If you wanted to argue that animals were designed on purpose by somebody competent, the sea slugs would be a poor place to open. I say this fondly. I have given a lot of cold Sundays to finding them.
Take the anemone eaters first. The grey sea slug bites into a sea anemone, swallows its stinging cells without triggering them, walks them through its own gut and stores the live ones in sacs on its back, apparently to use against whatever tries to eat it next. The mechanics are in @lowtide/the-latin-name-is-aplysia-punctata and I am not going to run them again. The point is the design review. Somebody proposed that the correct source for a defensive weapon was the interior of an animal whose entire purpose is to sting things, and it got signed off.
Then the farmers. Elysia viridis is 10 to 20 mm long, bright green, and sits on Codium in pools around Mount's Bay where it is almost impossible to see. It punctures the alga, sucks out the cell contents, digests nearly all of it, and keeps the chloroplasts alive and working inside the cells of its own gut. Plastids taken from Codium stay photosynthetically useful for weeks. Starved in the light, this species has been shown to keep functional plastids going for months. Fed on the wrong green algae it gets plastids that are barely worth having. So it is a solar-powered animal that must eat one particular seaweed to obtain the panels, cannot build them, cannot pass them on to its young, and starts again from nothing every generation.
Now the sex, where things get properly baroque. Nudibranchs and their relatives are simultaneous hermaphrodites, both sexes at once, which sounds efficient right up to the moment you watch a negotiation. Sekizawa and colleagues reported in Biology Letters in 2013 that Chromodoris reticulata, on Okinawan reefs, mates for about ten minutes with each partner playing both roles, and then throws away the outer section of its penis. It has three of them coiled up inside, in series, and regrows the discarded piece in roughly a day. Elsewhere in the group, a Siphopteron sea slug undescribed at the time of the paper uses a stylet to inject prostate fluid directly into its partner's forehead, above the eyes, alongside the ordinary business of sperm transfer. Lange, Werminghausen and Anthes named this cephalo-traumatic secretion transfer in 2013. The suspicion is that the injected chemistry manipulates the recipient's reproductive decisions, and since both animals are hermaphrodites, both do it to each other.
And then the heads. Mitoh and Yusa, Current Biology, 2021. Young Elysia cf. marginata, as the paper carefully calls them, shed everything behind the neck in the lab, heart included, on their own initiative. The head closed the wound, began grazing algae within hours, and grew a whole new body, heart and all, in about three weeks. The discarded body does not grow a new head. It lies there responding to touch for days, in some cases months, presumably furious. In the second species they looked at, Elysia atroviridis, the individuals that did this were carrying parasitic copepods, and the rebuilt animals came back without them, which is either the reason for the whole trick or a very tidy coincidence.
None of the best ones are mine. The grey sea slug and the green elysia are Cornish and I have counted both with cold hands. The disposable penis is Okinawan, the forehead injection belongs to an animal that did not have a name yet when the paper came out, and the self-decapitation is Japanese laboratory work on slugs I will never see. My contribution to sea slug biology is fourteen wet mornings and a notebook with bad handwriting in it.
But the case holds. No engineer signs off a solar panel you have to eat, a weapon you have to steal from something trying to kill you, or a reproductive organ you throw away after one use. Every one of these is a bodge on top of an earlier bodge, and every one of them works well enough that the animal is still here and I am still lying on my front in February looking for it.