes on the reef.

The Giant Mantis creates a fully lined, superbly constructed burrow for himself in soft sand. These holes seem to be lined with a cement made with spittle and bits of sea-grass, and they are extremely strongly built.

Smaller holes become the home of Commensual Partnerships between a prawn and a goby.

The Goby can see and hunt but can’t dig, and the prawn can dig but is blind and can’t hunt. So they team up, and form a mutually beneficial relationship.

The goby’s task is to find food, and the prawn shares his bounty. The goby acts as the attack alarm, and is incredibly sensitive to movement and light. Few people know to wait and watch their interaction but it can be hilarious.

The prawn’s job is to dig and shovel the debris out of their home as it accumulates, depositing it in the right place by following the spine of the Goby with a feeler. The goby moves to point in the direction where the prawn must shovel the next pile of sand. This works very well until an extra goby arrives, or another shrimp joins the team.

We watched for half an hour as a pair of industrious shrimp both shovelled sand, small coral pieces and bits of shell out of the hole under the direction of a rather harassed-looking goby. The problem came when they both shoved at the same long coral piece, and it got stuck in the mouth of the hole. Looking baffled, his line of retreat closed, the goby darted off and hid under a rock. The two prawns shoved vigorously until the mouth of the hole ruptured, and a full day’s work dropped back into the hole. The goby came home, and you almost hear him muttering with rage as he haughtily dived through the debris to seek sanctuary below.

Where there are two gobies and only one shrimp sharing a hole, it is even funnier, as the two gobies point the prawn in opposite directions. We watched a completely paranoid prawn shoving a pile of sand and shells out of the hole as directed by the first goby and then pushing the same pile of sand back into the hole by following the spine of the second goby. The hole never changed, and we left before the prawn gave up.

Look out for holes in quiet corners on rubble or sandy bottoms, duck behind a rock, and watch. It’s worth it.

Words: Jill Holloway

Pics: David Holloway

Copyright: Ocean Spirit

Jill Holloway

Jill Holloway

Jill Holloway lives in Mauritius and at Sodwana Bay Isimangaliso Wetland Park in South Africa. A PADI qualified Nitrox diver with over 1,500 dives, she is a passionate observer and preserver of the marine environment, and has a database of over 35,000 fish pics and hundreds of Gopro videos on fish behaviour, which she shares with her readers.

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