Is anatomical degeneration impossible in vertebrates?

Nature Talk
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Basically, I mean an animal evolving to radically simplify its anatomical structure, or losing several major body systems.

I will demonstrate some examples in arthropods, a phylum of animals with very complex anatomy and behaviors, in many ways analogous to vertebrates.

‘Degeneration’ as I will call it, happens mainly via two pathways in arthropod. One is hyper miniaturization. Many tiny arthropods, such as astigmatan mites, most copepods, and most ostracods lack any respiratory or circulatory system, being small enough that oxygen can just diffuse into their whole body.

The other is parasitism. Several parasitic arthropod taxa have essentially become blobs lacking any recognizable arthropod anatomy at all, such as rhizocephalans, dendrogastrids, cryptoniscid isopods, and some parasitic copepods.

Just, look at this!

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Those tiny white balls are isopods of the genus Liriopsis, parasitizing a Sacculina barnacle, parasitizing a crab.

A less extreme example of parasitic degeneration in arthropods are scale insects (plant parasites still count?). Depending on the species, females may lack eyes, legs, or antennae.

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Then there is the fact many insects lack functional (or any) mouthparts as adults, and have degraded digestive systems. This is much less grotesque than the above examples, but is still nigh unthinkable for vertebrates.

And its not just arthropods.

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Here is Enteroxenos, a gastropod that has lost its mouth, gut, gills, eyes, and digestive system.

I could also bring up examples of countless examples in worms, but I figure arthropods and gastropods are the most complex animals which include degenerated descendants.

So it begs the question, why have no such things happened among vertebrates? I suppose the male anglerfish could be an example, but it would be by far the only one. Are vertebrates just more constrained?

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Could it be partially due to mobility?

If you look at plants, they have way higher sexual diversity than vertebrates, for a simple reason: they can’t move. So they found other ways to compensate for that.

If you look at the examples you listed above — parasites — they don’t move that much. The anglerfish is also in a parasite type of situation.

Most vertebrates can easily change their environment by going somewhere else. If you’re going to give up some body parts, then you want to be in a stable environment (like sucking nutrients out of another organism) where you know that you’re not going to need those eyes, or legs, or whatever, ever again.

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Males of some deep-sea angler fish attach to the female and become parasites. Their bodies degenerate until they are little more than gonads. Pretty extreme.

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Well arthropods are plenty mobile creatures. Yet many of them have undergone anatomical degeneration. Why couldn’t a frog or fish evolve to lose their hearts like copepods and ostracods have?

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That is indeed a major example. But, for example, could a reptile evolve to lose its circulatory system?

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We don’t know, though I’d bet a lot against it. To loose its circulatory system, a vertebrate would have to become extremely small and thin (so gases and food would be able to travel everywhere by diffusion) and live in the water in a place with lots of nutrients and sufficient oxygen. Unlikely. Reptiles would be especially unlikely to do this because their hard skin is unsuitable for gas exchange. I suppose an amphibian would be more likely to do this, but we’re still in the realm of astronomically large probabilities against.

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some salamanders lack lungs, so it is definitely possible.

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Oooh, plethodontids. I didn’t even think of those, those are indeed a great example. While no vertebrate has approached the levels of degeneration seen in other animal phyla, amphibians and fishes come the closest.

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There are many species of reptiles and some of amphibians who have lost their limbs. Not sure if that fits your criteria, but it seems comparable to the scale insect example. In terms of gross anatomy, they have become worms, but they retain most of the internal complexity of their ancestors.

Axolotls have only rudimentary lungs, and there are fishes, reptiles and amphibians (many of them cave-dwellers) who have either lost their eyes over their evolutionary history, or have retained only rudimentary eyes.

Cetaceans have lost their hind limbs and have only a vestigial pelvis. Several species of birds, in different orders, have lost the ability to use their wings to fly, in some cases retaining only a vestigial wing.

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These all good examples of simpler forms of degeneration. Indeed limb loss is actually quite common among vertebrates, as is reduction of various skeletal elements. But it seems few if any of them have undergo extreme degeneration of most/all their organ systems. Male anglerfish are the only example, and perhaps plethodontid salamanders are the next closest.

