The male reproductive system in all tardigrades seems to be relatively simple. The single testis is located dorsally and the two seminal vesicles open latero-ventrally via two seminal ducts in an oval gonopore papilla (in heterotardigrades) or in the cloaca (in eutardigrades). Penile structures have never been found in any male tardigrade, and it is still uncertain how the sperm transfer occurs. However, it seems that some females have structures that can be inserted into the male so that the female can actively grab the sperm. This method of sperm transfer is very unusual in the animal kingdom.
The female reproductive system in heterotardigrades consists of a single ovary; there is a single oviduct opening in a six-lobed rosette gonopore system anterior to the anus. In eu-tardigrades, the oviduct opens into the cloaca. Two cuticular seminal receptacles are present in many arthrotardigrades, and a single internal receptacle is present in several eutardi-grades. The internal receptacle opens into the hindgut and it lacks a cuticle covering. The seminal receptacles are not homologous in heterotardigrades and eutardigrades. In Milne-sium tardigradum, a ventral fourth Malpighian tubule has been described. However, this structure is actually a single seminal receptacle.
All tardigrades have been considered egg-laying, but there exists a single unpublished record from an arthrotardigrade (Styraconyx sp.) collected in the deep sea that shows a larva coming out of the gonopore. If this is true, other deep-sea tardigrades may be viviparous as well.
Eggshell morphology has great taxonomic importance in Eutardigrada. The egg of Macrobiotus hufelandi was the first to be observed in a scanning electron microscope, and the details of the egg sculpture have fascinated scientists ever since. All echiniscids form cysts before they lay eggs. When a female hatches from the cyst, she lays the unsculptured eggs in the old exuvium.
Although information on the mating behavior in Arthro-tardigrada is extremely scarce, it has been observed that males and females in the species Parastygarctus sterreri mate venter to venter when the male ejects the sperms into the external seminal receptacles of the female.
In eutardigrades, the male clings with its first leg pair to the anterior part of the female. The claws on the first leg pair of the males may be strongly modified, as in Milnesium tardi-gradum. Internal fertilization is common in several eutardi-grade species, and these species always lay free eggs. However, some species lay their eggs in the old exuvium right after molting, so that the male can afterward spread the sperm into the exuvium.
Most tardigrades are dioecious, but hermaphroditism also occurs. Hermaphroditism is especially common in many genera of limno-terrestrial eutardigrades, whereas it has only been recorded in a single arthrotardigrade species, Orzeliscus sp. Besides usual sexual reproduction, many species are capable of reproducing by parthenogenesis (reproduction without male fertilization). In some species, males have never been observed and all reproduction is solely by parthenogenesis. However, it has been shown that some apparently partheno-genetic species sometimes have populations with males. It is not known how the production of males is triggered in these populations.
It seems likely that the evolution of parthenogenesis in tardigrades is linked to the evolution of cryptobiosis. This postulate is supported by the strong correlation between the presence of parthenogenetic and cryptobiotic capacities. The parthenogenetic capacities in tardigrades have evolved independently in the echiniscid heterotardigrades and the parachelate eutardigrades.
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