SOSA – Senckenberg Ocean Species Alliance

Marine Invertebrates

SOSA aims to discover, protect and build fascination for marine invertebrate species. But what are marine invertebrates?

Invertebrates are – simply put – all animals that lack a vertebral column, or backbone, in contrast to the cartilaginous or bony vertebrates. It is an umbrella term for an artificial (non-natural) division of animals – they are unbelievably diverse, comprising over 90 % of all animal species and divided among over 30 different phyla.

Whether on land, freshwater or marine water – invertebrates exist everywhere. We’d like to introduce you to the world of marine invertebrates.

Their size ranges from microscopic planktonic organisms to giant squids, which can grow over 12 m long. Explore these diverse creatures in the gallery below!

The anemone, Alicia mirabilis (Cnidaria), lives very secluded during the day with its tentacles retracted, resembling a bunch of berries. Only at night does this creature expand to its full size. Even though it is beautiful, never touch it! It´s body is covered with stinging cells.  
The Portuguese man o´war, Physalia physalis (Cnidaria), looks like a floating jellyfish but is in fact a colony of many organisms. It lives at the ocean’s surface – at the interface between water and air. With its venomous tentacles that can extend up to 30 meters, it preys on fish, squid, crustaceans and is also dangerous to humans.  
Aphrodita aculeata (Annelida) is also called a sea mouse, but this name is misleading as it is actually a worm. Although the sea mouse likes to bury its head in the sand of the ocean floor, when scientists discovered it, they also found that its brightly coloured bristles were an excellent model for fibre optic cables.
The Scaly-foot Snail, Chrysomallon squamiferum (Mollusca), is covered in hundreds of dermal sclerites – or scales – coated in iron sulphide, which not only create an armour-like appearance, but actually make this snail magnetic.
Grimpoteuthis imperator (Mollusca) belongs to the group of dumbo octopuses – and it is easy to see why. But what looks like its ears are actually fins that it flaps to swim in the highly pressurized deep-sea waters.
Nautilus belauensis (Mollusca) looks like a cross between a snail and a crustacean – but actually these are cephalopods (like squids and octopuses). Nautilidae are frequently referred as “living fossils” as this species has existed for 60 million years. However, this term can be misleading as today´s creatures show significant evolutionary adaptions compared to their ancestors. 
Phronima sedentaria (Arthropoda, Crustacea) are also known as “barrel shrimp” – for very unusual reasons. Female Phronimas attack salps – planktonic creatures resembling gelatinous barrels, eat the salps’ inner organs except for the barrel-shaped tests. They then “move in” and lay their eggs inside this barrel.  Phronima sedentaria might have also served as inspiration for the queen ‘Xenomorph regina’ from the sci-fi film series “Alien”. Whether this is really true or an urban legend remains unclear, however, can you see the similarities?

The selection above captures only a very small number of marine invertebrates. If you would like to explore more of these incredible creatures, visit Marinvert, our partner site with a growing list of European marine invertebrates.

Evolutionarily, invertebrates represent almost the entire animal kingdom and a simply astonishing range of body plans and adaptations. Comprising all animals except for the small group of the vertebrates, they span across an incredible range from basally derived, sessile and relatively simple groups such as sponges, to highly motile and complex cephalopods (such as squids) and arthropods (like crabs and lobsters).

This tree diagram shows the taxonomic relationships of the eight major phyla of invertebrates. Each branch represents a significant evolutionary event that led to an increase of complexity

Marine Invertebrates: Numerous but Neglected

Animals having a backbone are referred to as vertebrates. They belong to the group of Chordata and include, for example, fish, birds and all mammals. Although this group is the most familiar to us, it represents the smallest proportion in the animal kingdom – In fact, most creatures on this planet are invertebrates – an estimated over 90%.1

Estimates of known and unknown marine species1

The number of described species on land and in water is around 1,162,000; only 50,000 of these are vertebrates2. Although comprising the vast majority of animals, unfortunately, invertebrates do not get the attention they deserve and need. As a result, they are utterly underrepresented on the IUCN Red List (an inventory that showing the risk of extinction of species) and, thus, also receive less conservation action. This is despite many of them being endangered.

The Anthropocene, the current geological epoch characterized by significant human impact on Earth´s ecosystems, brings completely new challenges for the planet. Warming and acidification of the oceans, plastic pollution or deep-sea mining – all of these human-made threats lead to massive species loss that heavily impact marine invertebrates.

Species extinction is not only a great loss for biodiversity, but also matters for humans. Marine invertebrates are an important food source and part of an even bigger food web that humans depend on. They help to regulate the global climate through carbon sequestration and contribute to human health as some are source of important medical compounds.

Explore our gallery to find out how marine invertebrates contribute to our lives. 

References

1 Appeltans, W. et al.: The Magnitude of Global Marine Species Diversity. Current Biology 2012, 22, 23. https://doi.org/10.1016/j.cub.2012.09.036

2 Bertrand, S.; Escriva, H. The Evolution of Invertebrate Animals. Genes 2022, 13, 454. https://doi.org/10.3390/genes13030454