ADDITIONAL COLLECTIONS: From soil (sample No. 163) collected in South Island, New Zealand, May 1990. Conidiophores, conidiogenous cells, and conidia of the fungus may be produced on the dead host underwater but are produced more prolifically when the body of the host is exposed to air.

Conidiophores are simple, more or less cylindrical, and mostly less than 25 µm long by 1.2-2.4 µm wide. They occasionally reach up to 60 µm (Figs. 5-7). They are irregularly cylindrical along their length, frequently with a slightly swollen base and slightly inflated upper cells. Conidiogenous cells (phialides) are globose to subglobose or occasionally ellipsoid, especially when terminal on the conidiophore. The conidiogenous cell often proliferates at its mouth. Phialides measure 3.0-3.5 µm across by 3.0-4.5 µm long. The phialoconidia are triangular and more or less equidistant between the three apices (Figs. 1 and 8). They are 6.3-7.2 by 7.2-9.0 µm when produced on parasitized rotifers. When produced in culture, conidia are slightly larger (up to 11 µm in the longer dimension) and sometimes irregular in shape.

HABITAT: Parasitic in bdelloid rotifers.

Conidiophora simplica, irregulariter cylindrica per longitudinem, cum fondamento tumidulo frequenter at cella superiore subturgida, plerumque minora quam 25 µm (aliquando usque ad 60 µm) longa x 1.2-2.4 µm lata; phialides globosae vel subglobosae, aliquando ellipsoideae, 3.0-4.5 µm longae x 3.0-3.5 µm latae; conidia trigona, cum tribus apicibus plus minus aequidistantibus, 7.2-9.0 µm longa x 6.3-7.2 µm lata.

Conidia of the parasite were added to cultures of rotifers, and the rotifers were harvested and examined at 12-h intervals for several days. Microscopic examination showed that the conidia can lodge at a number of different locations inside the host. Occasionally they lodge in the mouth region and sometimes between the mouth and the mastax but more frequently in the lower gut near the anus. Conidia often lodged in several locations within the same rotifer (Figs. 2, 4, and 11). Germination of the conidium was the same irrespective of the site of lodging. The conidium produced a filiform germination hypha from each of the three corners of the conidium (Fig. 10). This extremely narrow hypha was up to 10 µm long and apparently penetrated through the gut wall into the body tissues of the host. After penetration, this hypha expanded rapidly into a branching system of beaded assimilative hyphae, which colonized the host (Figs. 2, 5, and 9). In later stages of infection, presumably when the host is dead or dying and its body resistance is lowered, the assimilative hyphae changed from the beaded to a filamentous type.
In the initial stages of infection the host very often demonstrated a marked response to the presence of the parasite. Around the tips of the filiform germination hyphae a thick deposit was produced that appeared blackish under phase contrast microscopy (Fig. 9). It is possible that this was a response at the point where the germ hypha broke through the gut wall into the body cavity of the host. The deposit often completely surrounded the tip of the invading germ tube (Fig. 4). Sometimes no further growth took place at this point, which may indicate that the parasitic attack had been overcome. The conidium is triangular, however, and the filiform germ tubes of the fungus penetrated the gut wall concurrently at three different locations. The fungus, therefore, often, broke through the deposit (Fig. 4, arrow) and proliferated as~beaded assimilative hyphae at one or more of these three fronts of attack. Sometimes the host continued to lay down deposits around the assimilative hyphae (Fig. 3, arrow), but this defence strategy was not maintained for long, and the body tissues of the host were then quickly overrun.
Empty, apparently dead hyphae were observed occasionally in some rotifers. These were apparently killed as a result of host resistance to the parasite. Whether or not the host can survive such an attack is not known, but even if the host succumbs, sporulation of the parasite will be prevented.
Harposporium species that attack rotifers fall into two distinct groups (Barron 1986). In H. cocleatum Drechsler, H. angularis Barron, and H. spirosporum Barron, the conidia are angular or helicoid and very sharply pointed at one or both ends. Conidia are designed to lodge in the host tissues during the feeding process. Such spores lodge very quickly after ingestion and never get past the mastax. This is similar to the method of attack described by Aschner and Kohn (1958) for H. anguillulae, which is operative in many Harposporium species attacking nematodes described by Drechsler and others (Barron 1977). Harposporium botuliforme Barron, H. cylindrosporum Barron, and H. trigonosporum, on the other hand, have conidia with rounded ends, and the shape of the conidium is not related to physical lodging. In all three of these species, the conidia lodge at any point between the mouth and the anus. It is possible that in these species a lectin-carbohydrate recognition mechanism (Nordbring-Hertz and Mattiasson 1979) is operative. Therefore, quite different methods of attack seem to be involved within the endoparasites of rotifers described under Harposporium, and the difference in method of attack is supported by differences in the conidiophores and conidiogenous cells between the two groups. Nevertheless, for the moment it seems appropriate to consider both groups in the same genus.

TYPE: Slides in herbarium (OAC 10855), isolated from sandy soil collected in beach area, north of Cairns, Australia, May 1989.