Saturday, January 14, 2017

The Case of the Case-carrying Worm

Dero (Aulophorus) vaga - 1mm - Linda Bower
I was following correspondence from Linda Bower asking Chris Barnhart for identification help after filming an eccentric worm that appeared to be dragging around a decorated case. Chris recognized that the case was partly made of Bryozoan statoblasts (the oval brown objects with a pale perimeter). Chris Barkau, Graduate Research Assistant at Southern Illinois University Carbondale was able to identify it as Tubificida: Naididae: Naidinae: Dero (Aulophorus) vaga.  I asked Linda to describe her find.

Case-carrying Worms dwell in ponds with Duckweed, but are often missed by traditional collecting methods. They are tiny – less than 1mm. It was difficult to find recent* detailed information on Dero vaga, also called Aulophorus vaga. We know that it forms protective tubes by means of a viscid secretion from their bodies. You can watch this video of the worm sliding in and out of its case as it searches for food. It is a fascinating dance you won't see elsewhere, and it is free!
Cut and whole Bryozoan colonies - Click to enlarge - MDC
The Case-carrying Worm is interesting enough, but combined with Bryozoan statoblasts? Wow! Bryozoans (aka Moss Animals) are really animals, but given their appearance, that is hard to believe. Bryozoans are a major animal group, having nearly 4,000 known species and only a few dozen of those live in freshwater habitats. They may grow on any submerged object, such as rocks, roots, and branches. They feed on protozoans, bacteria, and organic matter from the water. They are colonial, living in gelatinous blobs.

Most freshwater species produce resistant bodies called statoblasts that form in response to adverse environmental conditions and provide a means of overwintering. As they grow the statoblasts produce bi-valve shells made of chitin, the same stuff that makes the exoskeleton of arthropods (think insects and crayfish). More detailed information on Bryozoans is at this link.

I have filmed several Case-carrying Worms since this first find and here are three additional videos. Do not resist your temptation – follow these links:
Editor's note: You can bail out now or follow along for the details of the complicated life of D. vaga (or if you prefer A. vaga). These tiny (1mm) worms can be found floating within a mass of duckweed or clustered in the algae of the pond bottom, moving up or down based on the availability of food.  Like all other Oligochaeta (worms) D. vaga is a hermaphrodite. It is capable of sexual reproduction or fission. This was described in an 1899 paper, The Natural History and Morphology of Dera vaga.

The period of sexual reproduction occurs during the first two weeks of July, when the body cavity posterior to the clitellum is crowded with eggs.  Asexual reproduction by fission takes place throughout the year, but most rapidly during warm weather, when it may occur as often as three times a week. Three fission zones have been observed in one individual at the same time.
Click to enlarge

As the animal grows in length, the case which it inhabits is extended, and after fission the two daughter worms divide it by placing their heads together at its middle and forcibly breaking it, each worm then swimming away with one-half of the old case. The fission zone is formed near the middle of some segment, usually back of XVII and in front of XXII. The new head and tail are almost completely formed before separation takes place. The number of somites in the new head is constant, being five, while twelve to sixteen segments are visible in the tail before a second fission begins.
Worms divided by cutting regenerate the missing part, though only enough segments are regenerated at the anterior end to complete the cephalized portion, i.e., the first five. Thus if two are removed but two regenerate, while if seven are taken away only five new segments are formed. At least three or four segments in addition to the five in the cephalic region are necessary for the regeneration of the tail. 
There are detailed descriptions of this Oligochaete available when searching name variations.