CRITTER CONFIDENTIAL
Gotcha !
Paul Naylor reports on his observations of the suprisingly ferocious habits of small spider crabs – Inachus spp .
I have always been fascinated by the behaviour of these small and rather quirky-looking spider crabs , particularly their habit of ‘ decorating ’ themselves with pieces of sponge that grow into a complete coat and their tendency to live next to or within large snakelocks anemones . There are two very common species ; Leach ’ s spider crab ( Inachus phalangium ) and the scorpion spider crab ( Inachus dorsettensis ) plus the more rarely found Inachus leptochirus . They are , however , extremely hard to tell apart underwater , especially when characteristic carapace features are obscured with decorative sponge . This is why they are often simply known as Inachus spp . Whatever their exact identity , an impressive albeit rather gruesome recent event made me regard these crustaceans with even more respect !
When I dive in shallow areas with a seabed of muddy gravel , and a good population of painted gobies , I love to watch how gangs of these feisty fish dart around , interacting with one another and harassing other animals such as whelks ,
18 sea slugs and crabs . Their boisterous activity seems to be aimed at getting any pieces of food that these passing invertebrates disturb as they move across the seabed , or even stealing their meal entirely . Hermit crabs are particularly popular victims of the painted goby gangs and the fish tend to lunge into them and knock them over or at least push them off course on a regular basis .
On a recent dive to the painted goby ‘ mean streets ’ we found an Inachus spider crab walking out in the open across the seabed . The crab was totally covered in orange sponge but there was no corresponding sponge nearby , so it was very obvious . I settled down with my camera to see if the painted gobies would ‘ rough up ’ this spindly crab in their usual fashion . Sure enough , a good-sized painted goby soon bashed into the crab but , to my utter surprise , the quick-reacting crab grabbed at it and held on . I expected the crab ’ s success to be brief but , after some frantic writhing and twisting enveloped in a cloud of disturbed sediment , the water cleared to reveal the goby still grasped
This crab has been busy attaching pieces of sponge to its armour , where they are growing to form a complete coat