Rhizopus arrhizus Fischer   

Kingdom: Fungi
Phylum: Zygomycota
Subphylum: Mucoromycotina
Order: Mucorales

British distribution: Terrestrial, probably widespread
World distribution: Cosmopolitan.

Pictures of spoilage moulds on this site tend to have their origin in the author's somewhat relaxed attitude towards keeping his refrigerator tidy. This one is rather different. Strawberries bought in for a practical class resolutely refused to go mouldy and our technician gave up and put some in an incubator. Result: semi-stewed strawberries covered with an abundant, white mycelium. Rather than the Botrytis cinerea expected, the fungus was evidently a zygomycete of the order Mucorales, as indicated by the broad, coenocytic hyphae.

The day after the practical (naturally), the fungus produced sporangia and could be identified as the thermophilic species, Rhizopus arrhizus.

Under the medium power of the microscope (original photograph at × 125), the typical Rhizopus structure is apparent, with often multiple sporangiophores arising from a single point on the mycelium. Despite the vigour of the mycelium, the sporangiophores in this species are quite short, less than 1 mm.
Here the intact sporangium is seen to the right, but the sporangial wall has already dissolved and the mass of sporangiospores is beginning to lift away. The left-hand sporangium is already losing its spores, exposing the almost globose columella.

The scar left by the sporangial wall is visible towards the base of the columella.

At higher power (original photograph at × 500), the sculptured surface of the columella can be seen. The creases can be regarded as simply an artifact of the slide mount, resulting from the pressure of the coverslip.

The dark spores can be seen more easily. In this species they are ovoid to more or less globose, 5 – 9 µm in diameter.

Rhizopus is a small genus, with only ten species treated by Zycha et al. (1969). Amongst related species are:
R. stolonifer (= R. nigricans), a ubiquitous species that can rapidly colonise all kinds of substrates; sporangiophores 1.5 – 3 mm, spores typically ovoid, 10 – 15 × 7 µm, sometimes larger;
R. oligosporus, similar but with spores globose to ovoid, 7 – 12 µm. Some strains of this species produce chlamydospores in culture; these can be confused with zygospores by the unwary.

Photographed 1998.

Zycha, H., Siepmann, R., & Linnemann, G., (1969). Mucorales, eine Beschreibung aller Gattungen und Arten dieser Pilzgruppe. J.Cramer, Lehre.

© A.J. Silverside
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