On a Moss Safari, one of the larger organisms you will see are Moss Mites. They are the first and largest of the Moss Safari multicellular Big Five because they measure under one millimetre, the largest organisms of the Big Five. Here I take some time to share what I have learnt about finding moss mites and what their lives may be like within moss.
What Moss Mites look like under the microscope
When they are active, Moss Mites are quite easy to spot. They have eight hairy legs that they wave about slowly (Image A). They often seem quite clumsy, lumbering around in the field of view. There are usually two longer legs pointing more backwards, each side of the dark, pear-shaped body and two pointing more forward past the head and snout. The other two pairs point out to the sides. They will often grip onto debris such as microscopic stones or parts of the moss. I get the feeling they prefer the dark and try to move from the spotlight of the microscope.
When then are not active, moss mites are easy to miss under the microscope. They are often just dark, tear-drop shaped objects, with their legs tucked into the sides of their body (Image B). By stopping at objects that are potentially moss mites, you can increase the light intensity and reveal if it is indeed a Moss mite. Their dark ‘shells’ will only transmit some light, but they will glow as amber, reds or oranges (Images C and D). These are the distinctive shapes to look for (see below).
At higher magnifications and using a bright light from above or below, many of the features of moss mites can be observed in more detail. Since they are often still or slow moving, spending some time studying them is fairly easily. Particularly focus on the hooks at the end of their legs (Image E), the mouth parts and the ‘plates’ of their shell’.
Sometimes you may find the empty ‘shell’ of a moss mite (Image F), often an amber, dark case that you can study carefully to observe it’s structure.
What are moss mites?
Moss Mites are classified by scientists as Oribatid mites. They are sometimes called ‘beetle mites’ because they are beetle like in so far as they have a hard exoskeleton and jointed legs, but they are not insects as they have eight instead of six legs. Others have called them ‘snout mite’ due to the snout-like shape of their mouth parts.
Moss Mites are in the phylum Arthropoda. They are arthropods (along with crabs, insects, centipedes, scorpions and spiders), but because of their size, they are often referred to as ‘microarthropods’ (these include any arthropods that is difficult to see with the naked eye including thrips, spring-tails and pseudo scorpions). All of these are fascinating to observe under the microscope.
All arthropods have a hard exoskeleton (I referred to it as a ‘shell’ above, but will use the correct term ‘exoskeleton’ from now on) and pairs of jointed legs. Mites fall into this category. There are many other mites you might have heard of including dust mites (found in homes and beds chomping through dead skin cells), spider mites (that feed on plants and cause crop damage) and scabies mites (that cause an itchy, but treatable, skin infection).
Moss Mites are part of a very big group of oribatid mites. There are thousands of known species and there are estimated to be hundreds of thousands in total. It is thought that they outnumber all other soil arthropods. That’s a lot of discoveries still to be made! Scientists who study mites are called ‘acarologists’ and their research is known as ‘acarology.’ There’s whole university departments and books on the subject.
Oribatid mites are mostly found in soil, in fact they are considered highly important in decomposition in soil and understanding them is essential for the future of agriculture. However, only a few oribatid mites seem to have adapted to life in moss.
Moss Mites are usually much less than 1 mm in length, more usually 0.2 mm – 0.5 mm (200-500 micrometres). In general Oribatid mites have a life cycle of 1-2 years (but some species live for 4 years and in cold climates up to 7 years). They have a low metabolic rate, so they grow and develop slowly and do not reproduce very quickly (compared to other arthropods).
What’s their life cycle like?
They lay eggs and these eggs hatch into a pre-larva, which then develops into a larva. These larvae look like miniature versions of the adult, but with six legs instead of eight and with much softer exoskeletons. The larvae go through three stages as nymphs (scientifically: proto-nymph, deuto-nymph and trito-nymph). These move much more rapidly than the adults, and have a synchronous movement of their six legs together, as if they are doing breast stroke with all six legs. This may be the origin of their name ‘chiggers’ (note in the US, harvest mites are called ‘chiggers’ which can bite humans). Between each stage the mites moult, shedding their exoskeleton, until they finally become the eight legged adult mite. This is why you may find empty mite shaped exoskeletons in your Moss Squeeze samples.
