The next interstitial annelid family, I would like to present to you are the polygordiids. Even though these worms are part of our MeioSkag project investigating meiofaunal species in the Skagerrak, they are not truly meiofauna as their body length ranges from 1-10 cm. Hence, they are quite large. However, due to their exclusive occurrence in very coarse sediments they are considered interstitial as they crawl through the crevices in the sediment without moving the sediment particles. Accordingly, they are part of interstitial families in Annelida and hence of our project.
At the first glance, they do not look like an annelid and more like a large nematode. They move like a nematode and one cannot recognize their segments from the outside. They do not have segment boundaries, parapodia (lateral body appendages) or chaetae. However, they can have up to 200 segments, which can be easily recognized internally. Like the nematomorph Gordius, they like to form a knot by entangle themselves into each other. Instead of just one specimen forming the knot, it is several individuals who aggregate and form the knot; hence, the name Polygordius. In both cases, one is reminded of the Gordian knot, which can only be disentangled by cutting it into pieces.
Polygordiids have a head with two sensory palps, a nuchal organ and a mouth. Then follows the long and smooth body with internal segmentation. At the end, they have a pygidium, which is the most important structure for their taxonomy. It can comprise a ring of different numbers of adhesive glands as well as different numbers of appendages. Despite having a thick cuticle, these worms easily break apart during collecting. Hence, anterior ends can often not be identified using morpholy only as the posterior end is lacking. The worms are usually whitish or pinkish, but they are also very shiny. This is due to the thick, but transparent cuticle, which breaks the light and causes an iridescent effect generating all colors of the rainbow.
Another interesting aspect can also be found in the development of some species. All species have first a trochophora larvae, which can also be found, for example, in many annelids and mollusks. Some species have then the typical development to a segmented annelids with segments being added to the developing larval bodies. However, other species show a catastrophic metamorphosis. Within the trochophore larva a new segmented endolarva is formed. Most of the original larval tissue is then cast away and digested. The new larva continues to develop to the adult stage. Similar catastrophic changes of the body during development can, for example, also be seen in nemerteans or the annelid family Oweniidae.
Finally, given their simple appearance with only internal segmentation, no parapodia and chaetae it was originally thought that these worms show the ancestral condition of Annelida. This assumed an evolutionary trajectory from simple to complex. This is also known as the Archiannelida hypothesis. Nowadays, it is very well established that they are highly derived annelids and that the seemingly simple organization is an adaptation to their environment from a large polychaete ancestor with parapodia, chaetae and external segmentation. In general, it is shown now more and more often that seemingly “simple” organisms are derived from “complex” organisms as an adaptation to a habitat imposing high selection pressures on the shape and size of the organisms and hence are highly adaptive.
