Psammodrilidae is the last of the interstitial annelid families that shall be presented here. They are not known yet from Norway or Sweden but they can be found in Greenland and hence may potential turn up as part of MeioSkag project. In any case, they are quite peculiar interstitial annelids. All families presented so far have very similar body units called segments throughout their body and no different body regions can be observed. However, in psammodrilids three different regions can be clearly differentiated.
The first is an extended head consisting of the prostomium with the brain and peristomium with the mouth. The peristomium is usually differentiated into two components so that they appear as separate segments, but they are one. The latter part of the two is also called a collar and has two muscular diaphragms at each end. These two are connected by muscles with each other. When these connecting muscles are contracted the collar is shortened and the pharynx within it is elated. This causes low pressure in the mouth, which in turn sucks in very in front of it. Hence, it works like a vacuum cleaner and it has been observed how specimens vacuum-clean diatoms from the surface. Hence, one could also call these worms, the vacuum-cleaner worms. In some species, the collar is completely surrounded by epidermal cells generating a distinct honey-comb like pattern around the body. Hence, it the appears of a collar around a neck.
The next body region is called the thorax but has nothing to do with our thorax or the thorax of insects. Thorax is often used for the first anterior body region that is not the head (or a neck). In Psammodrilidae, the thorax can comprise three to six segments each of them bearing a pair of cirri on the back. Within the cirri, one can find a flexible internal supporting chaeta, called acicula. Such supporting chaeta can also be found in other annelid, especially in the major group Errantia.
The last body region is the abdomen. Like for the thorax, it does not imply with similarly named body regions in us or insects. This regions of six to sixteen segments bears no body extensions but each one can a few to many short hooked chaetae, called uncini. Again such uncini can also be found in other larger annelids, but this time predominantly in the other annelid group Sedentaria. Hence, Psammodrilidae shows characteristics of each major annelid group and surprisingly it is still debated where they are to be placed in the annelid tree of life.
Another peculiarity of Psammodrilidae in comparison to the other interstitial annelid families is that they build a thin mucous tube that can be covered by detritus and sand grains. The others move all freely in the space between the sand grains while psammodrilids live at least part times in their tubes. It can speculated if the uncini have the same function in these worms as they have in the larger annelid; that is anchoring the worm in the tube, for example to resist predators. One of the larger psammodrilids, Psammodrilus aedificator, even builds small pyramid-like houses on the surface in which they live and then feeding from the surrounding environment.
Psammodrilus aedificator occurs in the Arctic like Disko Island in Greenland. Once, I went with colleagues to the marine biological station of the University of Copenhagen to sample other annelids. However, we got the opportunity to rent their ship for day to go to a bay further away, known as the type locality of P. aedificator. In preparation to go there, we collected all the information on where they could be found and one record included GPS coordinates where they had been found before. Nowadays, this is considered a requirement but at the time of the record it was not. Hence, we felt well prepared and we got the GPS coordinates and took a sample with a grab. However, when we looked at the sediment sample it seemed very likely that they were not there. The original description had a very detailed description of the habitat in which the species occurred and this included grain sizes. The sample we had taken had these grain sizes in it is lower part but it was covered by a layer of 5 cm of fine silt. As the species was supposed to live on the surface and build the house with the sand grains around it, it seemed to very likely that they would be there. Hence, we decided to slowly row again from the beach into the open sea and take samples at regular intervals. The further we got out the smaller the covering layer of silt became until it disappeared. Later, in the lab it showed that our assumption was correct, the worms appeared in the samples as soon as the covering silt layer disappeared. Hence, as helpful as GPS coordinates are for many purposes a good description of the habitat is also important. Only with such a description it is possible to see if the habitat is still present at the provided coordinates. In the case, a small river had its opening into this bay and a recent washout had probably deposited a layer of silt on top of the original habitat and had not been dispersed yet. Hence, while the coordinates were correct, the dynamic nature of nature almost played a trick on us. Hence, GPS coordinates do not substitute a good habitat description but rather complements it.
Pictures are from Worsaae K, Kerbl A, Domenico MD, Gonzalez BC, Bekkouche N, Martínez A. Interstitial Annelida. Diversity. 2021; 13(2):77. https://doi.org/10.3390/d13020077.
