Neale Monks
Though hardly ever conspicuous, echinoids – generally often known as sea urchins – are among the many most vital invertebrates within the sea. They’re typically very quite a few and, specifically, the herbivorous species could be critically vital because the grazers that maintain giant seaweed species, similar to kelp, from taking up shallow water communities.
The Pacific shoreline of North America has been significantly effectively studied and robust connections between kelp, sea urchins and sea otters have been revealed. The ocean urchins eat younger kelp crops and the ocean otters eat the ocean urchins and, between the three of them, a steadiness is maintained with sufficient mature kelp crops to create a posh habitat that the varied fish and marine invertebrates can exploit. When sea otters had been hunted for his or her fur, the ocean urchin populations went up and the kelp crops had been decimated, ruining the ecosystem. As soon as looking stopped, sea urchin numbers went down and the kelp recovered.
An identical ecological response was noticed within the Caribbean just a few years in the past when an as-yet unidentified illness killed off 97% of the mature Diadema antillarum sea urchins. With the ocean urchins gone, the quantity of algae on coral reefs massively elevated, making it unimaginable for larval corals to quiet down and type new colonies. In a short time, the fish fauna modified and a number of the herbivorous fish that did effectively additional broken the reef by destroying established corals while they were feeding.
Echinoids can be found throughout the marine realm, from the intertidal zone down to the deepest abyssal plains. There are essentially three different types:
- The roughly spherical sea urchins proper.
- The more heart-shaped sea potatoes.
- The flattened, almost coin-shaped, sand dollars.
As is so often the case, the taxonomy of the group is a bit more complicated that this, but, for the casual fossil collector, recognising these three basic types is all you really need to understand their ecology and mode of life.
Morphology
Sea urchins are echinoderms, making them close relatives of things like starfish and sea cucumbers. Like starfish, they have a five-fold symmetry when viewed from above, but, instead of five arms, they have five paired rows of tube feet running up the sides of their bodies. There is some variation though and, as we shall see with sea potatoes, bilateral symmetry can be superimposed on top of their fundamental five-fold symmetry.
The skeleton of a sea urchin is interesting and highly distinctive, and this is one reason they are such recognisable and often rather attractive fossils. The body is mostly enclosed by a hollow, shell-like structure known as a test. If you look closely at the test, you will see that it is made from hundreds of small pieces known as plates, which connect together rather like a jigsaw puzzle. Upon closer inspection, it is possible to see that some of the plates have paired holes called pores through which the tube feet emerge. On most sea urchins, these pored plates are arranged in five sets of paired rows, known as ambulacra, each pair corresponding to a single arm of a starfish. So, while sea urchins may look spherical at first glance, they do actually have the same five-fold symmetry as starfish.
When a live sea urchin is examined from the outside, the most obvious structures are the tube feet and the spines. Pairs of tube feet poke out through the two pores of those plates that make up the ambulacra. The tube feet serve two main functions. Firstly, they are used for locomotion, pulling the sea urchin along the sea floor. Individually, they are not terribly strong, but, working in unison, they are very effective. Indeed, sea urchins, in intertidal habitats such as rock pools, are able to hold themselves in place, despite the strong currents and crashing waves. The other main function of the tube feet is for respiration. All the tube feet are interconnected by something called the water vascular system and, as water is pumped in and out of this system through the tube feet, oxygen is absorbed and carbon dioxide released.
Sea urchin spines are diverse and serve a variety of functions. At their most basic, they make a sea urchin difficult to eat. While some do this by making the sea urchin a prickly mouthful, the blunt, more robust spines seen on the so-called pencil urchins probably work by making the sea urchin too big for a fish to swallow whole and so more difficult to crush and eat. Some urchins augment their spines with venom glands, most infamously perhaps the tropical Diadema species that frequently sting careless tourists exploring coral reefs without proper gloves or footwear. All sea urchins can move their spines, but Diadema are sensitive to light and will wave their spines in a rather threatening manner in the direction of any moving shadows. Quite how they manage this feat is a bit of a mystery, since sea urchins have no eyes and an extremely simple nervous system.
