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Roundworms
Roundworms
We saw that nematodes (roundworms) were around 50 million years previously arriving in water films. Now these small unsegmented worms have probably developed a few different mouthparts to cope with other food sources, particularly piercing mouthparts to suck juices from plant roots and other animals.
Annelids
Annelids
There is a massive phylum called Annelida, They are also known as the segmented worms, with over 22,000 living species including ragworms, earthworms, potworms (below) and leeches.
The main thing to note here is that earthworms were not around at this time.
They do not arrive for another 200 million years.
Potworms (Enchytraeids)
Potworms (Enchytraeids)
Potworms are oligochaetes called Enchytraeids. Enchytraeidae are small (1-30mm), unpigmented and resemble small earthworms. One branch is closely related to the Lumbricid earthworms common in many soils these days (Erseus 2010). This may provide a clue to what happened a couple of hundred million years after this time.
Where do these potworms fit in?
Where do these potworms fit in?
Enchytraeids would have liked the peaty swamps, just as they do today.
Since mid 1990s, cladistic research sees leeches as a sub-group of oligochaetes (mainly terrestrial worms inc both earthworms & potworms) and these oligochaetes as a sub-group of polychaetes (all marine).
Despite all the new technologies “The adaptation of marine annelids to freshwater and land is still largely hidden in obscurity”(Erseus et al 2020).
Potworms may well have a significant impact on soil formation, but how does this relate with the other worms? Despite much hard work by morphologists and molecularists, things are still not clear. Let's use these worms to use the other detective tools - their environment then and their habitats now - to throw some light on the matter
"Phylogeny of Clitellata, derived from transcriptome analysis (Erséus et al., 2020) for 63 clitellate species plus 10 outgroup taxa (polychaetes). Only the main lineages are shown (aquatic species were condensed to families). The habitat use indicated was adapted with slight changes, from ancestral state reconstructions in Erséus et al. (2020). The terrestrialization events in Crassiclitellata, Enchytraeidae and Hirudinida are indicated by asterisks (not implying that the whole lineage is terrestrial). The family Haplotaxidae is not yet resolved (polyphyletic in this phylogeny)". (von Straalen 2021)
Cocoons
Cocoons
Clitellate annelids, earthworms, leeches and numerous others are an abundant and diverse animal group in these systems and comprise at least one-third of all described species of Annelida in the world (Erséus, 2005). At this time, there would have been no earthworms, those large earth movers, as we know them today.
Clitellates are hermaphroditic and share a ring of glandular epidermal cells (the clitellum) at sexual maturity. This clitellum secretes a protective cocoon around the embryos; free-living larvae never occur. This reproductive strategy has been regarded as an adaptation, or at least a pre-adaptation, to life on land (Purschke, 2003, Purschke et al 2000 Westheide, 1997).
Origins
Origins
While the Mesozoic origin hypothesis for enchytraeids is supported by current molecular data, the possibility of an earlier origin in the Late Devonian or Carboniferous periods cannot be ruled out and remains an intriguing area for further research.
“It is also obvious from this figure (based on 2020 data Erseus et al 2020) "Enchytraeids (potworms) have an origin different from crassiclitellates (most earthworms) and they are not a sister group of the earthworms. Instead, enchytraeids descend from another freshwater group. A molecular phylogeny (family tree)suggests that enchytraeids even colonized the land more than once. One group (species of the genera Enchytraeus and Lumbricillus) specialized on a life in sea weeds washed ashore in intertidal flats and beaches, another group (Mesenchytraeus, Cognettia, Fridericia, BuchholziaandAchaeta) turned to feed on organic litter of inland forests. In addi-tion, maybe because they have retained more aquatic characters than earthworms, we also see several freshwater and marine regressions in this group. So, the terrestrializations among Enchytraeide are more difficult to track than in earthworms".(von Straalen 2021)
Enchytraeids inhabit all continents and are perhaps the most widespread representatives of worms with a collar round them (clitellum), even more than earthworms. They are found in a wide range of habitats, broadly represented in soil and the splash zone on seacoast as well as freshwater and marine sediments.
