Saprolegnia is an oomycete, or water mold, often mistaken for a fungus due to its appearance and growth habits. This organism is widely known for causing “cotton wool disease,” a common ailment affecting freshwater fish, their eggs, and amphibians. It presents as white or grayish cotton-like tufts on the surface of its host. Microscopic identification is definitive because visual signs can be misleading. Accurate identification is the first step to manage its spread in aquaculture or aquarium settings.
Preparing a Sample for Viewing
The first step is to properly collect a sample. Using sterile forceps, remove a small portion of the cottony growth from an infected fish or culture plate, ensuring you obtain a piece of the mycelium, the vegetative part of the organism.
To create a wet mount, place a drop of water onto a clean microscope slide and transfer the sample into it. For enhanced contrast, a stain such as lactophenol cotton blue can be used instead of water to make structures more visible. Gently lower a coverslip at an angle to prevent air bubbles.
Identifying Asexual Structures
Under the microscope, the most prominent features of Saprolegnia are its asexual reproductive structures. You will observe long, transparent, tube-like filaments called hyphae. A key characteristic of these hyphae is that they are generally non-septate, meaning they lack the regular cross-walls found in many true fungi. This gives the filaments a continuous, uninterrupted appearance. These hyphae branch out, forming the visible mycelial mat.
At the tips of these hyphae, you can find the structures responsible for reproduction: the zoosporangia. These are typically elongated or club-shaped and are visibly distinct from the vegetative hyphae they grow from. Inside these zoosporangia, numerous tiny, motile zoospores are produced. When mature, the sporangium releases these zoospores, which can be observed as a swarm of swimming spores if the timing is right.
Observing Sexual Reproduction Features
While less commonly seen than the asexual structures, the features of sexual reproduction are definitive for identification. This process occurs under specific environmental conditions and involves the formation of male and female reproductive organs. The female structure, called an oogonium, is typically spherical and contains one or more eggs, or oospheres.
Fertilization is accomplished by the male antheridia, which are smaller, often club-shaped structures that attach to the surface of the oogonium. The antheridium penetrates the oogonium to deliver the male nuclei. This fertilization process leads to the development of thick-walled resting spores known as oospores inside the oogonium. Observing these spherical oogonia with attached antheridia or the resulting oospores within them provides strong evidence for the presence of Saprolegnia.
Distinguishing Saprolegnia from True Fungi Microscopically
It is common to confuse Saprolegnia with true fungi, which can appear similar macroscopically. The most reliable microscopic distinction is the structure of the hyphae. As noted, Saprolegnia hyphae are non-septate, while the hyphae of common molds are septate, meaning they are regularly divided by internal walls.
Another fundamental difference, though not visible without specialized techniques, is the genetic makeup of the hyphae. The vegetative hyphae of Saprolegnia are diploid, containing two sets of chromosomes (2n). Conversely, the hyphae of most true fungi are in a haploid state, with only one set of chromosomes (n). This biological distinction underpins the many structural differences seen under the microscope.