Mites

145 - 66 mya Cretaceous

Insects Springtails Woodlice Worms

Mites, particularly oribatids, are one of the three dominant groups of soil invertebrates. In the last period I called the recently emerged higher oribatids the 'denizens of the dirt' to express how they play an active role in litter decomposition, vertical translocation of organic matter for deeper soil humus formation, in smaller pores, and dispersion of decomposed debris in their poo, to help stick mineral soil particles together. In this period they differentiate further and spread out more

Origins

2 separate groups of mites

We saw the emergence earlier of the ‘stem group’ of mites, when Endeostigmata were around 400 mya (Arribas et al 2019). Many sorts of mites appeared in this Cretaceous period. Some are now known as "deep soil mites' which tend to be quite small and soft-bodied and may be elongated to allow for movement through very tight spaces between mineral soil particles. Most are effectively aquatic because the deep soils are often saturated, with the interstitial spaces filled with water. This indicates the sort of establishment of mites at this time - getting into the new nooks and crannies.

There are two quite separate groups of mites - the Parasitiformes and the Acariforms, consisting of Trombidiformes and Sarcoptiformes. Morphology suggests the more direct ancestors of Endeostigs are the Sarcoptiformes. As usual the systematics, taxonomy, and phylogeny of the ‘notoriously complex Acariformes’ need a lot of untangling. Using endeostigmatans’ family tree, we can improve the accuracy of time estimations on fossil-calibrations.

Oribatids (Lower)

We first encountered oribatids 400mya. They didn’t change much - even after the great 250mya extinction, but ‘branched’ out in this period around 100 mya. This period was a time of increased dispersion and radiation of soil mites due to the new leaf litter, new rhizospheres that led to new food sources.

We saw in the previous period that higher oribatids are confined to the Palearctic realm. What does this tell us about many soils in the world that do not have them? We take high number of higher oribatids for granted and they are seen as essential for soil building and decomposition. What replaces them? Termites?

Indian/African/Asian & Australian soils do not have 'higher' oribatids

This continues the story of oribatid evolution

400-360 mya (Late Devon)
330-300mya (Late Carb)
300 -250 mya (Permian)
Higher (200-145 mya Jurassic)

Oribatids (Higher)

If the earthworms became the engineers of the earth, then higher oribatids are the denizens of the dirt. This period is when the vitally important higher oribatid mites which appeared in the previous period, really became widespread, building stronger soil structures, but also changing their feeding habits. They chew and poo to produce glue to help stick the soil into macro-aggregates.

Half of all their radiations correlate with the advent of angiosperms. We saw that the oribatids in the previous were smaller and less hairy than their ancestors.

in the later part of this period they changed trophic levels (red arrows) and developed from lichen and primary decomposers to secondary decomposers, and predators or scavengers.

Schaefer & Caruso 2019red arrows = changes in trophic levels

(Schaefer & Caruso 2019)

Oribatid evolution 500-0 mya

From (Schaefer & Caruso 2019) Brown stacks =evlutionary events, orange & black arrows = vascular plant changes, red = mite mouthparts changing

Size & Spatial niches

“Small soil animals move in the pore-space of an architecturally complex soil-matrix, and their body size and form determine access to specific segments of a three-dimensional space (e.g., small vs large pore spaces), which provides not only resources but also some enemy-free space. Thus, the current diversity observed in soil animal body size and form could evolve functionally only when the three-dimensional complexity and structure of soil also evolved in terms of aggregates of different forms and sizes, which created pore spaces of different forms and sizes…

Analogous to the evolution of soils, where new kinds of soils add to the existent ones rather than replacing them, the evolution of oribatid mites can also be viewed as the addition of new lineages to existing ones, with emerging combinations of body sizes and forms, rather than the replacement of pre-existing lineages…

According to our results, the general notion of oribatid mites being generalist is to be revised because these animals inhabit a wide range of spatial niches but each species likely covers a specific trophic range in a specific spatial context." (Schaefer & Caruso 2019)