Earthworms are not distributed all over the world, indicating late evolution - after the breakup of Pangea. Enchytraeids are all over the world so predate the formation of Pangea.
"It is also obvious from this figure that enchytraeids (potworms) have an origin different from crassiclitellates and they are not a sister group of the earthworms. Instead, enchytraeids descend from another freshwater group. A molecular phylogeny developed by Christensen and Glenner (2010) suggests that enchytraeids even colonized the land more than once." (von Straalen 2021)
Despite all the morphological and molecular evidence over the years, this figure does not clear up much. The paper says it used 'habitat use' to help allocate relative -positions. But this seems much more general than we have quoted here, where we go into feeding habits and where to find them.The 3 asterisks in the figure would stand out more it it indicated 200 million years between enchytraeids and lumbricids
Then
Then
1.Pot worms are thought to have arisen in cool temperate climates where they are commonly found in moist forest soils rich in organic matter. They are also dominant members of soil communities in cold and wet versions like peat bogs and moorlands, there becoming the main ecosystem engineers. They are more predominant than earthworms in cold, wet, and organic-rich ecosystems with low pH, implying that this is the sort of terrain they first inhabited.
This present distribution may tell us about their origins. They would have lived in peat with some litter, but little soil to burrow into.
2. “They are unable to consume leaf matter and detritus like earthworms and instead gain most of their nutrients by consuming fungal hyphae and bacterial material, together with the frass or excreta from other soil animals.”
Again using this to indicate what may have gone on in the past, it fits the notion they move round in under the litter/peat layer, and rather than eating it, they eat the fungi below.
3. Perhaps the most interesting is “A molecular phylogeny(family tree) (developed by Christensen & Glenner 2010) suggests that enchytraeids even colonized the land more than once.” (van Staalen 2021). Did these creatures play a vital role in soil creation? These small potworms are in a position that may be vital in the next phase of soil development.
This indicates life in and out of the water, so they may have contributed greatly to stabilising the shore, and covering land with their casts creating new habitats.
Now
Now
They increase the availability of organic carbon as dissolved organic carbon in the surface (0–4cm) microcosms (Cole et al 2001) They may also help pore formation as their bodies are small enough to create pockets of air when they move. Their size (about 1mm diameter) creates tunnels fit for small arthropods to run through, but their strength is not up to moving mountains. Their excrement, can fuse into undifferentiable forms which contribute between 0-40% of aggregates (Davidson & Grieve 2006) Their tunnels would provide routes for springtails and mite and other fast running arthropods. They would also have been mixing the organic matter with the mineral matter, a vital process in early aggregation.
Enchytraeids are found to eat minerals. 21–35% of the enchytraeid population contained mineral grains. The estimated transport of mineral material in the upper 0.4 m amounted to 0.001–0.01% of the bulk soil per year. In experimental cores with enchytraeids present, the air permeability, volume of pores in the size class corresponding to the enchytraeid body width, and the proportion of aggregates corresponding to the size of their faecal pellets were higher than in cores without enchytraeids. (Didden 1990).(Didden 1990).
Enchytraeids are large enough to contribute to soil porosity through tunneling. The enormous amounts of casts produced and the mucus secretions they leave behind hugely influences the bacterial and mineral content of the region they inhabit- known as the drilosphere.(Erseus 2020)]
This shows that enchytraeids can make tunnels which would be vital in making some sort of structure on top any underlying rocks.
By looking at their present-day habitats, it is relatively easy to see how these enchytraeids could have created new soil-like structures all those years ago, and many times since. There are millions of them doing a lot of work and helping other small creatures find new places to live. Nowadays they live in damp peat, indicating that they are not doing much humifying, which is left to their cousins 200 million years later.
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