Like corals, sea anemones are animals that live in symbiosis with microscopic algae, which gives them their color, as well as with certain species of fish. Clownfish protect themselves from predators by sheltering among the anemones' tentacles, and each month lay eggs at their base. Equally, the anemones are also protected by the clownfish that they host.
Every other day, from October 2015 to December 2016, researchers and students visited thirteen pairs of clownfish and their host anemones in the coral reefs of Moorea Island (French Polynesia). This monitoring was conducted before, during, and after the El Niño event that in 2016 triggered a warming of the Pacific Ocean (+2°C on Moorea Island compared to the 2007-2015 average -- a combined effect of ongoing global warming and the El Niño episode) and a coral bleaching episode worldwide. Half of the anemones monitored in this study bleached as they lost their microalgae. Among the clownfish living in the bleached anemones, the scientists observed a drastic fall in the number of viable eggs (-73%). These fish were laying eggs less frequently and they were also laying fewer and less viable eggs -- while these parameters remained unchanged among fish hosted by unbleached anemones.
Blood samples taken from 52 pairs of clownfish (including the 13 previously mentioned) showed a sharp increase in the level of cortisol, the stress hormone, and a significant drop in the concentrations of sex hormones (the equivalents of testosterone and oestrogen). The bleaching of the anemones due to increased sea surface temperatures is thus a stressor that reduces the levels of sex hormones and thus the fertility of the fish. These links have been found for the first time in the natural environment in which the fish live.
The health of the anemones and the fish improved between three and four months after the end of the warming event, long after the temperatures had returned to normal. But would this have been the case had the warming episode been more intense, or longer? And, faced with a new warming episode, will the clownfish that have already suffered this initial stress be better acclimatized, or on the contrary more fragile? To provide some answers to these questions, the team has decided to continue to monitor each individual during the next El Niño episode. Such monitoring is possible due to the fact that clownfish have a fairly long life expectancy and are sedentary (they move very little from their host anemone).
The clownfish are not an isolated case: 12% of the coastal fish in French Polynesia depend on anemones or corals to feed or to find protection from predators. In cases of prolonged bleaching, like that of the Australian Great Barrier Reef in 2016 and 2017, the renewal of all of these populations could be affected, and with them the stability of the ecosystems.