Very informative and nicely done! The amount of species we have yet to observe is certainly a lot more than I expected, which I am sure is caused by high biodiversity in the more remote places of Earth. Thanks for the info!
Itâs also proof that carcinologists need to get moving!
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Here is arthropod classes as a whole, compared to each other and to vertebrates. Something interesting to note is that, despite insects being so much more specious than other arthropod classes, they are also better sampled too. Around 16% of total insect species have at least one research grade observation on inaturalist, as opposed to around 10% of arachnids, 7% of myriapods, or less than 1% of ostracods. The gap between vertebrates and arthropods is vast and has been discussed elsewhere, but it is still interesting to see.
The real takeaway from this, is that the oligostraca (ostracods and some related groups), are the biggest losers in all this
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I moved the above four posts to this topic and reopened to keep all the discussion together.
I didnât even know that was possible, but thanks I guess
If you ever want to open an old thread, you can request from a specific mod or just mention forum_moderators.
Anyway, we compared arthropods to vertebrates, and arthropods to each other. How does the rest of the animal kingdom stack up?
I did it mostly by phylum, but I aggregated some smaller related phyla into larger groups.
Scalidophora: Kinorhyncha, Loricifera, Priapulida
Mesozoa: Rhombozoa, Orthonectida
Gnathifera: Rotifera, Acanthocephala, Gathostomulida, Micrognathozoa, Chaetognatha
Lophophorata: Brachiopoda, Phoronida, Entoprocta, Bryozoa
Nematoida: Nematoda and Nematomorpha
I also split chordata into vertebrates and the non vertebrate ones (tunicates and lancelets, mostly tunicates)
In the observed species category, thereâs not many surprises here, other than perhaps there being more observed cnidarian than annelid species.
Vertebrates being the most complete is no surprise, but ctenophores being in second place is a bit of an upset. I guess low number of species plus a highly dedicated group of specialists can do wonders. Mollusks, echinoderms, and tunicates also have higher completeness than arthropods. This is likely because they have fewer species, and the species in those groups tend to be larger than the average arthropod and easier to find in their respective habitats. It certainly helps that most of them are benthic rather than pelagic. But, all things considered arthropoda isnât doing too bad. Itâs more complete than cnidarians, sponges, nematodes, and annelids.
For number of observations, arthropods, vertebrates, and mollusks are the only groups that exceed one million observations, as they contain many common, macroscopic, terrestrial species that the average person can easily run into. But with that said, annelids are strangely low. You think there would be a lot more for annelids considering how common and relatively easy to find earthworms are, yet they have fewer observations than echinoderms and cnidarians. I donât know what could possibly be the reason for this.
Vertebrates on top here, as expected, but ctenophores are very close behind! A whopping 91% of observed species of ctenophores have at least one research grade observation under their belt! Like I said earlier, this is likely the work of a small but dedicated group of specialists. Kudos to them! Then is mollusks and echinoderms which are nearly tied, with arthropods not far behind. Again, annelids being so low is rather surprising, but perhaps whatâs not surprising is nematodes being last place.
Mollusks and echinoderms on average I think must be easier to identify than arthropods because of their larger size and perhaps less specialized reproductive anatomy. But holy hell, whatâs wrong with you annelids? You have a smaller percentage of research grade observations than nematodes!
And here have the final category, research grade species as a percentage of known species. Ctenophores impressive at their second place, for reasons mentioned above. Then is molluscs are next, then tunicates and echinoderms are nearly tied, and then arthropods.
So, what are the takeaways from all this? The best sampled invert group, over all, seems to mollusks. On some level, all non vert groups suffer from similar limitations. But mollusks are in the best position, because:
- A large portion of them live in accessible habitats, such as land, freshwater, and shallow reefs and tide pools
- They arenât too specious, so the magnitude is not overwhelming like it is with arthropods
- They tend to be fairly large in size, which makes them easier to notice, easier to photograph, and easier to identify. There arenât any truly microscopic mollusks. There are some that border on it, but they are way less compared to other taxa.
- They have a certain charisma to them, shell collecting and identifying is very popular. Cephalopods have their own brand of popularity, and land snails are quite well liked as well.
- I suspect the lack of sclerotized reproductive structures means that species delineation based on obscure genital features is not much a thing with mollusks like it is with arthropods.
Echinoderms share in all of these points except with mollusks, except losing to them on the first point. There is no echinoderm analogue to the common land snails and freshwater clams. But the other characteristics put them ahead of arthropods. Tunicates share with mollusks in all ways except the fourth point. How do arthropods score on these points by comparison?
- While many arthropods do live in inaccessible habitats, as I covered above in this thread, when taken in aggregate, these difficult taxa make up a fairly small portion of the group. There is no shortage of abundant, easy to find arthropods on land and in shallow waters.
