The Lives of Our Forgotten Marine Invertebrates

Do you know what the most abundant creatures in the oceans are? It may surprise you to hear that shimmering fish is the wrong answer. The millions of seabirds which swoop over our coastlines is incorrect also, whilst the thousands of dolphins which leap over wave crests is still not close. In fact, the most abundant creatures in our oceans are marine invertebrates or, in other words, those without backbones. These animals often fail to make it onto conservation posters and into ocean protection campaigns as many believe they provide a fairly boring and ugly alternative to our magnificent humpback whales, polar bears and hammerhead sharks. Yet marine invertebrates make up an astonishing 92% of all life in our oceans, with many species playing important roles which keep our ecosystems healthy. These forgotten creatures nibble parasites off fish, take carbon dioxide out of the atmosphere and remove harmful algae from coral reefs. We know so very little of these animals’ lives, so let’s get to know a few of them a little better.

The term ‘marine invertebrates’ covers a colossal range of species, from urchins to anemones, snails to shrimp and octopuses to oysters. Sponges are not only one of the oldest invertebrates but one of the oldest species on the planet, evolving alongside some of Earth’s first animals. In fact, these unassuming sponges have played a critical role throughout our planet’s history and the five mass extinction events which have occurred within it. Several times, a large majority of our ocean life has been swept from the sea due to events such as global warming or high volcanic activity, yet sea sponges are thought to have prevailed, helped settle down sediment and create a home for small organisms which then provided a food base for larger animals. Today, turtles, fish and sea snails feed on these organisms whilst octopuses, anemones and shrimps call sponges their home. One particular sponge found growing in the Ross Sea off the icy coast of Antarctica is thought to be between 15,000 and 23,000 years old, making it the longest living animal ever to be recorded on Earth! Their day to day lives are a little less dramatic however and are spent filtering water through their array of holes.

Krill are yet another unassuming creature which is actually essential to the health of our oceans. Krill are small shrimp like animals which often cluster in large groups called swarms, with their bright bodies turning the surface of the ocean into a pinkish-red hue. Sometimes these swarms can be so enormous that they can actually be seen from space! Despite their small size of only two inches, these creatures feed some of the largest ocean species on our planet, with humpback whales travelling all the way from Australia and Peru to feed on these Antarctic dwelling critters. Amazingly, a single humpback whale will consume around 2,000 kilograms of krill in one day! Many species which live in the Antarctic year round also feed on krill, from penguins to leopard seals. Yet krill are in danger from melting ice sheets as they depend on the ice for the algae which grows there as well as a sheltered place to raise their young. Not only this but Antarctic krill are increasingly being targeted by the fishing industry to be used as feed for growing aquaculture farms or to be used in health supplements taken by humans. If krill numbers start falling, the numbers of our whales, seals, fish and seabirds will plummet too.

A more familiar marine invertebrate are crabs, found the world over in hidden rockpools, often surrounded by a bunch of eager children armed with nets and buckets. In total, Earth holds 4,500 species of crab, from the Japanese spider crab with a four metre long leg span to the pea crab, being only the size of a ten pence piece. Whilst crabs are renowned for having an iconic side shuffle, they also have a range of dancing skills including flapping their pincers and drumming their claws, all of which aim to help the male attract a partner. Yet crabs are already suffering from the effects of climate change as our oceans become more acidic due to an increased level of carbon dioxide. When the water becomes more acidic however, the number of carbonate ions decreases, yet these are a key component in crabs’ calcium carbonate shells which protect them from predators. This means crabs struggle to build and maintain their shells as they simply start dissolving into the water.