Burrowing sea urchins usually have much shorter spines than the surface-dwelling varieties. Instead of being used for defence, burrowing sea urchins often have spoon-shaped spatulate spines that work like little shovels. Other spines, known as fascioles, form a bristly coat that keeps them from becoming smothered by mud and silt.
The grazers: regular sea urchins
Regular sea urchins can be immediately recognised by their approximately spherical shape. They are further distinguished by the locations of the mouth and anus – the mouth is underneath the animal, close to the ground, while the anus is at the top of the test, where water currents can carry away waste most easily.
Most species have long, sharp spines, though some, often called pencil urchins, have relatively short, blunt spines that do, in fact, look a bit like stubby pencils. In the past, people used their spines for writing on slates and stones – like the rest of their skeleton, the spines of a sea urchin are made from calcium carbonate and this works just as well for writing on a slate as a stick of chalk!
The mouth of a regular sea urchin is equipped with a complex structure known as Aristotle’s lantern. Essentially a set of jaws composed of five elements, this structure is very good at scraping things from hard surfaces, such as rocks. Besides algae, regular sea urchins consume a wide range of other organisms as well, including sponges, bryozoans, sea squirts and even small shellfish like juvenile mussels. They also consume a certain amount of carrion and organic detritus.
One of the most fascinating things about the regular sea urchins is the intimacy with which other organisms live alongside them. Since sea urchins are rather difficult to eat, they can be a very good hiding place for smaller animals. The most famous such hitchhiker is probably the razorfish, Aeoliscus strigatus, a small fish that swims head-downwards and is often found hidden among the spines of Diadema sea urchins. Besides a number of other fish, additional hitchhikers include various shrimps, worms and even crabs.
Regular sea urchins are not common in the fossil record. The main problem is that their favoured habitats – rocky shores and other solid substrates – are not places where fossils readily form. Virtually all the animals that die in such places will be eaten by scavengers, pulverised by water currents or otherwise destroyed long before they can be fossilised. However, regular sea urchins do sometimes live in other types of habitat and, in the limestone we call Chalk, regular sea urchins of the type known as cidaroids are not uncommon. Among other things, fossil cidaroids can be distinguished by their big bosses on those plates where their robust club-like spines were attached. In fact, these spines are rather easier to find than the sea urchins themselves.
The burrowers: sea potatoes
Sea potatoes, sometimes called heart urchins, are well adapted to a burrowing lifestyle. Like other sea urchins, sea potatoes have the standard five paired rows of tube feet, but, unlike regular sea urchins, they also have a front end and a back end. Typically the blunt end of the sea potato is the front and the tapering end is the back. Somewhere at the front, usually closer to the bottom than the top, is the mouth. At the back is the anus.
When burrowing sea urchins dig, their tube feet gather up organic material and pass it along the ambulacra and into the mouth. This all passes through the gut where it is sifted and inedible material is passed out through the anus. Although the way they work is different, in ecological terms sea potatoes are doing much the same thing as earthworms do on land. Presumably, because they’re feeding on tiny particles of food (mostly detritus) sea potatoes do not have an Aristotle’s lantern.
Sea potatoes use their tube feet for movement as well as for moving food around. They also use specially modified tube feet to produce a channel up to the top of the sediment a bit like a snorkel. Through this, they keep clean water flowing into their burrows, allowing them to breathe and dispose of metabolic wastes. Other tube feet create a sanitary drain behind the animal into which faecal material can be dumped well away from wherever they are feeding.
Unlike regular sea urchins, which depend on their spines for protection, sea potatoes avoid their enemies by staying out of sight in their underground burrows. In fact, they have rather delicate shells and are easily damaged if clumsily unearthed. However, because they live in soft sediments, they have a much higher chance of being fossilised than other types of sea urchin. In fact, most fossil echinoids are of precisely this type, including such well-known, Cretaceous sea potatoes as Hemiaster and Micraster species.