Trophic Niches

Can we learn anything about distribution in Cretaceous times with what is happening now. Please note that when most present-day Western researchers refer to 'oribatids' they are referring 'higher oribatids'. Nevertheless the results are not what I expected, bearing in mind how much broad-leaved leaf litters had begun to play. . "Oribatid mite densities in the four forest types were highest in the coniferous forests (~120.000 ind./m²), lower in the 30 years old (~60.000 ind./m²) and the 70 years old (~50.000 ind./m²) beech forests and lowest in the natural beech forests (~30.000 ind./m²). High density of oribatid mites in coniferous compared to deciduous forests of the temperate zone have been reported before." The researchers were also surprised and suggested "in our study oribatid mite density correlated closely with the mass of litter layer suggesting that the thickness of the litter layer rather than the humus form determines oribatid mite densities " Erdmann et al 2012)

"Oribatid mite community composition differed between the coniferous and beech forests and between the young (30 years old) and the old (natural) beech forests. This suggests that the different forest types provide different niches for oribatid mites. Different niches are likely to be related to oribatid mite nutrition. It has been shown using a number of methods that oribatid mite species occupy very different trophic niches.. " (Erdmann et al 2012)

Mites v worms

Soil mite biomass is similar to earthworms, but they get through a lot more metabolism. But they are not friendly

Earthworms "detrimentally affect oribatid mites via bioturbation i.e., by reducing the thickness of organic layers and by mixing litter and mineral soil...Oribatids were also low in high-N areas, which "might be due to high densities of macro-decomposers (earthworms) at those nitrogen-rich sites". (Eardmann et al 2012)

Oribatid mites also "reach high densities in base-rich soils not colonised by earthworms indicating that the presence of earthworms reduces the density of oribatid mites". (Migge-Kleian et al 2006)

Oribatella spp

Predation

While originally hard to get at, their wide distribution must have meant they were prey to some. Predation has been an important factor throughout the evolution of oribatid mites, contributing to morphological diversity and defence mechanism evolution. Despite their hard cases, ants emerging in this period found a way to kill them. They were not the only ones...

Mesostigmatids

This order of large fast moving mites are predators, and belong to one of the two big mite divisions - Parasitiformes. They would now have the tunnels to run round, protected from the elements and strong predators. The larger ones munch springtails, while the smaller ones eat nematodes – 3-5/day and nymphal mites. The aggregated layers would now provide rich pickings.

Forest floors. with abundant litter, provide a large spectrum of the larger mites, like Pergamsus. Mesostigmatid mites like Pergamsus sp. are large and fast moving and prey on springtails (left, own photo) and eat arthropods and their eggs

Mesostigmata is what we expect to evolve in this period. They are predators on other mites sucking plants and springtails

Click to view Mesostig munch

Such is their role on soils, I have sponsored P crassipes in the outstanding ‘Tree of Life’. Do look to see its position in the family tree of life. They may well have been around before this period living in the surface layers, but are now ready to pounce on many more creatures. Smaller, colourless Mesostigmata – like Rhodacarids and relatives – inhabit the lower soil layer and eat nematodes. The size of Mesostigs diminishes with increasing depth. It is easy to see how this group could have evolved down through the layers.

Oldest

The oldest known record of the group is an indeterminate Sejidae deutonymph from the mid-Cretaceous (Albian-Cenomanian) aged Burmese amber of Myanmar” (Omid et al 2021) This record traces the Sejida (= Sejina) back to the mid-Cretaceous (ca. 100 Ma) and represents not only the oldest valid record of Mesostigmata, but it is also one of the oldest examples of the entire Parasitiformes clade. The fossil record of Mesostigmata is surprisingly sparse. Only nine fossil families from the lineages Gamasina and Uropodina (Mesostigmata: Monogynaspida) have been recorded and no valid fossils of the seemingly more early derivative Sejida and Trigynaspida have been found. Other recorded fossil mesostigmatans are from between 50 -15 mya

Astigmata

The other big mite group - Sarcoptiformes - includes Oribatids, which we have been around for a long time, and Astigmata, which we have not seen up till now. For a long time the Astigmata was treated separately from oribatids, following the idea of the influential mite morphologist, François Grandjean (Pepato & Klimov 2015).

However, it seems pretty certain that Astigmata evolved from Oribatids. It is likely that the conflict resulted from differences in the substitution rates among acariform lineages, especially comparing slowly evolving Oribatida with rapidly evolving Astigmata. “The data revealed the Crotonioidea as the source of astigmatan radiation and the sexual family Hermanniidae as the sister group, which generally supports previous morphological hypotheses". (Dabart et al 2009)

That is saying clearly that the Astigmata rose from an oribatid group, namely Desmonomatan oribatid mites, found in soil and litter, and now considered the sister group to Astigmata. These are a diverse group found in all sorts of places, like stored product mites, house dust mites, mange mites, feather and fur mites. Did some oribatids find that hitching a ride was more productive than chewin’ dead matter?