Mussels are facing a similar fate along with the rest of the mollusc community. As the ocean becomes acidic, baby mussels struggle to form the shells which will protect them and allow them to feed. These creatures are filter feeders which means they take in water, consume any nutrients within it and then expel the waste water. In a single day, one mussel can filter feed 65 litres of water, however this could soon be a problem as our ocean fills up with teeny microplastics. Mussels and other filter feeders such as clams and oysters accidentally take in toxic pieces of plastic alongside nutrients which can then cause a whole range of internal damage and disease. Yet losing this species would be a massive shame. Did you know that they have tiny beards which they use to cling onto rocks? These beards are produced as a liquid which hardens when it touches seawater and forms a bond so strong it can even grab on to ‘non-stick’ Teflon. Due to their incredible properties, mussel beards are now being examined by scientists as they try to develop a mussel-based adhesive to use within eye surgery. Dog whelks are one of the main predators to mussels as they drill their pointy shells through mussels’ blue coat before eating the squishy creature inside. Once the mussel realises what is happening, they often use their super sticky beards to glue itself to the dog whelk, ensuring it takes its enemy down with it as the whelk is left trapped on the side of the dying mussel shell for the rest of its life.

Our ocean’s squishy animals are also classed as invertebrates, with creatures such as jellyfish being found all over the world. Jellyfish again form one of the Earth’s oldest species, with their presence dating back to at least 500 million years ago. In fact, one particular species with the Latin name ‘turritopsis nutricula’ is actually able to renew its cells time and time again, a process which may be described as the ‘fountain of youth’ which allows it to, potentially, live forever. As many invertebrates are simple beings in comparison with our marine mammals and fish, some have developed stinging cells which allow them to catch prey and protect themselves from predators. Jellyfish are no exception, with box jellyfish being named one of the deadliest species on the entire planet. Not only do they have a poison which can send a human into cardiac arrest with only one sting but they are thought to be one of the only jellyfish species which have eyes and can purposefully move themselves in a direction rather than float with the currents. Whilst many people are not fans of these multicoloured species, jellyfish as a whole serve an important role within the ocean, providing food for turtles and a sheltered place for small fish to hide in areas of the ocean where there is little other protection. Even tired young crabs can hitch a ride atop their rounded body.

Octopuses are one of the most famous invertebrates, with many of us knowing some of the weird facts about them such as they have eight legs, three hearts and blue blood. Their blood is actually blue because it is copper-based, rather than our iron-based red blood which is less efficient at transporting oxygen at low temperatures. Octopuses are also fairly unique in the invertebrate world as they are thought to be some of the most intelligent creatures on the planet, with their brain making up for their lack of backbone. Over the years, they have developed multiple mechanisms which allow them to escape predators, these include spraying dark ink which blinds enemies and dulls their sense of smell whilst they can also camouflage themselves to any colour or texture, allowing them to disappear in seconds. Octopuses can even escape from humans, with Inky sneaking out of his New Zealand aquarium tank, down a 164 foot drainpipe and out into the sea surrounding the building. Other octopuses in captivity have decided to have a little fun by cheekily escaping from their tanks at night in order to eat tropical fish in the neighbouring tank before moving back home and falling into a contented sleep. Yet these species are also threatened by a loss of habitat as well as a decline in food as molluscs and crab numbers fall due to pollution, climate change and overfishing.

As you can see, invertebrates have far more interesting lives than we had ever previously imagined, yet aside from wacky facts, these animals are essential to the function of our ocean systems as they act as the cleaners, builders and food providers of the underwater society. Many of these creatures are some of the most accessible ocean species we can see with our own eyes as barnacles, crabs, anemones and starfish fill our intertidal rockpools. Rockpooling is a fantastic way to get children involved in the battle to save our oceans and all you need is a bucket, a net, an identification guide and a ton of enthusiasm. Allowing children to explore and discover some of these secret invertebrate hideaways is a great way to educate them about the ocean’s creatures and the challenges they face. If you need a bit of help on the facts side, there are several great children’s books available to help answer all their burning questions. ‘Jolly Ollie Octopus’ is a recently published book which blends fun facts about ocean critters with messages about finding what makes us happy; the book even has an accompanying website with facts and puzzles to keep young ones occupied. ‘Crabs, Dabs and Rock Pools’ is another great starting point for filling your child’s head with funny facts and subtle educational messages.

With marine invertebrates totalling 92% of ocean species and performing many critical jobs within our oceans, we mustn’t forget these underrated creatures as we take action to save our planet’s seas. Why not pay them a visit in a local rockpool and try to spot some of their hard work taking place.

By Neve McCracken-Heywood