What got me wondering was the original Japanese version of that famous haiku by Kobayashi Issa:
katatsuburi soro-soro nobore fuji no yama
‘Snail, slowly climb Fuji's mountain!’
(You may prefer R.H. Blyth's translation: “O snail / Climb Mount Fuji, / But slowly, slowly!”)
The more common form for ‘snail’ is katatsumuri, so in Japanese the choice offered above is: Katatsumuri ka? Otamajakushi ka?
One thing katatsumuri has going for it is its five syllables (or moras), perfect for an opening or closing line of haiku. (Snails feature in 53 out of Issa's 8000 haiku.) But snails seem to lack any specific seasonal association, at least according to Yamamoto Kenkichi's The Five Hundred Essential Japanese Season Words.
Otamajakushi ‘tadpole’, on the other hand, has good seasonal associations. But its six syllables can only fit well into the middle line of haiku, with the help of a filler particle like ya. Maybe that's why Issa wrote 166 haiku about frogs (... tobu kawazu ‘jumping frog’, ... naku kawazu ‘croaking frog’, kawazu kana ‘frog ...’ etc.), but apparently none about tadpoles. It's not really that hard. Here's one I made up.
atsumono ni / otamajakushi ya / kawazu kana
In the broth / Is it a tadpole / Or a frog ...
Regardless of their poetic qualities, these two words have interesting etymologies.
お玉杓子 otamajakushi literally means a ball (otama) ladle, scoop, or rice paddle (shakushi), which well describes the shape of a tadpole. Well, okay, a giant tadpole.
蝸牛 lit. ‘snail-cow’ can be pronounced kagyū in its Sino-Japanese reading, but the kanji have no relation to the several native Japanese words for ‘snail’: katatsumuri, katatsuburi, dedemushi, dendenmushi. When I was a kid, I learned dendenmushi, but I had forgotten whether it meant ‘snail’ or ‘caterpillar’. If I had to guess at the native Japanese etymologies for the two principal words for ‘snail’, I would propose that dendenmushi comes from dandan ‘little by little’ + mushi ‘bug’ (i.e., a slow-moving creepy-crawly); while katatsumuri comes from kata- ‘hard’ + tsumuri ‘head’ (i.e., a hard-shelled creepy-crawly).
I await further instruction from Matt of No-sword.
UPDATE: And Matt comes through with a much more thorough and enlightening post! He finds support for the same etymology for katatsumuri (‘hard head’), but explains that de(n)de(n)mushi comes from a recent song appealing to the snail to come out (hence 出 de-) of its shell. Better yet, the latter term, the only one I had heard, seems to have arisen in Kyoto, where I spent my elementary school years.
UPDATE 2: Matt of No-sword follows up with a wonderfully clarifying post on one of the murkiest issues in contemporary herpetological poetics, the difference between two types of frogs in Japanese: the poetic kawazu and the aquatic pedestrian kaeru.
Before I repeat Matt's far more poetic conclusion (which Language Hat has already done,
Only three amphibians are native to the [Polynesia-Micronesia biodiversity] hotspot, and all are ranid frogs of the genus Platymantis. Two species are endemic to Fiji, the Fiji tree frog (Platymantis vitiensis) and Fiji ground frog (P. vitiana, EN), and one, the Palau frog (P. pelewensis), is endemic to Palau. All three species are related to other Platymantis species in the Solomon Islands and in New Guinea.This explains why, of all the Micronesian dictionaries I consulted, I could only find a native word for ‘frog’ in the Palauan Dictionary: bechébech, where the unaccented e represents a schwa (uh, er sound) and the ch represents a glottal stop. (By the way, the E[uro]speranto words for ‘frog’ and ‘toad’ are rano and bufo, respectively.)
So here's the enlightening conclusion to Matt's follow-up frog post, which cites one of the most famous haiku of all by perhaps the most famous haiku poet of all.
古池や かはづ飛び込む 水の音
Furuike ya/ Kawazu tobikomu/ mizu no oto
Old pond/ Frog jumps in/ Sound of water
Bonus fact: Bashō was actually consciously playing with the kawazu tradition here by attributing the sound to the water rather than the frog. The frog's implied silence, after centuries of naku kawazu [‘croaking frog’], is a crucial part of the stillness that allows the sound of water to make its impact.