Invertebrates (exclusive insects, myriapodes and Cheliserata)

When we think about animals, we usually get a vertebrate in mind (e.g., a mammal, a bird or a fish). However, the vast majority of animals are not vertebrates, invertebrates. For example, given the report by Elven & Søli (2020) in Norway 98% of all species are invertebrates and only 2% are vertebrates. Hence, invertebrates occur 49 times more often than vertebrates. By far the largest invertebrate group are the arthropods (insects, crustaceans, spiders and alike) with 30567 of 40999 Norwegian species and they might bias this estimate. However, even if we take arthropods out of this calculation completely, 91% of all species are invertebrates (without arthropods) and 9% are vertebrates. Even in this case, there are 10 times more invertebrate species.

Invertebrates are defined by the absence of two characters; that is the lack of a head and a backbone. As vertebrates are nested within invertebrates, invertebrates are actually not a natural group, but an overarching term comprising a huge diversity of body plans, sizes, life histories and ecologies. This diversity is substantially larger than the diversity observed in vertebrates. For example, this includes sessile sponges, flying butterflies, large squids and the smallest animals on Earth. Moreover, invertebrates can be found everywhere on Earth. Finally, there are 31 animal phyla and all of them are invertebrates. Vertebrata are not a phylum, but the largest subphylum within the phylum Chordata. However, the ancestral chordate was also an invertebrate. Hence, a typical animal is therefore an invertebrate and not a vertebrate.

In our collections, invertebrates are managed in five collections. The insect collection comprises the subphyla Insecta, Myriapoda (millipedes and centipedes) and most Chelicerata (spiders, scorpions, mites and alike, but not Merostomata and Pycnogonida). All the other 30 invertebrate phyla and the arthropod subphylum Crustacea plus the chelicerate classes Merostomata and Pycnogonida are part of the collections on this page. Together these groups comprise 11992 of the Norwegian species, which is 29% of the Norwegian biodiversity.

In specific, the Lophophorata collection comprises the three phyla Brachiopoda, Bryozoa and Phoronida and 387 species of these groups occur in Norway. The Crustacea collection holds species belonging to the arthropod subphylum Crustacea plus the chelicerate classes Merostomata and Pycnogonida as well as the phyla Tardigrada and Onychophora. Onychophora are not known from Norway, but for all the other groups 3002 species occur in Norway. The Mollusk collection consists "just" of the phylum Mollusca, but Mollusca is the second largest phylum on Earth after Arthropoda with respect to number of species. In Norway, 1075 species occur. Finally, all remaining 24 animal phyla including Chordata without Vertebrata (in specific, Cephalochordata and Tunicata) are put together the "Invertebrate" collection. 7528 species of these groups occur in Norway.

Some of the largest groups among invertebrates:

Annelida

Annlida is an animal phylum also known as ringed worms. This is due to their bodyplan, which consists of repeating bodyunits, which are usually identical to each other and are called segments. Hence, the body appears like a chain of rings. The best known annelids are probably earthworms, which everyone has seen in their garden or on the street after a heavy rain. Earthworms and alike are usually referred to oligochaetes. The second best known group of annelid species is probably leeches. They feature often in movies as bloodsuckers, but are also used for the very same propuse in medicine. The third large group of annelids are the polychaetes, which predominantly occur in the sea and have very different sizes, life histories and ecologies. With respect to bodyplans they are also the most diverse of the three annelid groups. There are about 20000 species in Annelida globably, which are distributed in about 2100 genera in more than 170 families. By far the most species occur in polychaetes with about 14000 species, then in oligochaetes with about 5150 species and finally leeches with about 850 species. In Norway, 30 species of leeches, 472 oligochaete species and about 1000 polychaete species supposedly occur.

Bryozoa

The phylum Bryozoa is called moss animals, as they grow like moss over surfaces. They are colonial animals occuring both in the sea and freshwater habitats like lakes and rivers. The individuals in each colony are only a few millimeters in size, but the colonies might grow quite large. The animals filter their food out of the water by using a tentacular crown they erect into the water, but which they can also rapidly draw into their body. The body is protected by a chitinous or calcified outer layer. The individuals of a colony can all have the same bodyform or in some groups individuals, though clones of the neighboring individuals, can have different bodyforms with different functions like feeding, protection or reproduction. Moreover, especially due to the calcified bodies this phylum of invertebrates has a very rich fossil record. The about 6500 extant species of bryozoans are grouped into four groups. Phylactolaemata occur exclusively in the freshwater and 11 out of the 80 species occur in Norway. The other three groups are grouped together as Gymnolaemata with about 6400 species globally and 360 in Norway. Stenolaemata are excusively marine and the body is protected by a calcified outer layer. Like Stenolaemata, Cheilostomata are excusively marine and possess a calcified body. Finally, Ctenostomata has a chitinous body and is almost exclusily marine with very few species occuring in freshwater.