- This is their biggest weakness here, there is over a million species
- Arthropods vary a lot in size, but suffice say most of them are big enough to be noticed by people, and to half decent photos without specialized equipment. But, there is also a lot of microscopic or near microscopic species. Copepods, ostracods, cladocerans, mites, springtails, psocids, a bunch of obscure orders, and even many flies, micromoths and parasitoid wasps. These are either hard to hard to notice/find, and you cant get a good picture of these with typical cell phone cameras. These tiny taxa really add up and drag down the performance of the phylum as a whole. Early in the thread, we can see there was a correlation between body size and sampling quality between different arthropod taxa.
- It is often said arthropods are uncharismatic and unloved, but really this is an area where they excel at. Compared to the other non vertebrate taxa featured, arthropods are really the most charismatic and popular of them all. Even aside the super obvious examples of butterflies, bees, and dragonflies, basically every major arthropod group has its own dedicated âfandomâ. Tarantulas, scorpions, roaches, mantids, phasmids, crabs, isopods, centipedes, millipedes, etc. If someoneâs favorite animal is not a vertebrate, it is almost certainly an arthropod. Unlike other non vertebrates, arthropods have recognizable limbs and faces, which makes them easier to relate to. People who fancy themselves as general bug lovers/invertebrate enthusiasts can identify or share info about all manner of different arthropods, but I doubt they will have much to say about nemerteans, brachiopods, or nematodes.
- Another big weakness here, lots of arthropod genera have species which can only be told apart by microscopic examination and/or dissection. Species can be based on very subtle features, often in the genitalia. Even high quality photos of living animals can be useless for species id if certain features are not in frame.
But, all things considered arthropods are not doing so bad. Even with millions more species, arthropods are better sampled than most other animal groups. I mean, compare with nematodes for example.
- Nematodes are live either deep in the soil, deep in aquatic substrate, or deep in some host animal/plant. The chances of encountering one while you are out and about is practically nill. Most nematode observations are of nematode caused damage on plants, and most people wont know to look out for that. The only nematodes that one can conceivable encounter in a regular outside walk that are big enough to notice are mermithids (nematomorphs as well), and those arent super common.
- The number of known nematode species is rather modest, it is less than the known number of fish species even (keep in mind I am not accounting for potential undescribed species richness, which is way higher). But itâs certainly not a low number of species, not nearly low enough to make up for all the other disadvantages.
- A substantial minority of arthropod species are microscopic, but a large majority of nematodes are. The few that arenât tend to be endoparasites of vertebrates that no one other than vets and doctors would encounter with any regularity, and I donât think vets and doctors upload the parasites they extract from their patients to inaturalist all that much.
- A bunch of people donât even know what nematodes are, and those that do often hate them. I would say the average human encounter with nematodes is far more negative than the average human encounter with arthropods. Sure, arthropods can be pests, spread disease, and damage crops. But people also have plenty of positive experiences catching bugs, looking at bugs, observing their behaviors, etc. Meanwhile, peopleâs interactions with nematodes tends to be only the negative aspects. They also just donât have the whimsy of arthropods. Arthropods come in so many varied forms, and many look very âcoolâ or âcuteâ. But nematodes are just kinda bland, generic looking worms. People simply lack also simply lack a basic familiarity with nematodes that they have with many mollusks and arthropods
- Speaking of looking bland and generic, nematodes can be extremely difficult to tell apart. Imagine the issues of arthropod id, but like ten times worse. You need a microscope to even tell what class or order of nematode you are looking at, let alone species. This is basically never an issue with arthropods or other larger non vertebrates.
To put it blunty, even mites outperform nematodes on inaturalist, and mites are some of the worst performing arthropods. Here are the completion scores
Trombidiiformes: 3.29%
Sarcoptiformes: 4.31%
Mesotigmata: 1.21%
Nematoida: 0.79%
Now, granted, most non vertebrate taxa arent quite as bad as nematodes. But a combination of obscurity, rarity, and difficulty of identification put them in a pretty bad place.
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On that note, if you were to look at Platyhelminths, I predict that you would find the observations heavily biased toward the large, terrestrial kinds, many of which are colorful and âjust look really coolâ; whereas the aquatic and parasitic ones would be rarely observed at all.
Yup. Most flatworms are either microscopic, or endoparasites. Geoplanids are the only ones people could ever reasonable come across
There are plenty of marine flatworms. They reach a respectable size and some of them are very colorful. They are not uncommon in the intertidal zone and on coral reefs.
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True, these are the most observed kinds of flatworms behind the terrestrial ones, I should have mentioned those too
Now, here is a question I have? Why are INat statistics summarized as birds, mammals, amphibians, reptiles, fish, arachnids, insects, molluscs and "âothersâ'? Why not do it by phylum instead? As in arthropoda, chordata, mollusca, cnidaria, etc
Thanks for these amazing analyses! I think delving into data is so delicious, particularly regarding life forms. I know iNat prefer us to just use the heart symbol to say Thanks, but what you have done needs an order of magnitude mire appreciation.
Beause most users would not understand those.
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If only people knew about phyla more
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