Horseshoe crabs have not evolved beyond the molecular level for 250 million years. From: National Wildlife Federation

by Trevor Regan

In our March and April newsletter, we discussed a newly discovered branch of marine life. This month, we are featuring a species of marine invertebrate that has remained frozen in evolution for around 250 million years, living fossils that now play a critical role in our health and modern medicine—the horseshoe crab. 

It should be noted that horseshoe crabs are neither crabs nor crustaceans (just as starfish are not actually fish), and share a closer link to spiders, scorpions, and other arachnids. The confusion stems from the similarities in habitat—shallow coastal waters, intertidal zones, jetties—and anatomy to crabs, lobsters, and other marine crustaceans. Their large, hoof-shaped chitin carapace or shell, a common trait across arthropods, shields their body, which comprises three parts—the telson, abdomen, and cephalothorax, the largest of the three. This anatomical structure is also shared by crustaceans. The Atlantic, Mangrove, tri-spine, and Indo-Polynesian are the four remaining species of horseshoe crab. Only the Atlantic and Mangrove live in non-marine environments, both being observed in brackish environments and estuaries. 

The first fossils resembling horseshoe crabs—the group Xiphosura—are dated to c. 480 million years ago, and, as mentioned, they would reach their modern form some 250 million years ago. If you pick up the shed exoskeleton of a horseshoe crab that washed ashore, you are seeing the same animal that dinosaurs would have seen scuttling around beneath them in the shallows. They have withstood every known mass extinction event, including the end of the Permian Era, which killed around 96% of marine life at the time. Horseshoe crabs are truly an outlier among the fauna of the ocean and the world at large, which raises the question of how they became and have remained so unbelievably resilient. Their physical traits, of course, play a role and are perfectly tailored to their habitat. And even though their chosen environment is relatively stable, surviving mass extinctions, shoreline shifts, continental drift, and intense, sometimes catastrophic climate changes implies unparalleled adaptability and, again, resilience. 

The sky-blue, copper-based blood of horseshoe crabs is a key cog that has kept these marine machines running for a quarter of a billion years. Amebocytes, which are specialized cells within their bloodstream, instantly clot when a bacterial toxin is detected, safeguarding them against infection. This adaptation is thought to have occurred around 400 million years ago, and each day, their blood is used to test and develop modern vaccines. Some 500,000 Atlantic horseshoe crabs are harvested every year. Their blood is drawn, and then the crabs are returned to the ocean. However, the practice has become increasingly controversial, as despite how beneficial it is to us, some horseshoe crabs do not survive the harvesting process. This, coupled with dramatic habitat loss for the four extant species of horseshoe crabs, has threatened the survival of these extraordinary ancient creatures. Pauses on harvesting and breeding centers have been sponsored by the United States and Malaysia, respectively, to allow for reproduction.

Sources

https://oceanservice.noaa.gov/facts/horseshoe-crab.html

https://www.nhm.ac.uk/discover/horseshoe-crab-blood-miracle-vaccine-ingredient.html

https://en.wikipedia.org/wiki/Horseshoe_crab

https://dnr.maryland.gov/ccs/pages/horseshoecrab-medical.aspx

The leopard sea cucumber. From: Natural History Museum

by Bethany Woo

One of the weirdest and most harmless-looking marine invertebrates is the sea cucumber! Sea cucumbers are in the phylum Echinodermata, which also contains sea stars and urchins. Although sea cucumbers have retained some similarities to these other creatures—for example, they also have tube feet like sea stars. Sea cucumbers look vastly different from other echinoderms. This is because sea cucumbers’ skeletal structure has evolved into a reduced form of small ossicles—small bits of calcium carbonate that lack rigid structure—giving them a weird “blob” or “sausage” like appearance.

