Milnesium dornensis, Roszkowska, Daniel Adrian Ciobanu Milena & Kaczmarek, Łukasz, 2015

Roszkowska, Daniel Adrian Ciobanu Milena & Kaczmarek, Łukasz, 2015, Two new tardigrade species from Romania (Eutardigrada: Milnesiidae, Macrobiotidae), with some remarks on secondary sex characters in Milnesium dornensis sp. nov., Zootaxa 3941 (4), pp. 542-564 : 559-560

Milnesium dornensis sp. nov.: secondary sex characters.

Observations of males in tardigrades (especially in terrestrial and freshwater species) are not very frequent and the sex ratio in natural populations of most of the species is ca. 1:25 or less ( Bertolani 1972; Ramazzotti & Maucci 1983; Suzuki 2008, Zawierucha et al. 2015). However, it should also be stressed that sometimes, especially in the spring, males are frequent but completely absent in other seasons ( Bertolani 1972).

In general, differences in external morphology between male and female tardigrades are more often observed in marine Arthrotardigrada and terrestrial Heterotardigrada, but less frequently in Eutardigrada (e.g. Murray 1910; Thulin 1928; Ramazzotti & Maucci 1983; Pollock 1975; Bertolani 1992; Claxton 1996; Rebecchi & Nelson 1998; Suzuki 2008). A smaller body size in males has been observed in some marine and terrestrial tardigrades (e.g. Kristensen 1980; Kristensen & Higgins 1984; Rebecchi & Nelson 1998; Suzuki 2008). Other morphometric differences have been observed in some Eu- and Heterotardigrada e.g. a) modified claws of I pair of legs (terrestrial eutardigrades— e.g. Rebecchi & Nelson 1998), b) modified claws of IV pair of legs (freshwater eutardigrades— e.g. Bertolani 1992), c) absence of spurs on claws (marine heterotardigrades— e.g. Kristensen & Renaud-Mornant 1983), d) more massive and elaborate epicuticle (marine heterotardigrades— e.g. Kristensen & Renaud Mornant 1983), e) larger cephalic clavae (terrestrial heterotardigrades— e.g. Bertolani 1992), f) different shapes of gonopores (terrestrial heterotardigrades— e.g. Claxton 1996), g) more robust lateral gibbosities of IV pair of legs (terrestrial eutardigrades— e.g. Bertolani & Rebecchi 1988, 1993) and h) flattened or not flattened body (marine heterotardigrades— e.g. Kristensen 1980).

In the genus Milnesium the best known and well described secondary sex character is the different claw structure of the first pair of legs. Additionally, a smaller size and more slender body in M. cf. tardigradum males was reported by both Rebecchi & Nelson (1998) and Suzuki (2008), with the later also observing possible sexual behaviour of a male’s interest in a female who had already oviposited in the old cuticle before ecdysis.

The presence of a number of Milnesium dornensis sp. nov. males in our investigation allows us to discuss some morphometric differences between males and females of this new Milnesium species. Rebecchi & Nelson (1998) suggested that the secondary sex characters (e.g. modified claws on legs I of Milnesium species) were invalid in evaluating the sex ratio of Eutardigrada populations, because these characters are present only in mature males. However, as we show below and in Tables 1 and 2 , sexually mature males and females of M. dornensis sp. nov. also differ in other less evident morphometric characters. Although the dorsal sculpture in both sexes in M. dornensis sp. nov. is similar, characters that differ clearly are: 1) males of the new species are more slender (especially between legs III and IV) and smaller than females (female mean length = 679 µm; male mean length = 474 µm); 2) males also have relatively shorter papillae; 3) a narrower buccal tube; 4) longer claws on legs III – IV; and 5) more robust cuticular bars under the claws of the I pair of legs. Additionally, we also observed small differences in the presence of eyes in males and females (50% of males and 73% of females had eyes).

Morphological differences mentioned above are probably connected not only with reproduction (modifications of claws and shape and size of the body), but also with feeding behaviour (buccal tube structure, papillae). What is known, the modified claws on the first pair of legs in males are used to hold the females during copulation ( Rebecchi & Nelson 1998). Relatively longer claws on legs III – IV probably allow males better support during the act of copulation. The slender and smaller body of males may be advantageous because they allow a better and faster reaction to female (or competitor’s) movements (compare with sexual behaviour described by Suzuki (2008)).

As mentioned above, differences between both sexes in M. dornensis sp. nov. were noted in the length of the peribuccal and lateral papillae and the buccal tube width (see Tables 1 and 2 ). Peribuccal and lateral papillae are clearly shorter and the buccal tube is distinctly narrower in males of the new species. All these structures may be connected with feeding behaviour. Peribuccal papillae are sensory organs that probably have a mechanoreceptive and/or chemoreceptive function ( Dewel & Clark 1973; Dewel et al. 1993; Wiederhöft & Greven 1999), as they have been observed immediately retracting in response to a noxious substance being added to the water ( Dewel & Clark 1973); however, the exact function of peribuccal and lateral papillae is still unknown. Better understood, but far from being completely understood, is the function of the buccal tube and its significance in feeding behaviour and the diet of tardigrades. Many different types of food sources have been reported for tardigrades i.e. plant cell fluids, algae, bacteria, protozoa and small invertebrates like nematodes, rotifers and other tardigrades (for the review see Schill et al. 2011). As was shown by Guidetti et al. (2012), the width of the buccal tube has an important role in determining the type of food consumed by tardigrades. This aspect was also discussed by Roszkowska et al. (2015), who showed that different Milnesium species with different buccal tube width had different type of prey in the gut. In M. dornensis sp. nov., females with a relatively wider buccal tube may feed on different types of prey than males with a relatively narrower buccal tube. However at present, these are only interesting possibilities which need confirmation by future behavioural studies.