Can we learn something from present distribution? Oribatids, as we have seen, are slow growing, largely parthenogenic and live in very stable conditions. Astigmata appear to colonise more rapidly more disturbed soil environments. “Oribatida influence decomposition and soil structure by comminuting organic matter; their fecal pellets provide a large surface area for decomposition, and are in turn an integral component of soil structure inorganic horizons. ….Astigmatic mites also facilitate the humification process…and also consume the liquified products of decaying organic material” (Behan-Pelletier 1998). This implies astigmata can exist in more anaerobic conditions. Astigmata is a megadiverse lineage of mites that expanded into a great number of habitats via associations with invertebrate and vertebrate hosts.

Hitching a ride

In this period, we’ve seen springtails like to hitch a ride. It seems that mites were also benefitting from hitching a ride. The evolutionary success of Astigmata is also linked to this - called - phoresy consisting of a commensal relationship where deviations may result in mutualistic or parasitic relationships. A phoresy -related metamorphosis, namely the origin of the heteromorphic deutonymph, is highly specialized form of phoresy or dispersal on hosts. “The origin of this instar is enigmatic since it is morphologically divergent and no intermediate forms are known. Here we describe the heteromorphic deutonymph of Levantoglyphidae family from early Cretaceous amber of Lebanon (129 Ma), which displays a transitional morphology.

It is similar to extant phoretic deutonymphs in its modifications for phoresy but has the masticatory system and other parts of the gnathosoma well-developed. These aspects point to a gradual evolution of the astigmatid heteromorphic morphology and metamorphosis. The presence of well-developed presumably host-seeking sensory elements on the gnathosoma suggests that the deutonymph was not feeding either during phoretic or pre- or postphoretic periods." (Klimov et al 2021)

It is this group of mites who have developed their phoretic capacities to human associates which include stored food mites, house dust mites, and scabies.

Prostigmata

One of the two big mite division of the Acariformes are Trombidiformes. Several groups of mites have evolved the ability to feed on living plant tissue, and this would have been when they evolved making the most of the new leaves. Most species belong to one of several lineages of Trombidiformes, each of which has independently evolved this capability, but all share the modification of the chelicerae into piercing stylets.

Trombidiformes include the very diverse group Prostigmata This group originated about 150 m years previously but would have come into its own in this period. Their sucking mouthparts were ideal for the new juicy leaves.

Give mites a bad name

Most Prostigmata are the ones which give mites a bad name Its members include medically important mites , like Demodex, the chiggers, and scrub-itch mites, and many agriculturally pests, including the gall mites, & spider mites - Tetranychidae. Most species are relatively host specific and do little damage, but some, such as the two spotted spider mite are well known pests. Other mites living on plants are beneficial as predators of plant eating mites. Chief among these are species in the gamasid family Phytoseiidae.

Pseudoscorpions

It was a real shock when I first saw one of these down the microscope.

Closely related to mites, we saw the first appearance of pseudoscorpions over 350mya. Since that period they seemed - like many springtails and mites - to disappear only to resurface around 100mya. Almost no other fossils of pseudoscorpions are known for over 250 million years until Cretaceous fossils in amber, all belonging to modern families, suggesting that the major diversification of pseudoscorpions had already taken place by this time. (Del-Claro & Tizo-Pedroso 2009)

They would have had a lot more to feed on in this period. They are predators of other small soil animals such as springtails, silverfish, fly larvae and nymphal mites. They would lie in wait in the leaf litter or under a stone, waiting for prey to brush against the sensitive hairs on their claws. They quickly grasp the prey in their pincers and inject venom into it, then pump digestive enzymes into the dead animal and suck up the externally digested fluids.

Curiously, they exhibit “ the extended parental care, division of labour, cooperative breeding and feeding, and the tolerance among members dividing the same share that could be considered enough to characterize a true social life" (Del-Claro & Tizo-Pedroso 2009)

First Oribatids (Late Devonian)

'Lower' (Carboniferous)

'Higher' (Jurassic)

The soil mites have now evolved many soil dwelling forms, of differtent shapes and feeding habits, exploiting feed resources in many ways, and providing food for others.

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