It's fishing
Endless expanses, tugging currents, bottomless depths - and in the middle of it all, the offspring of colorful fish are drifting far away from home. Will they find their way back into the protective hoard of familiar corals and if so, how? "Always follow your nose" must be one answer.
The anemonefish – now made notorious by the blockbuster “Finding Nemo” – has a very cramped home: it lives in and around individual sea anemones, which it rarely leaves voluntarily. Some coral gobies of the genus Gobidion live in territories that are only forty cubic centimeters in size. And even larger species such as the emperor angelfish (Pomacanthus imperator), which at least reaches a body length of around forty centimeters, is usually content with a home of only a thousand square meters, which seems to ensure its livelihood and reproduction.
Once in their lives, however, many coral fish go on a long journey - and that at a young age. Because at the latest as soon as they have hatched as larvae from the eggs, they drift away from the coral reef with the currents, which for young animals resembles a real shark tank. Relatively far from the place of procreation, they spend their childhood among the plankton of the open sea before settling back at home. But where actually: Are you drawn back to your parents' house? Or do the offspring fish monopolize the first coral reef they come across? Puzzles that may not only interest behavioral researchers, but also conservationists. For example, this could influence the recolonization of destroyed habitats in one direction or the other.
Uncontrolled expansion and spread of the animals should, however, ensure that neighboring populations mix in a genetically diverse manner and that the genetic material does not differ extremely from one another. But that is not the case, at least for some species, as marine biologists led by Gabriele Gerlach from the Marine Resources Center in Woods Hole have found out from genetic analyzes of some inhabitants of the Australian Great Barrier Reef. Döderlein's cardinalfish (Apogon doederleini), for example, spends around a month as a larva outside the reef in open water and is a poor swimmer at this time. Actually, it should settle down where the wind and waves are taking it – and consequently mix the individual stocks well, as long as the currents play along.
Within the five coral reefs compared in the so-called Capricorn/Bunker group, currents mostly flow from the southeast to the northwest, so that at least the northern cardinalfish could be actively enriched by young animals that were passively washed in from the south: Their habitats could therefore, according to one hydrodynamic model of the researchers can still be reached within the plankton stage of the fish, while a reverse exchange in this way is almost completely ruled out. However, the genetic differences between the individual populations are not limited to the deviations between a northern and a southern group that Gerlach and her colleagues expected.
In fact, there is a strict separation between them: Even if there were only three kilometers between their individual occurrences, the perch differ so clearly in their genetic composition that they hardly ever breed among themselves. Apogon doederleini washed away as larvae consequently return to the bosom of their mother reef despite their inhibited movement abilities, and are thus similar in their genetic distances to the swallowtail damselfish (Acanthochromis polyacanthus), which even as larvae remains steadfast on site and thus just as little with more distant ones Conspecifics mixed.
But how do the pubescent cardinalfish find their way back home when they shouldn't have any memory of "their" coral banks? Earlier investigations and experiments indicated that the background noise of the reefs could serve as a guide: the louder parrot fish grind the coral sticks, crabs clatter with their claws or starfish drag across the bottom, the more likely and in larger numbers there are those scattered across the sea juveniles again. However, noise can only provide the animals with a general direction, but the fine-tuning when navigating must be carried out by another messenger, which people probably associate with fish in a rather unpleasant way: scents.
Experiments in specially constructed two-channel measuring tubes and subsequent genetic analyzes showed that the returnees clearly preferred the water – or the olfactory signals dissolved in it – of their place of birth, while avoiding that of foreign places. The Döderleins cardinalfish and related species are similar to salmon, which also find their original home river from the sea in this way. In both cases, this could increase species diversity in the long run, as reproductive barriers often trigger the first step in species segregation.
According to the researchers, the tropical fish probably take in the scent of home right at the beginning of their journey, during which they bob around near their home reef for a few days before drifting further away. However, the nature and composition of the odors is still unclear to them; they mainly point to species-specific pheromones, with which similar species of cardinalfish have already been experimentally lured to artificial reefs. To really guide the offspring home, they're likely to blend with other essences from the reef community, all of which leave a distinct note in the water. However, one element that makes us wrinkle our noses is probably missing: dead fish are consumed too quickly in the species-rich reef.
