Saccocirridae is considered a meiofaunal or interstitial annelid family as all species are found in the space between the sand grains. While a few species might be quite small with 3 mm, several are quite large with up to 3 cm in length. Additionally, they can be quite broad with a body width of up to 0.5 mm. The width is the size of the body length of many meiofaunal species. Hence, why are they still called meiofauna or interstitial species?

The reason for this is that they live in coarse to very coarse sediments. Accordingly, the space between the sand grains or better said shell pieces and pebbles can be quite large and the species can move through these without large displacements of the sediment. The latter would be characteristic for a burrowing organism, while the former is the characteristic for interstitial ones. In consequence, Saccocirridae fulfill the criteria for interstitial species, but strictly spoken not for meiofauna. Meiofauna are usually species passing through a sieved with a mesh size of 1 mm, but not one with 45 µm. However, as interstitial and meiofaunal are often used interchangeable, especially in Annelida, these species are also called meiofaunal.

Saccocirridae possess a relative small head with comparatively large palps. The palps are clearly set apart from the body and possess a canal. Hence, they are clearly different from Protodriloidae, which we presented last month. Additionally, the head a pair of eyes as well as numerous unpigmented photoreceptor cells. Hence, strictly spoken the head has not just two eyes but numerous ones. The small head is followed by up to 200 segments. Another contrast to many other interstitial annelid families like Protodriloidae or Dinophilidae is that the species have small parapodia with simple chaetae that can be stout or forked. This is something they have in common with Nerillidae, which are substantially smaller. The last segments are often without parapodia and the animals end in two lobes. They have several adhesive glands at these lobes as well as long their body allowing them to stick to surfaces like most other interstitial species. Hence, despite being quite uncommon in their size, they have many similarities with much smaller interstitial species nonetheless.

With respect to our MeioSkag project, it would be surprising to find this family in Norwegian waters as they far have only been found in tropical and warm waters, but not in boreal and polar ones. On the other hand, several years ago we found some on the North Sea island on Sylt in one year, but unfortunately not in the following years and we have no good record of this first found. Hence, it still remains to be seen if these species also exhibit an expansion towards higher latitudes so many other species show due to the climate change.