Keeping Goldfish

[naiad]

Seems that bony fishes ancestors had mutations which selected for calcium/phosphorus accumulation in the internal cartilage (bones) rather than calcium/phosphorus in the external scales, leaving the scales more flexible but the bones firmer. (I had thought this was due to living in freshwater, but now I think it was more likely a chance mutation.)

Possibly. The change from cartilage to bone was a gradual process, so many mutations must have been involved. The lampreys, hagfish, sharks and rays have an entirely cartilaginous skeleton. The sturgeons, lungfishes and other primitive fishes have some bone in the skull, but the rest of the skeleton is cartilage. The most advanced groups of bony fishes have an entirely bony skeleton.

The pearlscale goldfish is an example of a mutation in the calcium/phosphorus accumulation process in the bones/scales. It has calcium deposits in the middle of each scale (the 'pearls'). The skeleton seems to have lost out in the process - it is the shortest of all the fancy breeds, and is the closest a fish can get to being spherical. Swimming and buoyancy control are major challenges for this breed. Some are even shorter than the ones in the pictures.
http://www.bristol-aquarists.org.uk/goldfish/pearlscl/pearlscl.htm

I'm wondering where the reptiles got their keratin...I think there is a frog which during the larval stage has a keratinous snout and/or keratinous pseudo-teeth, this too could have spread from the mouth out around the body as scales.

Some frogs do have claws (African and South American clawed frogs) so they can produce keratin.

Platypus males have poison ankle spurs, are there any fish with poison spines or glands on the rear non-caudal fins? (I forgot the proper name for the rear lateral fins) Catfish have poison spines on the front fins and dorsal fin but not on the rear fins AFAIK.

The rear lateral fins are the pelvic fins. I can't think of any fish with poison spines on them, except possibly the lionfish, which has poison spines on most of its fins. This could be because the other fins are more easily used for defence. The dorsal fin can be used by a bottom-dwelling fish to fend off attack from above, and the pectoral fins can be stuck out as the fish lashes from side to side.

 

[wet]

[I sort of randomly copied from science blog Tetrapod zoology 3+ page article on frogs.]

Ghost frogs (so named because one species comes from Skeleton Gorge in South Africa) are a small group (6 species) of southern African species, specialised for clinging to rock surfaces along mountain streams [adjacent image shows Table Mountain ghost frog Heleophryne rosei]. Their tadpoles have an oral sucker-like disc and lack keratinous beaks (though they do possess denticles). Suggested at times to be close relatives of the Australasian myobatrachids (more on this group later), ghost frogs were found by Haas (2003) to be outside of Neobatrachia, and by Frost et al. (2006) to the sister-taxon to the rest of Neobatrachia (a taxon they named Phthanobatrachia). Genetic data led Van der Meijden et al. (2007) to link ghost frogs with the cannibal frogs* (Lechriodus), conventionally classified within Myobatrachidae, and the Chilean helmeted water toad (Caudiverbera caudiverbera) [a species I covered briefly here], conventionally - and probably erroneously - included within Leptodactylidae.

* It's a dumb name, given that cannibalism is not exactly rare among anurans.

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While Seychelles frogs have (virtually) always been regarded as neobatrachians, workers have disagreed as to whether they are closer to hyloids or ranoids, and this is still an area of contention. The four extant species of Seychelles frogs (the newest was named in 2002) are all tiny (SVL 10-40 mm) terrestrial frogs of leaf litter or arboreal habitats in high altitude forest [adjacent image shows tiny Gardiner's Seychelles frog Sechellophryne gardineri*]. They all lay their eggs on land; the young either undergo direct development, or the non-feeding tadpoles are carried on the back of the mother. They lack external vocal sacs and middle ear ossicles, so should be both voiceless and deaf... but they aren't, as at least some species make calls (Gerlach & Willi 2002).
[by Darren Naish at Tet Zoo]
Tetrapod Zoology : Ghost frogs, hyloids, arcifery.. what more could you want?

Incredibly diverse frogs and toads! Yet all based on quite similar body morphology.