The sifters: sand dollars
Sand dollars and their relatives are very peculiar sea urchins with a test more or less flattened from top to bottom. The less compressed forms look a bit like buns, while the more compressed forms, the typical sand dollars, do indeed look rather like coins. If their tests are opened up, one obvious feature unique to this group is the presence of buttresses between the upper and lower sides of the test. While this might be to make their shells stronger, another hypothesis is that a denser shell is less easily buffeted around by water currents – a useful trick for animals that live in turbulent shallow water habitats.
While there is some variation, the typical sand dollar burrows through the sediment just below the surface. Unlike the sea potatoes, they do not go down very far into the sediment and they do not form snorkels or sanitary drains. In fact, sand dollars feed primarily on stuff at the very top of the sediment. Their spines are flattened and covered with beating cilia, and these waft water across the ambulacra and towards the mouth, where tiny particles of organic detritus and plankton are consumed.
Sand dollars sometimes half dig themselves into the sediment so that they are poking out of the sand at a slight angle. Supposedly, this makes it easier for them to capture plankton. In places where the water is more turbulent, they will lie flat on the sediment, their flat, streamlined profile ensuring water slips by them as easily as possible. Should the water get really choppy, they dig into the sand completely and hide away until things settle down again.
Evolutionary history
The evolution of the sea urchins is complicated, though, in general terms, the regular sea urchins evolved first and, from them, evolved the more specialised sea potatoes and sand dollars. Primitive sea urchins had appeared by the Ordovician Period, but for whatever reason, they are not a major part of marine communities at any point in the Palaeozoic. By the Permian Period, the first regular sea urchins of modern aspect had appeared, the group known as cidaroids, distinguished by their robust, club-like spines.
Throughout the Triassic, the regular sea urchins diversified, including the group known as the euechinoids or ‘true’ regular sea urchins. These are the ones with long, thin spines that are so common and so ecologically important today. Sea potatoes and sand dollars branched off from these at some point during the Jurassic, occupying new ecological niches on and within the sediment. However, it was from the Cretaceous onwards that the sea urchins really showed their potential and they are probably as diverse now as they ever have been.
Sea urchin fossils
At least in the UK, sediments of Cretaceous age are the best place to go hunting for sea urchins. Almost any fossiliferous exposure of the Chalk is likely to yield fossil sea potatoes as well as the occasional cidaroid spine or plate.
Micraster is perhaps the best known genus of Cretaceous sea urchin, with several different species known, some of which have been used for biostratigraphy. A species like Micraster coranguinum clearly shows itself to be one of the sea potatoes, with an obvious bilateral symmetry superimposed on its fundamental five-fold symmetry.
On most specimens, the ambulacra can be clearly seen, as can the mouth and the anus. It isn’t difficult to imagine these sea potatoes ploughing their way through the sea floor, but, even after death, they seem to have been important. Fossil Micraster are commonly encrusted with things like oyster shells, brachiopods, bryozoans and tubeworms. Given the muddy nature of the sea floor at this time, sea urchin tests were self-contained islands where animals that needed solid ground could find a place to settle down.
Very occasionally, whole cidaroids can be found, with the club-like spines still attached to the roughly spherical test. These are impressive fossils, but what is perhaps even more interesting is how cidaroids were viewed in the past. Fossil cidaroid tests were believed to be snake eggs. When breeding, snakes produce balls of foam and they periodically throw them into the air, catching them again with their teeth. Therefore, the circular bosses on the ambulacral plates were thought to be tooth marks. If a person caught a ‘snake egg’ thrown into the air before it hit the ground again, it could be used as a potent magical charm that provided skill in law courts, success in battles and immunity against poisons.
Cidaroid spines were also thought to be valuable. Having a vaguely bladder-like shape, they were used for spells against diseases of the urinary system, such as kidney stones. Fossilised sea potatoes were not without use either. Resembling, in some cases, loaves of bread, they were known as fairy’s loaves and considered useful charms for the housewife or baker to keep by the oven to ensure successful baking.
In this age of Jamie Oliver-this and Nigella Lawson-that, perhaps a fairy loaf sea urchin is the one fossil we should all keep about the house!
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