Protodriloidae is a family of only two species in the genus Protodriloides. Together, with the families Saccocirridae and Protodrilidae, they are part of the order Protodrilida. We will learn more about the other two families in the next two months. Hence, look forward to it.

Both Protodriloides species look very similar, but each has also distinctive features, which makes them easy to separate. The species can a bit more than 1 cm long and they have a triangular head with two anterior body extensions, called palps. These palps originate directly from the head and are solid in contrast to the other two families in this order. This also means that they cannot shed their palps like for example species of Protodrilidae like to do. The head is followed by numerous segments with adhesive glands. In the species Protodriloides chaetifer, these segments also bear a pair of short, sigmoid-like chaetae; hence, the name and the difference to the other species Protodriloides symbioticus. At the end, they have the pygidium, which also has adhesive glands. These large numbers of adhesive glands allow the worms to easily stick to surfaces like sand grains or the wall of a pipette. Many of them have been lost in pipetting them from one dish to another. Protodriloides symbioticus has additionally greenish inclusions along the whole body. Originally, they were thought to be endosymbionts and hence the name. Recent research showed that they are “just” greenish glands with unknown function.

Besides the differences above, the two species also prefer slightly different habitats. Both like to live in the space between sand grains of coarse to medium coarse sandy sediments. Protodriloides chaetifer likes a beach life and can be found at slope of sandy beaches with medium to high exposure to the sea. In contrast, Protodriloides symbioticus is tuned in on a more relaxing life in intertidal flats, with less wave action and more cover by the sea throughout the day. This strive for more activity of Protodriloides chaetifer can also be seen by that they are found around the whole globe with records from beaches in Europe, North America, Southern Africa and Eastern Asia, a real globetrotter. However, there is a string indication that these might actually be at least four different species that just look identical, so called cryptic species. Again in contrast, Protodriloides symbioticus seemingly also likes a more quite life here. So far, they have only been recorded from the Southern North Sea.

Why are they important? They are an examples to a different evolutionary route to colonize the space between the sand grains. One, progenetic evolution, we already presented when showcasing the family Dinophilidae, which we also work on in the MeioSkag project. In this case, it is rather a step-wise reduction in body size allowing the colonization of continuously smaller spaces between the sand grains. Within the Protodrilida, the hypothetical succession is suggested to follow a step-wise reduction in size from the large-sized species of Saccocirridae via Protodriloidae to the smaller ones in Protodrilidae. Hence, Protodriloidae allow a glimpse on how such an ancestor might have looked alike even though they are not the ancestors themselves.