The fabled Mediterranean Sea is under stress from overfishing, pollution, rapid warming, and the associated proliferation of invasive species that thrive in the warming waters. Two species of rabbitfish (Siganus luridus and Siganus rivulatus) crossed the Suez Canal into the Mediterranean Sea in the 20th century, and now make up about 95% of the herbivorous fish in rocky habitats along the Levant Basin off the Israeli coast. These fish are voracious feeders on macroalgae that live in the Levant, and they have become much more abundant during the past 30 years in association with increased water temperatures of 2-3 degrees C.
While the Levant has been warming and rabbitfish have been proliferating, things have not gone very well for the purple sea urchin Paracentrotus lividus. Previously, it had been a very important consumer of macroalgae within the Levant, but its population has collapsed within the past decades. For his Masters program, Erez Yeruham decided to investigate why the sea urchin population collapsed. Initially, he and his colleagues thought it was likely that sea urchins were competitively excluded by the invasive rabbitfish. These fish overgrazed much of the algal meadows, forming barren grounds along much of the Israeli coastline. However, during the experiments they did to check that out, they noted that sea urchin mortality occurred in two consecutive summers, but not in other seasons. That led them to explore how sea urchin survival was affected by both the impact of warming water and by competition with rabbitfish.
To investigate competitive exclusion of sea urchins by rabbitfish, the researchers bolted 25 metal cages (50 x 50 x 20 cm) to the rocks approximately 9 meters below the surface of the sea. They set up six different treatments: (1) fish only (F), (2) fish and sea urchins (FU), (3) sea urchins only (U), (4) no fish nor sea urchins (N), (5) cage control – a partial cage that allowed access to organisms (CC), and (6) no cage – an open control plot marked with bolts (NC). For the treatments with sea urchins (FU and U), the researchers introduced five sea urchins into each cage. For the treatments with fish (F and FU), the researchers cut oblong holes in the mesh large enough for rabbitfish to get through. There were five replicates of each experiment in the fall of 2011 and again in the spring of 2012.
Yeruham and his colleagues discovered that fish drastically reduced the abundance of soft algae, but that urchins had no discernable effect. The researchers suggest that sea urchin density in the cages was low enough that even though sea urchins were eating some soft algae, the effects were too small to be detected. Both fish and sea urchins had very little effect on the abundance of calcareous algae (algae with hard crusty surfaces).
The researchers compared the amount of food in sea urchin guts when they were caged by themselves, or in cages with fish access. Sea urchins had 40% more food in their guts when fish were excluded (left graph below). In addition, they had a 30% greater gonado-somatic index (GSI) when fish were excluded (right graph below – the GSI measures the relative size of the gonads – a high GSI indicates good health and high reproductive potential). So when rabbitfish could visit the cages, sea urchins ate much less and suffered poorer health.
The results of this experiment show that rabbitfish have strong competitive effects on sea urchin food intake and overall health. But do warmer waters also help to explain the collapse of sea urchin populations in the Levant? And might thermal stress interact with food limitation to influence sea urchin health? To answer these questions the researchers used seawater pumped in directly from the sea into tanks that housed eight sea urchins. Five tanks received ambient temperature seawater, while five other tanks received water that was chilled by 2 deg. C to mimic water temperatures before sea urchin populations collapsed. Each tank was divided in half by a partition so that four urchins could be fed (algae) three times a week, while the other four urchins were starved.
One important finding is that during the winter, feeding rates were similar when comparing sea urchins in ambient vs. chilled sea water (two left bars below – those differences are not statistically significant). However, feeding rates plummeted in the summer when water temperatures exceeded 29 deg. C in the ambient-temperature sea water.
Respiration rates (measured as oxygen consumption) are a good measure of metabolic performance. Highest respiration rates were measured in the winter with fed sea urchins (ambient was slightly higher than cold) and in the summer with cold fed sea urchins. Most notably, when sea water temperatures increased above 29 deg. C in the summer, the respiration rates were very low, even in sea urchins that were well-fed.
What emerges from this series of experiments is that sea urchins feed much more poorly and have lower respiration rates at high temperatures, independent of the effects of competition with rabbitfish. The researchers also found that survival rates were lower at elevated temperatures. Yeruham and his colleagues conclude that the direct effects of high temperature and the indirect effects of competition with rabbitfish are important factors that together conspired to lead to the collapse of sea urchin populations in the Levant. They expect that as sea temperatures increase, rabbitfish will become more dominant in other regions that are now a bit cooler than the Levant. As warming continues and competition increases, Yeruham and his colleagues predict that sea urchin populations will collapse in those somewhat cooler ecosystems as well, changing the structure and functioning of coastal ecosystems across the Mediterranean.
note: the paper that describes this research is from the journal Ecology. The reference is Yeruham, E., Shpigel, M., Abelson, A., and Rilov, G.. 2020. Ocean warming and tropical invaders erode the performance of a key herbivore. Ecology 101( 2):e02925. 10.1002/ecy.2925. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.