(Carrying over the Seadragon musing I did over on Twitter)
I've been thinking a bit about the biology of the Seadragonus Giganticus Maximus.
Mainly, the reason why there are so few of them.
I briefly considered the concepts for the movie franchise's Red Death, but it's a bit too bloodthirsty for my taste(they have a baby battle royale until one survives)
Anyway, I think Seadragons are born so incredibly weak that most of them don't make it to a big enough size to survive. Their chances of survival are so miniscule that their existances are by pure chance, twists of fate- hiccups, so to speak. Yes, sweet sweet parallels. Seadragons are born near the bottom of the food chain and work their way up the more they grow, until they hit their life's peak as the apex predator.
When that peak happens varies from dragon to dragon, probably relative to their nutrition and environment they grow in. Considering Furious was already baby elephant-sized when Hiccup the Second was 7 while Toothless was tiny all of Hiccup the Third's life, I like to think harsher environments spur the growth process for Seadragons to increase their chances of survival. Toothless is living the pampered life of a cat for the most part, while Furious grew up in the wild. I do think slower grown, healthier Seadragons might end up bigger and/or stronger when they hit their peak... so who knows how massive Toothless will get.
I also thought it was interesting to note that despite their names Seadragons that weren't full grown didn't really seem to live in the sea, or at the very least have sea-creature attributes like fins and webbed feet. No particular affinity for water either. Honestly, it might just be that they grow so big that the ocean is the only place they can comfortably roam and rest in. They probably don't even need to swim at that size!
That's about all I can think of for the time being. If anyone else has more ideas they would like to add on, or correct a mistake I might have made here, feel free to let me know!