Scaling waterfalls through torrents of water would be an impressive feat for any creature, but for tiny juvenile gobies returning to their homes in the upper reaches of island streams, it's an inevitable rite of passage. Many goby larvae are washed out to sea soon after hatching, so species that reside in the upper reaches of streams have no choice but to scale obstacles in their path on the return journey. However, the youngsters are well prepared for this trial. They have a sucker formed from fused pelvic fins on their undersides that they use for climbing. Adult fish also face the risk of being washed downstream. However, they seem to climb less than youngsters. According to Takashi Maie from Clemson University, USA, the adults' suckers may not adhere strongly enough to support the larger fish's mass, as their suckers only increase in size in proportion to their body length, whereas their mass increases much more. Intrigued by this possibility, Maie realised that he would have to measure the adhesive pressure and sucker suction force from fish with different lifestyles and of different sizes to find out how their adhesion varies as they grow. Teaming up with Heiko Schoenfuss from St Cloud State University, USA, and Richard Blob from Clemson, Maie travelled to the Hilo field station of the Division of Aquatic Resources, Hawai‘i, to measure how tightly goby suckers fasten to surfaces (p. 3925).
Scrambling up waterfalls to the gobies' homes and donning wetsuits to snorkel and catch the fish, the trio successfully collected five species of climbing fish ranging in size from 0.1 to 15 g, in addition to one non-climbing species collected at low altitude. Back at the field station, the team gently anaesthetised the fish and lightly touched their suckers to a simulated climbing surface. The suckers immediately formed a seal and stuck on. Measuring the pressure in the cavity between the sucker and surface at inclinations ranging from 45 deg to over 90 deg and comparing the sucker sizes and adhesive forces between the two lifestyles (climbing and non-climbing), Maie and his colleagues noticed that the non-climbers' suckers increased in size much more rapidly than if they were growing in proportion to the rest of the body, whereas the climbers' suckers only grew in proportion of the rest of their bodies. The non-climbers' suckers were relatively large for their size.
Next the team measured the sucker's adhesion as the fish ascended waterfalls. After building a gently cascading simulated miniwaterfall, Maie drilled a hole in the surface, inserted a pressure transducer to measure the fish's adhesion and waited for the animals to pass over the pressure transducer as they ascended the slope. Comparing the climbers' suction force with the passive force that they had measured when the fish were anaesthetised, Maie realised that the active suction forces were more than twice as great as the passive, and were sufficient to bear more than twice the fish's mass. In contrast, the non-climbers' adhesion forces were much weaker than those of their climbing cousins, despite the relatively large size of their suckers. The team suspects that the non-climbers enlarged suckers may compensate for their weaker adhesion.
Having shown that climbing gobies have smaller suckers than their non-climbing cousins but that the climbers' suction is much stronger, Maie explains that the climbers actively expand their suckers while in contact with the surface to increase their suction, and he and his colleagues are keen to find out how the muscles and ray structures that support the sucker alter with age to help growing gobies get a grip.