As I was walking out of the water after completing a survey of the historic Renouf sectors (first delineated in the early 1920s), I was stopped by a visitor to Lough Hyne. She asked if I’d seen much in the water, which of course I had. She went on to tell of how she’d heard so much about the diversity of Lough Hyne and so went out and bought a snorkel set to see it for herself. Excited and armed with her gear, she jumped in the water and began her snorkel around the North Basin, only to be sorely disappointed and find that she saw only sand and old shells. And so she asked me, where do you find the bounty of animals reported within Lough Hyne?
Amongst the rocks. Any animals out in the open are at risk of being spotted and consumed by a predator, so most animals hide. As a result, a casual swim may not, at first glance, reveal much life. But if you slow down and start to look in the crevices between rocks, under ledges created by rocks, and amongst the algae covering the rocks, you’ll start to see more life than you would expect. In fact, one of the most plentiful and densely inhabited areas is actually underneath and on the bottom of rocks. The tight space between the bottom on the rock and soft-sediment beneath limits access of larger predators, providing refuge for smaller or more vulnerable organisms. Often, juveniles of more mobile animals, such as crabs, will be found under rocks, hiding until they outgrow the space and hopefully are large enough to evade predators. Other organisms will inhabit these types of spaces for their entire lives, such as the squat lobster Galathea squamifera. Of course, that plan doesn’t always work out. Some predators also have adapted to inhabit the underside of rocks. For example, the predatory limpet, Emarginula fissura, can be found under rocks, feeding on sponges.
Amongst the rocks. Any animals out in the open are at risk of being spotted and consumed by a predator, so most animals hide. As a result, a casual swim may not, at first glance, reveal much life. But if you slow down and start to look in the crevices between rocks, under ledges created by rocks, and amongst the algae covering the rocks, you’ll start to see more life than you would expect. In fact, one of the most plentiful and densely inhabited areas is actually underneath and on the bottom of rocks. The tight space between the bottom on the rock and soft-sediment beneath limits access of larger predators, providing refuge for smaller or more vulnerable organisms. Often, juveniles of more mobile animals, such as crabs, will be found under rocks, hiding until they outgrow the space and hopefully are large enough to evade predators. Other organisms will inhabit these types of spaces for their entire lives, such as the squat lobster Galathea squamifera. Of course, that plan doesn’t always work out. Some predators also have adapted to inhabit the underside of rocks. For example, the predatory limpet, Emarginula fissura, can be found under rocks, feeding on sponges.
Of course, there are challenges to living under a rock. Mortality, particularly for sessile or sedentary organisms (i.e. organisms that don’t move), can be caused by other factors such as suffocation. If sediment collects in the space under the rock, the animals living on the rock can become smothered and die. If water flow becomes too low, oxygen can become quickly depleted by the animals living in the space, resulting in mortality. There are a few animals that are more resistant to these types of environmental challenges. In particular, worms (which can be segmented, ribbon, or flat) are relatively tolerant of such poor conditions. Some worms are able to burrow through soft-sediment and can therefore create an access point to more oxygenated water. Other worms, such as the keel worm Pomatoceros lamarcki appear to be tolerant of low oxygen environments (likely through physiological adaptations). Pomatoceros lamarcki is often the only organism found underneath a nearly anoxic rock.
Reduced water flow beneath a rock means not only reduced oxygen replenishment, but also a reduction in food particles passively flowing past. As a consequence, organisms that rely on capturing particles out of the water to feed will be far more successful if they can actively pump water (and the particles within). Not surprisingly, the undersides of rocks are frequently colonized by sponges, ascidians (sea squirts), and bryozoans, representing the most efficient filter feeders.
With the right adaptations, life under the rocks can be safe and bountiful. Many organisms, mobile and stationary, have developed strategies to take advantage of these spaces, making the space between a rock and hard place not so bad after all.
Sara Edquist
Sara Edquist