As part of our new ArtsDatabanken project MeioSkag, we are investigating interstitial and meiofaunal species occurring in sediments of the Norwegian part of the Skagerrak. Among others, this includes the so-called interstitial families within Annelida. In this series “Group of the month”, I will now present the different families, which have not already been presented before. Besides their small body size, these annelid families are usually characterized a common trait. They have no or reduced lateral body appendages (Parapodia), which larger annelids use for locomotion. As an example, you read about Dinophilidae or Parergodrilidae. In the space between the sand grains, the advantage of locomotion is outweigh by the disadvantage that such body extension might get stuck in the small crevices and holes through which these animals have to crawl. The family Nerillidae, I will present today is unusual in this respect.

Nerillids are small-sized worms with a body length of usually less than 2 mm. They have a head with one or two pairs of eyes, a pair of ventral palps (special sensory organs of polychaetes) and usually one pair of antennae plus an antenna between them on the dorsal side. At the end of the head, they possess a nuchal organ, which is possibly a chemosensory organ and hence it is kind of a nose. The peristomium of annelids bearing the mouth is reduced to the mouth and its lips. The mouth is followed by a cavity, which contains a tongue-like organ that can have stylets. With this tongue, nerillids feed, for example, on biofilms, organic debris, diatoms or bacteria. The head is followed by 7-9 segments bearing small lateral body appendages. As said, this is unusual in these interstitial annelid families. These lateral appendages bear two groups of chaetae and a cirrus between them. The chaetae can be simple capillary ones or compound ones consisting of a stalk and long slender blade-like part, called falcigerous spinigers. The first segment is usually different from the following ones in the number of chaetae on the lateral appendages and the shape of the cirrus. At the end of the body, the pygidium, one can find the anus and a pair of cirri with different length.

It is not known why the parapodia are not as reduced as in the other interstitial annelid families. Like in the other families, locomotion is primarily accomplished by gliding with the aid of cilia on the belly and not by the parapodia. However, besides sand, nerillids are also found in coarse sand, gravel and muddy sediments. These latter habitats could be a possible explanation why the parapodia are not reduced much more. In coarse sand and gravel, the space between the grains and particles are much larger. While not being active in locomotion itself, the parapodia could help steering the animal through these 3-dimensional structures by keeping contact with the grains and particles even in larger spaces. In contrast, a muddy environment is not truly an interstitial habitat any longer. An interstitial habitat is characterized by the possibility that an animal can crawl through it without burrowing (meaning moving a substantial amount of sand particles out of the way). In a muddy environment, the particles are too small to allow for large enough spaces to crawl through. Hence, animals including very small ones have to move particles out the way to crawl through the sediment. The parapodia could possibly assist in this.

Featured image is from Struck T.H. et al. (2014). Phylogeny and evolution of Annelida based on molecular data. In: Wägele J.W., Bartolomaeus T. editors. Deep Metazoan Phylogeny: The backbone of the Tree of Life – New insights from analyses of molecules, morphology, and theory of data analysis. Berlin: De Gruyter. p. 143–160.