Olavius algarvensis is a species of gutless oligochaete worm in the family Tubificidae which depends on symbiotic bacteria for its nutrition.

Instead, the body cavity contains the ventral nerve cord (inside a muscular sheath) and two blood vessels which are surrounded by a "fluffy" layer of chloragocytic cells.

[2] The symbiotic bacteria are located between the cuticle and epidermis, and also in vacuoles within epidermal cells, which often show signs of lysis.

The bacteria are absent from the anterior part of the worm and the pygidium, but are found from segment VII or VIII onwards.

There are five different species of bacterial symbionts in O. algarvensis, which are located under the cuticle of the worm: two sulfide-oxidizing Gammaproteobacteria, two sulfate-reducing Deltaproteobacteria, and one spirochaete.