| Concerning the reproduction of rotifers there are two groups: While the Bdelloidea or Digononta (which have two germovitellaria) reproduce exlusively asexually by obligate parthenogenesis the monogonont rotifers may reproduce either by parthenogenesis or sexually. |
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| Digonont rotifers (Bdelloids) |
| Bdelloidea / Digononta reproduce always asexually, either viviparously or oviparously. |
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| Vivipary occurs mainly in the genera Dissotrocha and Rotaria (the above image shows a specimen of Dissotrocha macrostyla, with two trophi: the trophi of the daughter is marked by the an arrow, the trophi of the mother is marked by arrowhead). |
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| Ovipary occurs mainly in the family Adinetids, Habrotrochids, Philodinavids, and the genera Macrotrachela, Mniobia, Philodina, Pleuretra. The image shows Adineta editae with egg. |
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| Monogonont rotifers |
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| The Monogononta therefore have different types of eggs. The eggs that are produced while reproducing parthenogenetically are diploid (red: 2n) and are either carried by the female, for example Anuraeopsis, Brachionus, deposited near the sheath (eg. Collotheca; Ptygura) or they develop inside the mother until the daughter is born (eg. Asplanchna; Rhinoglena). These eggs are called amictic eggs, becaues they develop without mixis (fertilization by a haploid sperm cell). Parthenogenetic reproduction enables the population to increase rapidly because there is no need for finding a sexual partner, which is always an advantage if the distance between specimen is large, eg in lakes or ponds. As a consequence the rotifer population can more effectively exploit food resources, for example planktonic algae which start growing with increasing day length / light in spring. |
| The other type of reproducing is sexually by mictic eggs. When a so-called "mictic stimulus" ("Mixis stimulus" in German) occurs in the environment, which could be for example changes in the chemical composition of the surrounding water or the temperature, the females may produce haploid eggs (green: 1n). If these eggs are not fertilized they develop into haploid males (blue) which are usually pretty much smaller (there are exceptions, for instance Asplanchna) and look different from the females (sexual dimorphism). The males can now fertilize the mictic eggs of mictic females (blue+green: 1n+1n). These fertilized eggs (brown) are called "resting eggs". The resting eggs have a mechanichally and also chemically very durable shell which is species-specific and enables the egg to survive bad environmental conditions like ice in ponds or dry seasons. |
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| Cephalodella sp., mating behavior. The copulation organ of the male is visible in the inside of the female (arrowhead). |
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| Cephalodella sp, male specimen. |
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| Asplanchna priodonta, male specimen. In Asplanchna, the males are not much smaller than the females and look pretty similar (except for the trophi and sexual organs). |
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| Asplanchna priodonta, male; genital apparatus. |
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| Asplanchna priodonta, sperms / spermatocytes. |
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| Brachionus sp., male |
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| Brachionus sp., male; reproductive organs: F: foot, which is ventral of the copulation organ (KO) and the penis (P). Sp: sperms; Z: toes. |
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