Cnidaria

Cnidaria comprise among others jellyfish, sea anemones and corals, but also parasites. Except for the parasites, cnidarians are filter feeding, even the jellyfishes. They occur both in freshwater and marine environments. Their most prominent characteristic, and usually the kind of encounter we have with them, is that they can nettle their prey or us. For this, they have specific cells called cnidocytes. This makes some cnidarian species the deadliest animals on Earth. Originally, the life cycle of cnidarians comprises two stages: a sessile stage, which reproduces asexually, and a pelagic stage, which reproduces sexually. However, this life cycle has been strongly modified in several groups ranging from entirely sessile to entirely pelagic. Moreover, cnidarians can be solitary individuals or colonies. Globally about 13200 species are described, which are split into three larger groups. Anthozoa, the sea anemones, corals and alike, comprises about 6200 species with 157 occurring in Norway. Medusozoa, including the jellyfishes, has about 4800 species including 305 Norwegian ones. Finally, the third group is the parasitic group, Myxozoa, with about 2200 species globally and 200 in Norway. 

Crustacea

Crustaceans comprise among others crabs, lobsters, shrimps, beach hoppers, pillbugs, barnacles and copepods. Of all arthropods, they have probably the most diversity in body forms and also the best known invertebrate group besides insects. In principal, their body consists of three regions, the head, the thorax (usually with the legs) and the abdomen (usually with the reproductive organs). However, some parasitic crustaceans are as adults just a lump and string of cells without any body structures like a head, legs or gills. Each region called tagmata comprises different numbers of segments. They are predominantly marine, but also often occur in freshwater, but more rarely in terrestrial environments. In the ocean, individual species of crustaceans can make up a substantial biomass. For example, the antarctic krill species is estimated due be 500 million tons of biomass. This is close to the biomass of all ant species on Earth. Like invertebrates, crustaceans are not a natural group as Insecta are placed within them. Or with other words, insects are terrestrial crustaceans. Globally, about 70000 species of crustaceans are described, of which 2498 supposedly occur in Norway. Given the large diversity of body plans, crustaceans are divided into 11 classes and it is not certain how they are related to each other. In Norway, most crustaceans are from the classes Malacostraca, which includes, for examples crabs, lobsters, and shrimps, and Maxillopoda, which includes copepods.

Echinodermata

Echinodermata are also often relatively well-known as the phylum comprises among others sea urchins, starfish, sea cucumbers, sea lilies and brittle stars. They are exclusively marine and properly among the more bizarre animals. Like us, most animals have a left and right side. This means they are bilaterally symmetrically. However, for Echinodermata this is not the case. They do not have a left and right side, but rather have five sides. This is best seen in seastars and brittle stars, but if one looks closely this can also be seen in sea urchins or sea cucumbers even though they often look like balls or sausages. Echinoderms can be very small with less than a 1 cm across, but also very big with some sea cucumbers reaching length of 3 m. To date about 7300 species of Echinodermata are known, but they also have a rich fossil record with twice as much species. In Norway, only 164 species are recorded so far. Echinodermata are split into five classes and all occur in Norway as well. Crinoidea comprises the sea lilies and feather stars, while the starfishes are in Asteroidea and the brittle stars are in Ophiuroidea. Sea urchins and alike are group into Echinoidea and sea cucumbers are Holothuroidea.

Mollusca

Besides crustaceans, mollusks are properly the most eaten invertebrates on Earth as they comprise mussels, scallops, clams, snails, slugs, squids, and octopuses. Mollusc shells have also been used for other purposes including as ornaments or as currency. Mollusks like many of these large invertebrate groups comprise a huge diversity of bodyplans. They can be just a few mm in body length or up to 13 m as the giant squid. The body can generally be divided in three regions: the head, the foot and visceral mass covered by the mantle. The mantle generates the shells. However, all of these regions can be strongly modified or reduced across the different groups. While most mollusks possess shells, not all do. In some groups, the shells have been lost, are internal, or are reduced to scales or small spicules. Another example is Bivalvia (mussels and alike) with two shells, in which the foot and head are reduced to support the filter feeding. Mollusca comprises about 80000 species on Earth and 1075 in Norway. Mollusca is divided into eight classes, of which seven occur in Norway; only Monoplacophora is lacking. The most species-rich classes are Gastropoda and Bivalvia, both globally and in Norway. In Norway, about 650 snails and 290 bivalves occur.