Due to their soft, squishable bodies, sea cucumbers developed unique defense mechanisms against predators, which include various species of fish, sea stars, and crustaceans. One common defense mechanism uses a specialized organ called “Culverian tubes”. These are white thin tubes that the sea cucumber expels out of its body that deters predators. Culverian tubes vary between species, with some sea cucumbers having sticky tubes that can entangle predatory fish and others having toxic tubes. Another defense mechanism is “evisceration” - where a sea cucumber will expel out its guts or internal organs for the predator to eat. While the predator is busy consuming these body parts, the sea cucumber is able to move away and later regenerates the lost organs. Lastly, different sea cucumbers produce toxic chemical compounds that not only can be toxic to predators, but can also act as anti-bacteria and anti-microbial protection. Many species of sea cucumber can do one, if not multiple of these mechanisms!

Why is sea cucumber survival important? Because sea cucumbers play a vital role in recycling cycles of ocean ecosystems around the globe! Sea cucumbers typically are benthic - they meander along the ocean floor using their tube feet to move, but to also suck up benthic substrate and sediment. The sea cucumbers suck up the ocean floor in search of detritus (decomposing organic material), algae, and other nutrient sources and release the rest of the substrate as more fine sediment/sand in a process called “bioturbation”. This process is key in creating more accessible forms of calcium carbonate and other inorganic compounds rich in nitrogen and phosphorus that can be used by other creatures. Despite their unassuming and unusual looks, sea cucumbers play a vital role in ocean health!

Sources

https://www.nhm.ac.uk/discover/what-is-a-sea-cucumber.html

https://schmidtocean.org/cruise-log-post/creature-report-deep-sea-cucumbers/

https://marinesanctuary.org/blog/sea-wonder-sea-cucumber/

https://pmc.ncbi.nlm.nih.gov/articles/PMC7587958/

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An octopus camouflaging itself onto a piece of coral. From: ThoughtCo

by Madeline McCormick

This month’s spotlight science article will give us a new perspective on camouflage in marine environments. What marine organisms use camouflage, you may be asking. Well, there is no shortage of players in this game of hide and seek! Great white sharks have a white-tinted underbelly to resemble the pelagic, or open ocean, water surface. Leafy sea dragons float almost entirely invisible amongst seaweeds and kelps with the help of leaf-like extrusions on their bodies. But what about more complex methods of disguise? Let’s discuss chromatophores, iridophores, and instant pattern shifts!

A chromatophore is a cell that can change color, and many species of cephalopods–a taxonomic group including squid, cuttlefish, and octopi– have millions of chromatophores covering their bodies. These specialized cells allow them to evade predators, sneak up on prey, and even to communicate with nearby cephalopods. In fact, both squids and octopuses have been observed having social interactions where skin patterns designate messages such as a predator threat, mating motives, or aggression or submission in a fight. The process of expressing these color-changing patterns might be more complex than you’d think. Envision each chromatophore as an ink-filled sac. If you push, pull, or stretch the sac the ink might shift to be more or less visible. Well, these cells are interconnected with a complex muscle tissue tied to the nervous system. Because not every chromatophore has the same pigment, when the muscle stretches certain cells, “show colors” become more obvious in the form of patterns! Beneath the chromatophores are iridophores, or reflective cells. These cells don’t contain pigment, but reflect surrounding light like ocean blues that aren’t a pigment contained in the chromatophores. And just like that, a cephalopod has disappeared before your eyes!

These organisms have evolved to be able to either control instant pattern shifts, or do them naturally. Because each chromatophore is controlled by a nerve, the brain can send signals to the muscles to stretch and display a certain pattern. Likewise, in a fight or flight scenario, the nervous system may naturally affect pattern change without conscious intent. A cephalopod’s ability to camouflage is just one of the many markers of their advanced intellect. But we don’t want to give cephalopods all the glory as many fish and crustaceans have been confirmed to contain chromatophores such as flounders and lobsters! Can you think of any other marine organisms that might utilize camouflage?

https://ocean.si.edu/ocean-life/invertebrates/how-octopuses-and-squids-change-color

https://youtu.be/efL6B8x9MaM?si=BKnULhj483d2MXP0