At Sea
The bright blue tropical Atlantic sparkles around us again as we begin our last ocean crossing of the voyage, from Ascension Island to Senegal. This is also our last day in the Southern Hemisphere; already we have covered almost 50 degrees of latitude and our long journey is well on its way to a very successful conclusion.
The seas around Ascension are another of the strange and wonderful islands of shallow-water habitat that we have encountered along our route north. The clear blue waters here are full of active, colorful tropical fish, but a close look reveals that this is quite an unusual community. To begin with, there is very little coral at all. Reef-building corals could thrive here, and some do, but Ascension is such a remote island that very few species of coral have managed to succeed in the long journey from source habitats like the Caribbean. In place of the more common coral animals, the rugged volcanic rocks that form the shoreline of Ascension are covered below the waterline with pale pink and white coralline algae; grown into miniature fairy castles and forests of tiny stalagmites it makes a lovely underwater landscape like no other in the world.
The fish that dwell in this unique habitat seem, at first, to be quite a normal group of tropical reef species. Triggerfish, moray eels, groupers, hawkfish, squirrelfish, and many other familiar families are represented. But diversity is low; there is only one common grouper, a single species of squirrelfish, a couple of kinds of butterflyfish and so on. By contrast, numbers of individuals of some species are astoundingly high; the black triggerfish fills the water over the reef, thousands upon thousands of them in a swirling spectacle of life that defines the nature of Ascension underwater. When any kind of animal successfully colonizes a remote habitat like Ascension, they often find an easy life, free from their usual predators and competitors. This opens many doors and often the new arrival will be able to achieve a much larger than normal population as it expands into new niches and ways of life, a process called ecological release.
Moray eels are another example of this phenomenon at Ascension. There are four common species here (perhaps larval morays are well suited for the difficult journey to remote islands?) and a couple of them, the black-eared moray and the spotted moray are absolutely everywhere. Their population density on the rocky reefs here is probably close to twenty times what they achieve on the coral reefs of the Caribbean. And their behavior is different as well. They are often out in the open, swimming and hunting actively during the day, something that morays don’t usually do.
Finally, and best of all, there are the endemic species, those found here and nowhere else. My favorite is the white hawkfish, a pretty little predator that has abandoned the pink and orange colors that camouflage its close cousins among corals, in favor of a pure white that suits it perfectly for the strange and wonderful reefs of Ascension Island.