It appears that you will not observe any mating in Moss Mites behaviour because they are all female and reproduce by parthenogenesis. This means that the egg can develop into embryos without fertilisation from a sperm, it is a very common form of asexual reproduction within organisms that live in moss. Note that I am not completely sure that Moss Mites do not reproduce sexually, but I will update if I find out different.
What are the adaptations of Moss Mites?
Anatomy
Moss Mites have a body plan that is made up of two parts: the head end with complex mouthparts and a light sensitive organ (the lenticulus) that allows Moss Mites to detect light. Their exoskeleton is made up of a series of plates, which can been seen under the microscope. On the underside, starting from the head end, you can see the sternal plate, the genitoventral plate and the anal plate (the anal plate has a hole in it, the anus). The plates have markings on them called striations. The sides of the exoskeleton have two plates called pteromorphs (wing structures). Sometimes you can see tint holes down the sides of the Moss Mites – these are the tubes through which it can gas exchange, the holes are known as spiracles. You will also notice that the legs and top of the body has a number or elongated hairs that are probably sensory and defensive.
Adaptations
Moss mites have some fascinating adaptations, these are just some we are learning about. I suspect there are many more.
Sucking mouth parts – Moss Mites use their mouth parts to pierce through the moss cell walls and suck out the nutritious cell contents.
Pausing – between the last two nymph stages and the stage between nymph to adult, oribatid mites can hibernate. This is known as a ‘diapause’ when their already slow metabolism can reduce to the absolute minimum.
Playing dead (thanatosis) – when disturbed Moss Mites can stop moving to appear dead, there is some research suggesting they may also produce chemicals that suggest they are dead as well.
Raising hairs – to deter predators, Moss Mites can raise the hairs and adornments on their bodies.
Hard cuticle (exoskeleton) – is thought to reduce both desiccation and predation.
Synchronised reproduction – in environments where there are predictable seasonal changes in rain, some species synchronise their reproductive life cycle with the seasons to maximise survival.
Freezing tolerance – to avoid damage from ice formation in below zero temperature, Moss Mites produce ‘antifreeze’ chemicals in their bodies.
UV protection – the red/orange colour of the exoskeleton contains carotenoids that provide UV protection (reduces cellular and DNA damage). It is also thought that some Moss Mites migrate to mosses to gain shelter from UV radiation, but move on when conditions deteriorate.
Visitors or dwellers?
Finally, I am interested in whether Moss Mites I find live exclusively in moss as their home, like permanent residents or whether they just visit moss canopies for shelter, protection and food.
I imagine in a nice damp, dense moss cushion, a Moss Mite is likely to stay moist, have a good food supply, face relatively few predators and has protection from UV light. Some species (Erythraeoidea spp.) live more permanently in tree bark and possibly like to take a stroll through any moist moss that happens to be growing there. It seems that some of the permanent moss dwelling species (genra Dameaeus, Belba and Metabelba) are fungal eaters. You will see many fungal spores in moss and leave it damp in a warm spot too long, you will see fungal growth. However, there are other Moss Mite species that are predatory, eating two of our Big Five, rotifers and nematodes. So perhaps a visit to moss would provide quite a banquet for them!
Further Reading and References
For younger explorers: https://kids.frontiersin.org/articles/10.3389/frym.2020.545263
To find out more about the incredible animals that live in soil, with brilliant photographs visit https://www.chaosofdelight.org/all-about-mites-oribatida
Pfingstl, T., & Schatz, H. (2021). A survey of lifespans in Oribatida excluding Astigmata (Acari). Zoosymposia, 20, 7-27.
Seniczak, S., Norton, R. A., & Wang, H. F. (1998). The Morphology of Juvenile Stages of Moss Mites of the Family Thrypochthoniidae (Acari: Oribatida), and the Taxonomic Status of Some Genera and Species. Zoologischer Anzeiger, 237(1), 85.
Smrž, J. (2006). Microhabitat selection in the simple oribatid community dwelling in epilithic moss cover (Acari: Oribatida). Naturwissenschaften, 93(11), 570-576.
Wehner, K., Heethoff, M., & Brückner, A. (2018). Seasonal fluctuation of oribatid mite communities in forest microhabitats. PeerJ, 6, e4863.
Small print
I’m a novice, I am learning. I am doing Moss Safari’s for pleasure and for education. I am a teacher and learner and although I am doing my best to be accurate, I will sometimes get things wrong. Do let me know if you are more informed than me and notice any errors big or small. I will make any changes needed to my blog.
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