Sommerstorffia Spinosa Arnaudow

SUMMARY

Sommerstorffia spinosa, a predaceous species of the family Saprolegniaceae which captures and devours rotifers, was collected in August, 1950, in soil from an alpine tundra stream at an altitude of 6500 feet in the Talkeetna Mountains in Alaska. This is the third published report of its occurrence in nature. It consists of 1 to 6 or rarely more, straight or slightly curved, 60–200 μ long by 5.6–10.4 μ broad, comparatively rigid, non-septate, tubular branches which are terminated by a specialized, narrow tapering peg or blunt spine filled with homogeneous refractive substance. These pegs presumably secrete an adhesive substance when taken into the mouth of the rotifer, which is then unable to free itself.

The fungus invades and absorbs the internal organs of its host, and usually forms an endobiotic sporangium whose content undergoes cleavage into zoospore initials. The latter emerge singly in succession without flagellar action from long exit tubes and encyst nearby in clusters, and later germinate to form elongately reniform, 6–6.9 × 9–10.4 μ, laterally biflagellate zoospores. These eventually come to rest on the underside of a suitable floating substratum and develop into flask-shaped, 6.8–12 × 13.6–20.4 μ sporelings which adhere strongly to the substratum and hang down in the water. Their necks are filled with a refractive substance and are as capable of capturing rotifers as the predaceous pegs of the mature thallus.

Sporelings which have captured rotifers usually develop into a dichotomously branched epibiotic thallus and an endobiotic sporangium. Occasionally, they give rise to a completely epibiotic thallus or only an endobiotic sporangium. Oospore development was not observed in the Alaskan material.

Species of Monostyla, Distyla, Colurus were readily captured in large numbers by S. spinosa, but members of Euchlanis, Habrotrocha and Philodina were never found captive. Entosiphon ovatum, a protozoan, was frequently caught by the sporelings and predaceous pegs of the mature thallus.

Thalli of what may prove to be another species of Sommerstorffia were found in the Alaskan material. These thalli are broadly or narrowly fusiform, stalked, vary from 15 to 24 μ in overall length by 5 to 10 μ in greatest diameter and are subtended by a small oval or circular, 2.5–3.6 μ diam., flattened foot or holdfast, which is strongly adherent to or embedded in the substratum. They lack structurally specialized organs of prey, and apparently are capable of capturing rotifers by contact with any part of the body. They give rise to an endobiotic sporangium which forms zoospore initials, cystospores, and zoospores as in S. spinosa. No epibiotic development of thalli was observed.