Two different forms of cryptic species-complexes in mites of the Histiostomatidae (Astigmata) from bank mud and bark beetle-galleries and their significance for applied biodiversity research
Phoretic carriers of the species Histiostoma maritimum, H. litorale, H. palustre and H. sp.
H. palustre was found in the form of adults in the mud substrate in the Berlin gravelpit area and was then cultivated. In the same substrate, there were individuals of a beetle species of the Hydrophilidae, which at that time was assigned to the genus Cercyon and was assumed as being the putative host for H. palustre (Wirth, 2003). The species H. sp. was found in different stages of development in the mud substrate, a long-term culture did not succeed, only a few deutonymphs were formed. There are therefore no permanent preparations of deutonymphs. There is also a lack of clear morphological studies of the deutonymphs. The species is mentioned here because I originally confused its deutonymphs with those of H. maritimum. These deutonymphs were only studied under superficial light microscopic conditions. They resembIed H. maritimum in outline, proportions, and the shape of the palposoma. I carried out videographic studies on the movement of the gnathosoma in adults. These are so far published as online video film only. A scientific peer-reviewed publication on this species does not yet exist, and the phoretic transporters of the species are still unknown. The species is therefore only mentioned in the margin in my later ecological explanations.
According to Scheucher (1957), the phoretic host list of H. maritimum and H. litorale is long. Overlapping of the hosts with regard to the occurrence of the two mite species is common. According to Scheucher ’s host list, H. litorale is associated with 39 beetle species of the Carabidae of genera Acupalpus, Agonum, Amara, Anisodactylus, Bembidion, Carabus, Elaphrus, Epaphius, Europhilus, Lorocera, Patrobus, Platynus, Pterostichus, Stenolophus. H. maritimum shares with H. litorale 23 of those hosts. Additionally Scheucher reported it from Tachinus sp. (Staphylinidae), Hydrophilus caraboides (Hydrophilidae) and Heterocerus marginatus (Heteroceridae).
I isolated H. litorale in Berlin as predominantly adult stages from the sapropel mud. Therefore the phoretic hosts belonging to it are not known to me. Only a few mites have been found on Bembidion, but I assigned them to H. maritimum through breeding. Otherwise, the only available host of the Carabidae, which according to Scheucher (1957) carries H. litorale and H. maritimum, was Elaphrus cupreus with often numerous deutonymphs. I always assigned them to H. maritimum in Wirth (2004). However, it cannot be ruled out that H. litorale deutonymphs may have been underneath and were overlooked due to their morphological similarity.
H. maritimum could in those earlier times regularly be found in the examined Berlin wetlands, always in the form of its deutonymphs. There they were found in large numbers on E. cupreus, on Bembidion sp. in smaller numbers and in beetles of the genus Heterocerus in very large numbers. However, these were exclusively Heterocerus fenestratus (especially many mites) and Heterocerus fusculus (less mites). Heterocerus marginatus was not found in the area. The two first named Heterocerus species were completely unknown as phoretic hosts up to this point in time (Wirth, 2004).
Since it cannot be ruled out that H. palustre with not exactly known hosts and H. sp. with completely unknown hosts occur even together with H. litorale and H. maritimum, the risk of confusing the species on the basis of their deutonymphs would be very high. However, breeding would lead to adult stages that are distinctly different and can be easily assigned to the individual species.
Since it always seems suspicious if the same species is using an almost infinite number of hosts for distribution, it makes sense to record the developmental stages of a species morphologically as detailed as possible in order to have comparison options even if theoretically the suspicion arises that, for example, a cryptic species complex might be actually hidden behind H. maritimum, despite of its very distinct adult morphology. I in Wirth (2004) referred to attempts at crossbreeding between H. maritimum strains from Heterocerus and those from Elaphrus cupreus. These, however, not representative extensive experiments suggested that it was always the same mite species. Crossing attempts on arrhenotokous histiostomatid species are problematic, because males may hatch from unfertilized eggs and can initiate cultures by mating with their mother. The experiments in 2004 showed, however, that isolated female tritonymphs did not lay eggs at all in the adult stage. This would have to be confirmed again on a representative basis.
On the basis of the H. maritimum species, it is shown below that high-resolution optics (here SEM) reveal features that were not yet known from previous light microscopic drawings or photos. On the basis of such detailed representations, different H. maritimum strains can be compared better with each other in the future.
The ecological conditions of the find situation, which have changed over the years, are also discussed in somewhat more detail, since details on ecology are generally also suitable for precisely characterizing a species.
The former gravel pit site „Im Jagen 86“ changed its mosaic of habitat types within 2002 and 2021 remarkably seemingly mostly due to natural conditions. From 2000 to at least 2006 three ponds (a bigger one and two smaller ones) laid close to each other with grassland areas in between and an adjacent sand dune (Fig. 3 A: sand dune in 2018).
Hosts with H. maritimum (Heterocerus fenestratus, rarer H. fusculus, and Elaphrus cupreus) were regularly found around the small ponds (around 30 and 50 m in diameter including shore areas) . They preferred contiguous areas of sapropel with a flat surface of at least two square meters being exposed to sun warmth. H. palustre and H. litorale appeared under similar conditions in the same habitats.
Within years the small ponds dried out and became replaced by dry meadows (Fig. 3 A, B). The big pond (ca. 70-80 m in diameter including shore areas) persisted, but with a shore, mostly being densily overgrown by vegetation, such as reeds. Only a small open sapropel area exists up today (s. a. = sapropel area, Fig. 3 B). None of the above mentioned mite species was found in random samplings around the big pond, after the small ones disappeared. Only Histiostoma n. sp. could be isolated from that small sapropel area of the remaining pond, exactly from a mud sample collected close to a moss cover in early spring 2019.
The life strategies of H. litorale and H. n. sp. are unknown. H. palustre reared well on decomposing potato pieces (Wirth, 2003). The necromenic strategy of H. maritimum was never mentioned before in a peer review journal, but it was closer described in my phd thesis (Wirth, 2004), according to which seasonal differences in occupation frequencies on H. fenestratus and H. fusculus were found with low mite numbers in May to June and a 100 percent frequency for both beetles in August (studied in 2000), when high numbers of freshly hatched young beetles appeared close to each other. H. fenestratus was found carrying significantly more mite deutonymphs than H. fusculus.
Deutonymphs of H. litorale, H. palustre and H. maritimum resemble each other remarkably regarding total shape (Figs. 2 A,C,E), pattern of ventral apodemes, dorsal and leg station and shape of the palposoma (drawings and descriptions in Scheucher 1957, Wirth 2003). A conspicuous common feature is the narrow and protruding palposoma (Fig. 4 A, deutonymph of H. palustre, SEM). The outline and proportions of the H. litorale deutonymph differs according to Scheucher (1957) from the other two species, but differences appear being gradually and depending on the preparation. The deutonymph of H. n. sp. is not well studied, but seemed to be of a similar type.
Despite of the similar deutonymphs, adults of the named species possess unique and distinct morphological characters (Figs. 2 B,D,F) that differ them clearly from each other and from other known species (Scheucher 1957, Wirth 2003). H. maritimum was described by Oudemans from its deutonymph and redescribed by Scheucher (1957) based additionally on its adults. The redescription is detailed and describes most relevant features of the adults. But my SEM studies are more detailed than Scheucher’s drawings and show additional structures on the dorsal bodies of both genders, which are described herewith for the first time with a special focus to the dorsal side of the male.