I would be first to admit that sea cucumbers (holothurians) are far from my favorite group of organisms. These generally soft-bodied echinoderms well deserve their name; they look like warty pickles. They are capable of changing their external muscles so that they can be rigid or flaccid. A flaccid sea-cucumber is not the most pleasant thing to handle. If you dissect it, as I did as a graduate student, their interior anatomy is absolutely confounding. And, as is demonstrated in virtually any invertebrate zoology class, when suitably provoked they will eject their guts and other internal organs (these will eventually regenerate). The ejected parts contain sticky threads which can immobilize predators; I suspect any others will just be nauseated and leave the animal alone.
Sea cucumbers are considered a health-promoting delicacy in many parts of Asia (I ate one once and was not impressed; perhaps it was how it was prepared). And there is the problem. The demand for holothurians has led to a rapid rise in prices, which in turn leads to overfishing. As reported in the New York Times on October 5, overfishing in the Yucatan of two species, Isostichopus badionotus and Holothuria floridana, has led to the collapse of the populations. This has happened remarkably fast: heavy exploitation by diving did not begin until 2012. The population collapse occurred within five years. There has also been a human toll; at least 40 divers have died and many others have been injured and/or suffered the bends. According to the Times, these two species are not alone; the International Union for the Conservation (IUCN) lists 23 species as endangered or threatened.
The species of sea cucumber are not the only ones threatened by human exploitation. In contrast to sea cucumbers, I am very fond of the American horseshoe crab, Limulus polyphemus, an aquatic relative of spiders and scorpions. They are truly amazing beasts; they can swim, walk, and burrow and can survive a huge range of environmental conditions. They are found from Maine to Florida. One of the most fascinating things about them is their mating behavior. They come up on shore en masse to reproduce. As the large females reach the beach at high tide, the smaller males grab on to her back. She digs a nest in which she deposits her eggs, which is then fertilized by the males. The huge natural abundances of reproducing animals and the resulting enormous egg deposits have historically provided a rich source of food for many other animals, especially migrating birds such as the red knot (the interaction between the red knot and the horseshoe crab Is beautifully described in Deborah Cramer’s book The Narrow Edge).
Horseshoe crabs are not eaten by humans. This has not kept them from being exploited. Their vast numbers made them a ready source of fertilizer, as well as hog and chicken feed. Apparently, animals fed horseshoe crabs were inedible due to their taste and had to be given other feed before being sent to market. This usage has almost vanished. The major impact today on Limulus is that they are used as bait. But a growing threat comes because the blood of horseshoe crabs is remarkable. First, it uses copper based hemocyanin to carry oxygen, rather than iron-based hemoglobin. As a result, it is blue in color. More importantly, it carries amoebocytes that induce clotting when encountering bacteria. This has led to the development of a compound, called Limulus amebocyte lysate (LAL), which allows the detection of bacteria and is standard method in the medical industry to detect bacterial contamination. As recently reported in Business Insider (https://www.businessinsider.com/why-horseshoe-crab-blood-expensive-2018-8), a gallon of horseshoe crab blood is worth some $60,000. Hundreds of thousands of crabs are drained of about 30% of their blood before begin returned to the ocean. The mortality is not known exactly but is somewhere in excess of 10%, perhaps as high as 30%. The combined impact of harvesting for bait and the biomedical industry has led the IUCN to categorize the species as Vulnerable to extinction. This does not take into account its potential impact on species dependent on the crabs and their eggs for survival.
Invertebrates, especially marine invertebrates, are rarely mentioned in our discussions of the ongoing extinction. The notable exception are coral reefs, although even in this case the concern is focused on the entire ecosystem than on individual species. Instead, most of the attention has been paid to threatened and endangered terrestrial vertebrates. The IUCN is notoriously underrepresented in terms of marine invertebrates. The unfortunate aspect of this is that most of our understanding of extinctions in the geologic past have focused on marine organisms, especially invertebrates. This includes the iconic five mass extinctions that led to the coining of the phrase “The Sixth Extinction.” It is very difficult therefore, to meaningfully compare today’s ongoing extinction episode with those in the ancient past.
But what really saddens me is that the threats to holothurians and horseshoe crabs are mostly flying under the radar in terms of public attention. They are not charismatic; some people may consider them positively ugly (okay, I agree with this about most sea cucumbers). But their aesthetic appeal does not make them less important or deserving of protection. In our concern for the continuing erosion of the natural world, we need to care just as much about Limulus and Holothuria as we do pandas.