People have been preserving food with salt for millennia, drawing water out of fish and meat for long-term storage. However, creatures that reside in the sea protect themselves from drying out by allowing the salts in their bodies to balance those in the surrounding water. But sea-bottom dwellers that make their home close to shore can be at risk of swelling when rivers flooded by a deluge on land surge into coastal waters, reducing their salinity. And how will such creatures cope as previously icy regions melt, further diluting coastal waters? Yet, few studies have looked at the long-term effect of seawater dilution on seabed dwellers, such as the European sea urchin (Echinus esculentus). Nicholas Barrett, Elizabeth Harper and Lloyd Peck from the University of Cambridge and British Antarctic Survey, UK, and Kim Last and Helena Reinardy at The Scottish Association for Marine Science (SAMS), decided to check how well the urchins cope when the salt in their environment plummets.
Barrett visited the SAMS aquarium, home to a colony of urchins, where he placed some of the animals into full strength seawater [31 parts salt per thousand (‰)], in addition to plunging others into watered-down seawater (26‰, 21‰, 16‰ and 11‰ salt), for a day to find out how they coped with a sudden drop in salinity. The most dilute seawaters (16‰ and 11‰) turned out to be lethal for the urchins, but the animals all survived in the more concentrated seawaters, leading the researchers to conclude that the urchins could just about survive in seawater at 18.5‰ salt. But how well would the urchins cope after an extended stay in reduced strength seawater?
This time, Barrett gently diluted some of the urchins’ seawater to 26‰ salt, while reducing the seawater in a second enclosure to 21‰, before monitoring the echinoderms’ health over 25 days. The urchins in the most dilute seawater lost ∼4% of their body mass, and when Barrett looked at the general health of the urchins, it was clear that the animals in the most dilute seawater were not well. They lost most of their spines, their tube feet deteriorated, they had difficulty feeding and they found it difficult to right themselves after toppling over. An extended stay in dilute seawater was clearly a threat to the animal's health. However, the urchins in the intermediate strength seawater coped better and by the end of the period they were just as healthy as urchins in regular seawater; they seemed to be able to adjust and tolerate slightly lower salinity. But might a period of preparation at both of the reduced salt concentrations prepare the urchins for a sudden, and potentially life-threatening, drop to 18‰ salinity?
Barrett plunged the two groups of urchins into the 18‰ seawater for 6 h to find out how 25 days in more dilute seawaters had prepared them for the shock. The metabolic rate of the urchins that had been residing in 21‰ seawater fell, while the 26‰ seawater urchins’ metabolic rate increased; they were both struggling in the fresher water, but in different ways. However, two of the urchins from the 26‰ seawater were less affected by the sudden drop, successfully righting themselves after toppling over. The period in 26‰ seawater seemed to prepare the urchins for a life-threatening freshwater shock, as if a swollen river had suddenly surged into the sea.
In the long term, sea urchins can cope with slightly diluted seawater (26‰ salt), but more extreme freshwater influxes can be dangerous for the animals, making it challenging for the urchins to survive in coastal regions that are regularly inundated with freshwater and are becoming less salty.
