Waters by Israel and Lebanon have warmed by over 3 degrees Celsius and the invading rabbitfish are eating all the algae: The urchin population has collapsed
Ruth Schuster | Feb. 10, 2020

Purple sea urchins have effectively vanished from the waters of Lebanon and reports coming from Turkey indicate they are endangered there as well. On the rocky shores of Europe, Paracentrotus lividus is still hanging on. But in Israel, from as many as 10 per square meter in the shallows five decades ago, now they’re almost impossible to find.

Observant Jews do not eat of the sea urchin because it isn’t kosher, but in other cultures it’s an openly acknowledged delicacy. This doesn’t mean nonobservant Israelis never consumed the spiky seafood. “There had been a big market for them here too, once upon a time,” Erez Yeruham, a doctoral student at Dr. Gil Rilov’s lab at Haifa’s Israel Oceanographic and Limnological Research Institute tells Haaretz. Well, the dilemma about whether or not to eat them is over for Israelis: they’re gone.

In the Eastern Mediterranean, purple sea urchins were slammed by two insurmountable problems. One is competition by invasive rabbitfish, which swam up the Suez Canal from the Red Sea about a century ago and wreaked havoc on the Mediterranean ecology – including by decimating the algae forests that used to exist but are also now gone. The purple sea urchin is also a herbivore, and by devouring the algae, the rabbitfish literally ate them (and many other species) out of house and home.

That problem might be partially reversible by restoring algae forests, for example by target fishing the rabbitfish, although that’s hardly a trivial pursuit, Rilov points out.

The second problem isn’t reversible: ocean warming because of climate change. Warming sea surface temperature is especially extreme along the coasts of Israel and Lebanon, which are already the warmest part of the Mediterranean.

Both problems have been building up for decades, but it is in the last 20 years or so that the sea urchin population of the Eastern Mediterranean reached the tipping point and began to collapse.

Thirty years ago, Israelis would have to tread cautiously in the shallows unless they wanted to get stabbed in the feet: the sea urchins numbered between 2 to 10 per square meter.

In hundreds of dives between 2010 and 2014 along 80 kilometers (50 miles) of coastline, including a marine reserve in Rosh Hanikra, Rilov and his team at the Israel Oceanographic and Limnological Research Institute found exactly 19 sea urchins, they reported in Nature Scientific Reports in 2015.

Functional extinction

Is the purple sea urchin of the Mediterranean going extinct? “Extinction” in the sea is somewhat different from extinction on land, Yeruham explains. “On land we can know if an animal such as the dodo went extinct, but at sea it’s difficult to know for sure,” he points out. Almost all the world’s waters are alien territory to us: Note the rediscovery of the coelacanth, a giant fish that had been thought to have gone extinct 66 million years ago, but was found alive and swimming in the deep Indian Ocean in the 1930s.

So, while it’s almost unheard of to declare extinction at sea, there is such a thing as ecological or functional extinction, Yeruham says: When an animal or organism has reached such low density that it has no effective function in the ecological system – and that is the situation of the purple sea urchin in the Eastern Mediterranean today.

What ecological function did the purple sea urchin play? It was a key element in the food chain, less so the spiny adults (except to non-Jewish humans) and more so their larval and young stage, Yeruham says. In any case, when they were thronging the coastlines, they were prey to fish, especially but not only big ones, and other animals such as sea worms. And as adults they were important herbivores, before the rabbitfish came from the Red Sea and took over.

Leaving aside the moral and ethical issues involved in any species’ extinction, the disappearance of sea urchins has in and of itself abetted the decimation of the Eastern Mediterranean fishery.

Once upon a time you couldn’t wade in the shallow seas off Haifa without fear of being stabbed and/or slipping on algae while admiring the fish. Now what you see afloat is mainly garbage, though Rilov points out that some biodiversity hotspots still remain.

Aside from starving fish, the sea urchins’ disappearance is symptomatic of a much bigger problem. It’s easy to discern the disappearance of a formerly very common edible animal. “But that just shows there are others we’re not seeing, other species that had been common and are now gone – but we haven’t noticed it,” Yeruham explains. Such as the black sea urchin, Arbacia lixula. It wasn’t edible and we didn’t notice it, necessarily, but it’s gone too. There are others, he says.

Warming seas

While sea urchin populations have definitely collapsed along the coasts of Israel and Lebanon, and there are reports from Turkey that where algae disappeared so did the urchins, there’s very little data emerging about the Egyptian coast, for instance. But with the water trending warmer, that may be a matter of time.

Meanwhile, the warmest, saltiest part of the Mediterranean is Israel and Lebanon, and it is evidently no coincidence that this is where the urchins have vanished. This year, Yeruham and colleagues reported the culmination of years of research in the Ecology journal. Studying starving sea urchins (the algae they graze on being gone) in elevated water temperature, they found the animals could hardly breathe, let alone procreate.

The tipping point for them is 30.5 degrees Celsius (86.9 degrees Fahrenheit) sea surface temperature. Above that point, Yeruham says, the sea urchins die off. Since sea surface temperatures in our part of the Mediterranean have risen by more than 3 degrees Celsius on average and now reach 31 and 32 degrees Celsius in shallow water in summer – the urchins simply cannot hang on.

“Our summer temperature does not enable their population to exist anymore. This is a new thing, a change in recent years,” Yeruham says.

In Europe, the sea urchin populations are fluctuating but haven’t collapsed yet.

Theoretically, the algal populations of the Mediterranean could be restored: While we never did have kelp forests like the ones dying off California, we had our own weedy waters. Crucial to this would be to ban fishing altogether, to help recreate the natural habitat by not eating the predatory fish that would eat the herbivorous invasive rabbitfish that ate the algal forests.

But nothing Israel does can reverse the trend of ocean warming at this point – and certainly not fast. “The wheel can’t go backward on that,” Yeruham sums up.

It is true that a Spanish team by the University of Barcelona, the Spanish National Research Council and the University of Tromsø in Norway reported this week in the Diversity and Distributions journal that genetic analysis of Paracentrotus lividus from 11 different areas concluded that although the larvae swim about during their weeks of life before becoming semi-sessile adults, the animal does not evince high dispersal. Populations of urchins do not mix and match. That is bad.

On the upside, they did find high genetic diversity in genes related to tolerance of temperature and salinity, which could indicate a propensity to adapt. That is good. But is it good enough? That depends whether it really helps, how much the water warms – and if they can find food. Even if we restore algae to the battered Eastern Mediterranean, the sea urchins may never return.