The vegetative hyphae are hyaline, septate, and more or less uniform in width, varying from 1.8 to 3.0 µm. Conidiogenous cells arise directly from the vegetative hyphae and are short, hyaline, simple and 8.5-12 (20) µm tall by 2-2.5 µm wide (Figs. 4 and 10). They taper to a narrow truncate apex about 1.2 µm across. In most cases each conidium is solitary and borne at the apex of the conidiogenous cell. Occasionally a second conidium is borne on a sympodially extending conidiogenous cell. Conidia are of several morphological types. Many of the conidia are scolecospores and 50-85 (95) µm long by 3.5-5.5 µm wide. They are hyaline, multiseptate, and more or less cylindrical but taper towards the apex and the narrowly truncate base (Figs. 1, 6, and 9). The most striking type of conidium produced by D. heterospora originates in the same manner as the scolecospores. After producing a short stalk, however, this conidium branches twice dichotomously at a wide angle to form four secondary branches, with the lower branches often reflexing down and elongating past the apex of the conidiogenous cell (Figs. 2, 7, 8, and 12). The conidiogenous cell, the stalk, and the four arms are all in the same plane. The stalk of the conidium is 14-25 µm long and up to 4.5 µm at the widest point and tapers below to a narrow attachment point with the conidiogenous cell. In overall dimensions the conidia are 30-45 µm long and 14-32 µm wide across the arms. In most cases the two lower arms are wider apart than the two upper arms, but occasionally this relationship is reversed. In some conidia one or two of the arms abort before full development.
In a number of cases the second dichotomy is omitted, but the conidium continues development. This results in a triradiate conidium (Figs. 3 and 11), but these are not as common as the scolecospores or the five-armed spores.

HABITAT: Parasitic on the eggs of bdelloid rotifers and nematodes.

Hyphae crescentes, hyalinae, septatae, aequabiles plus minus in latitudinem, 1.8 usque ad 3.0 µm in latitudinem; conidiophora brevia, hyalina, simplicia, 8.5-12 (20) µm longa, attenuata usque ad apicem angustum et truncatum circa 1.2 µm in latitudinem; conidium singulatim in apice conidiophori, constans ex caudicula 14-25 µm longum dein bicorne dichotome in angulo lato formans quattuor ramos secundarios; conidia matura 30-45 µm longa in totum et 14-32 µm lata; scolecosporae 50-85 (95) µm longae x 3.5-5.5 µm latae, hyalinae, septatae, cylindricae plus minus sed lineamento irregulari et terminalibus attenuatis.

The fungus was first observed as fine hyphae lacing sparsely over the agar or through the water phase of rotifer plates seeded with soil. Occasionally a parasitized rotifer or rotifer egg was found associated with the hyphae. The fungus was never observed to sporulate underwater. If the aqueous material was drained off of the water agar plate on which the rotifer had been captured, then occasionally conidia were produced over the drying surface of the agar. If the drained plates were placed under long wave ultraviolet light (black light), conidial production was improved. In those plates that were not exposed to black light, conidiation was extremely poor, and in plates where a thin film of water covered the surface, conidiation was not observed even after several weeks of incubation.
The fungus had the ability to parasitize rotifer eggs. Eggs are often laid close to or directly adjacent to the hyphae of the parasite. In such cases a short branch produces a terminal, swollen, appressorium-like structure that attached to the shell of the egg (Figs. 5 and 13). After penetration a subcuticular vesicle was produced from which assimilative hyphae develop to digest the contents of the egg.
Occasionally mature rotifers were found attached to the hyphal system. It was not clear how such rotifers had been captured. It seemed likely that parasitized mature rotifers were either senescent or dead at the time of penetration. It is possible, however, that the fungus can attack immobile, encysted rotifers in the same manner as eggs. Experiments using nematode eggs instead of rotifer eggs showed that these were attacked and consumed in the same way as rotifer eggs. The parasite was not a vigorous one, and at, any moment only a very small percentage of the eggs available were in fact parasitized. Drechsler demonstrated that T. quadridens attacked testaceous rhizopods and T. aphanopaga attacked small nematodes, so the host range of this group of species may be wider than is usually reported for such parasites.
The production of several morphologically different spore types of D. heterospora is important and significant. Phragmospores have been reported previously for Dwayaangam yakuensis by Matsushima (1989). Phragmospores were also reported by Butterfield (1973) in her report on morphological variation in Dicranidion fragile Harkness in culture. Dicranidion is apparently closely related to Dwayaangam and differs principally in that there is only one dichotomy, rather than two, from the primary stalk. The similarities between Dwayaangam and Dicranidion, however, are more striking if we examine the illustrations of Ando and Tubaki (1984a) for their Dicranidion fissile, discovered in rainwater. In this species many of the conidia bifurcate a second time to give a Dwayaangam-like spore. The main difference is that the first dichotomy is not at a wide angle, and the arms are not, therefore, reflexed backwards towards the stalk of the conidium as they are in Dwayaangam. This, however, is a difference of degree rather than kind. Also for Dicranidion fissile, Ando and Tubaki (1984b) described and illustrated phragmospores similar to those described by Matsushima (1989) for Dwayaangam yakuensis and described herein for Dwayaangam heterospora. On the basis of the morphology of its conidia, Dicranidion fissile is somewhat intermediate between Dicranidion and Dwayaangam and could have been placed in either genus.
Dwayaangam heterospora possesses a whole range of spore types from typical Dactylella-like phragmospores and scolecospores, through triradiate Dicranidion-like spores to five-armed conidia typical of the genus. Further studies may reveal other species more or less intermediate between Dicranidion and Dwayaangam, and the validity of separating the two groups into separate genera may become questionable.
Based on their biology and morphology, this group of fungi seems most closely related to, and possibly derived from, species such as Dactylella spermatophaga and Trinacrium subtile discovered by Drechsler (1938) and reported by him to parasitize the oospores of Oomycota.

TYPE: Diagrams in text. Isolated from organic debris collected in rain forest, Te Anau, South Island, New Zealand, May 1989.