The Life History of Merocystis Kathae in the Whelk, Buccinum undatum
Authors: Patten Ruth 
Published: June 2009

1. The life history of Merocystis kathae shows that development follows a seasonal course. The youngest stages may appear in the kidney any time between March and June. Growth proceeds and sexual differentiation takes place but mature gametes are not found until November, when the first stages of sporogony also make their appearance. Mature spores containing sporozoites are formed in January and become increasingly common up to May. Degenerating stages of spores are common in summer.

2. The smallest parasites within the cells of the kidney are about 10μ. Growth occurs in the intracellular condition. The karyosome is complex, and contains a chromatinic body; in older parasites no chromatin reaction is given by the nucleus outside the karyosome.

3. Sexual differentiation is not perceptible until growth is almost complete.

4. In the female the intrakaryosomic body emerges, though a large karyosomic remnant remains until after fertilisation. Very fine granules of chromatin reappear in the nucleus, and form a fine network, which incorporates the intrakaryosomic body, and later forms the characteristic prefertilisation ‘cobweb’ nucleus. This network contracts in the mature macrogamete. Meantime the parasite has become free of its host cell, and the cytoplasm stains more intensely.

5. In the male, the elongated karyosome breaks down, generally completely, leaving the centrally placed chromatin body surrounded by a ring (in sectional view), of deeply stained material containing granules of chromatin, into which the central body passes as in the female. The male is released from the host cell, and, as in the female an alteration of the cytoplasmic staining occurs.

6. The nucleus, in the male parasite, moves to the surface, and undergoes repeated divisions until approximately 32 nuclei are formed. Cleavage of the cytoplasm brings about the formation of uninucleate cytomeres; within each of these are produced many nuclei, which lie near the surface, and finally become the microgametes.

7. After the macrogamete is fertilised, a thick membrane is formed outside the zygote. The nucleus assumes the characteristic spindle shape and subsequently divides.

8. Nuclear divisions proceed, until finally a great number of cone-shaped nuclei are found on the surface. This becomes folded and the nuclei sink in, and each is surrounded by a portion of cytoplasm to form a sporoblast. A single nuclear division then occurs, and the two nuclei elongate to form sporozoites each with a small cytoplasmic body.

9. Nuclear divisions are mitotic, but the longitudinal splitting of the chromosomes occurs without contraction. It is probable that ‘amitotic’ appearances are due to the incomplete incorporation of these long chromosomes at the telophase poles, and the multipolar divisions to the subsequent division of such ‘amitotic’ resting nuclei. Six chromosomes can be seen at the anaphase poles, but each is looped back so that it is V or J shaped. This chromosome number is characteristic of the divisions of both microgametogenesis and sporogony.

10. There is good evidence that, as in other Coccidia, reduction division occurs in the zygote, and that this is therefore the only diploid phase.