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Fascinating speculation, thanks for suggesting it. I think it becomes more conceivable if we avoid having tetrapods constantly jump to mind as canonical examples of vertebrates. For whatever reason, the tetrapod developmental program is quite constrained (although naked mole rats are truly, truly strange and if there’s an evolutionary radiation from them, all bets may be off). There is far more developmental plasticity among non-tetrapod fish. Some have larval stages very different from adult, some can change sex, and well, there are anglerfish.

So, could a parasitic fish come to be where all life stages, not just adult males, are extremely reduced? I think it’s quite plausible. And if one has previously existed, it’s unlikely to have left fossil traces (small, and not numerous – parasites are less numerous than their hosts and have a limited range of hosts). So who really knows?

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Brook lampreys also have degenerate, nonfunctional digestive systems as adults.

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In the advanced snakes, pretty much all of the paired organs are reduced to one…one lung, kidney, ovary/testes. Similarly, in Order Gymnophiona, caecilian amphibians have lost at least one lung. The commonality seems to be the tubular body plan is too long and narrow to warrant duplicates.

Also consider all of the convergent degeneration in cave taxa. No light, so no need for eyes, pigment, etc. This transcends vertebrates and occur in verts. and inverts. alike.

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The best example I can think of is canine transmissible venereal tumor. It is a clump of dog cells that no longer behaves like a dog, instead acting as a parasite on other dogs. Apparently it has been around for thousands of years now.

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Oh wow, this is totally new to me! I had no idea snakes had this level of organ reduction.

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Fish do seem to have the greatest capacity for this. Male anglerfish ofc, some fish changing sex, and some adult lampreys having non feeding adults as mentioned by someone else.

I guess the real question is, why do fish have so much more developmental plasticity than tetrapods?

And for that matter, crustaceans also seem to have much more developmental plasticity than terrestrial arthropods. All the horrifying parasites, hermaphroditism is common crustaceans but extremely rare in insects (only in a few scale insects) and so far unknown in arachnids and myriapods. Perhaps the colonization of land for both vertebrates and arthropods came with the addition of developmental constraints?

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Not sure you can say that terrestrial insects have less capacity for developmental changes than crustacea. I mean … You have complete metamorphosis, with the larvae sometimes having much less morphological complexity than the adults. Some even have hypermetamorphosis, where they undergo radical changes even before morphing to adults – for instance parasitoid wasps where the first instar is mobile, later ones adapted to feeding within a host. Cecidomyiidae (gall midges) have paedogenesis, where their reproductive organs mature but the rest of their body remains larval. You have extreme parasitic adaptations, like strepsiptera, where adult females lose eyes, mouth, most organs, the ability to move – essentially becoming a sack of eggs.

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True true, but no insect can quite compare to rhizocephalans and dendrogastrids, or cryptoniscid isopods. Female streps get close but even they still somewhat resemble an insect. And the point of hermaphroditism being extremely rare in insects but common in crustaceans still holds.

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Disagree on a “larva” in metamorphosis having less complexity: the genes that function for being a tadpole get completely superceded for almost every organ system as the sexual adult slowly develops; the lungs aren’t built/restructured from the gills, for instance-- and where do the bony legs come from?
For the schistosome cercarial parasite, it’s got the enzymes, phototaxis and swimming tail to be able to penetrate the vertebrate host, traits not expressed in adults. A miracidium hatching form the egg has cilia to swim to the snail host, and the right membrane structure to maintain cellular homeostasis as it transits from a saline to fresh-water environment.
As far as flatworms having “lost” organ systems, don’t assume they had ever developed: a successful ancestral game plan may persist through amazing geologic, meteorologic and biologic upheavals. Less complexity often doesn’t mean “degeneration”, so much as a more complex arrangement wasn’t needed for survival success.

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Others have brought up the examples of cave-dwelling organisms (many taxa including spiders, beetles, salamanders, etc.) which have reduced or lost eyes and pigment. The common term used for such changes is “regressive evolution”, not “degeneration”.

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