Life in the intertidal zone is rough, with waves continually pounding anything that chooses to live there. Some intertidal residents huddle in crevices for protection, while others simply anchor themselves to the rocks to ride out the surge. Northern clingfish (Gobiesox maeandricus) grip on to rocks with a suction disk on their bellies: ‘They have an impressive ability to stick to substrates with a huge variety of roughnesses’, says Petra Ditsche, from the University of Washington, USA. However, rocks on the shore are rarely pristine. They are usually coated in a carpet of slippery microorganisms and algae, which made Ditsche and her colleagues Dylan Wainwright and Adam Summers wonder how the slimy layer would affect the tiny fish's cling power (p. 2548).
But before the trio could test out the fish's grip, they had to produce a series of consistently rough surfaces, varying in grain size from silt to fine gravel, so Ditsche turned to sandpaper. ‘I had used moulds of sandpaper in former studies’, she explains, adding that casting the moulds in the same material also eliminated material property differences. And when she ran out of course grades of sandpaper to test, Ditsche resorted to gluing different grades of fine gravel to cardboard to produce the roughest moulds. Then she hung multiple samples of each surface in the sea at Friday Harbor Laboratories for 6 weeks until they were covered in algae and other microorganisms, ready to test the impact on the tenacious little fish.
Back in the lab, Ditsche teamed up with Wainwright to measure the force required to tug fish ranging in mass from 1.5 to 15 g free from the clean surface, and saw that the fish found it harder to stick to the smooth surface than to the fine-grained surfaces. They suffered the greatest failure in performance when the grain size increased from 500–1000 μm to 1000–2000 μm, although the largest fish fared better than the smallest fish, which couldn't hang on to anything coarser that 1000 μm. They also found that fish with small discs (up to 13 mm diameter) could not stick to anything coarser than a 269 μm grain-size surface, while the larger fish with 34 mm diameter discs were able to secure themselves firmly to the coarsest surfaces.
Next the team tested the effects of the slimy coating on the fish's grip, and was impressed to see that the fish could cling on to the fouled smooth and rough surfaces with impressive forces of up to 14–15 N, which is 150 times the body weight of the fish. And even though all of the fish could still cling to the fouled surfaces, the suction forces that they could generate were reduced, affecting their attachment to the finest surfaces by over 20% while their ability to cling on to the 269 μm surface only fell by 6%.
Considering the material properties of the microorganisms and algae that had grown on the test surfaces, the team suggests that the slim component of the microorganism film acts as a lubricant between the fish and their rocky resting place, reducing friction between the suction cup and the rock by preventing the tiny hairs that line the circumference of the disc from interlocking with the rock surface. Ditsche says, ‘The growth of biofilm or other fouling organisms on the original substrate can have a considerable impact on the attachment of aquatic animals, and fouling has to be taken into account when considering the attachment of aquatic animals in general.’
- © 2014. Published by The Company of Biologists Ltd