Milnesium biology and dispersal
Considering that microscopic size is a key requirement for passive LLD,
two factors are likely to limit dispersal abilities of Milnesiumspecies: body size and oviposition strategy. The great majority of
apochelan species represent the largest known tardigrades, with some
individuals exceeding 1 mm in length (e.g. Morek et al. 2020a). Although
tardigrades shrink when they enter the cryptobiotic tun stage,
milnesiids are still on average larger than other tardigrade genera,
which may reduce their dispersal potential. Moreover, Milnesiumdeposit their eggs inside the shed cuticle, so it is the size of the
exuvia, that are only slightly smaller than the adult female, instead of
the egg itself that determine the dispersal potential. Thus, oviposition
strategy may also have a prominent role in the reduction of the
dispersal potential in Milnesium , in contrast to multiple
parachelan genera that freely deposit their eggs (e.g. species of the
superfamily Macrobiotoidea Thulin, 1928). Apart from the size of animals
and their propagules, also cryptobiotic abilities and reproductive mode
have been hypothesised to affect dispersal potential of microscopic
animals (Hörandl, 2009; Fontaneto, 2011; Incagnone et al., 2015).
Specifically, greater cryptobiotic survival (both in terms of time and
suboptimal environmental conditions) and parthenogenesis are predicted
to enhance dispersion. Although Milnesium inceptum , the
only species of the genus in which cryptobiosis has been investigated
(Schill, Steinbrück, & Köhler, 2004; Wang, Grohme, Mali, Schill, &
Frohme, 2014) exhibits high cryptobiotic abilities (and large geographic
distribution for that matter), this cannot be easily extrapolated onto
other species in the genus. Although many milnesiids are found in
xerothermic habitats, which suggests good cryptobiotic abilities,
variation – as in any other phenotypic trait – should be expected.
Therefore, until cryptobiotic capabilities in a number ofMilnesium spp. under a controlled common garden design are
assessed, the effect of this trait on dispersal cannot be evaluated.
Furthermore, in the great majority of investigated populations
represented by at least ten adults, no males have been found (72/89;
81%), which suggest (at least facultative) parthenogenesis. If this
reproductive mode indeed prevails in the genus, the lack of evidence for
a widespread LLD suggests that this trait is not a key factor that
shapes geographic distributions of Milnesium species.