Marine Iguanas: yeah
Gary, there is a reason why they only stayed evolving in that one particular spot in the whole world! Still marks a question, if that means they are "rare" just because that's the only spot to see them anywhere... ? A matter of opinion I suppose, since it's remote for "you" to go there. But by the numbers of them it's not really, the same... M IG's are not in decline either.
I could say the same thing about the "gilled" goat from Vietnam.
Or Antarctic Penguins, are they considered rare, because that's the only place in the world you'll find them? uh, ok guys...
"Rare" to a biologist is something that is only a few hundred to a short few thousand strong and clearly declining. Possibly on it's way to exstinction. Any of these animals are threatened currently by changing conditions and man made pollution. Changing their habitat. However, none are on their way to becoming extinct as of yet.
(Sigh) A matter of distinct opinion, I suppose.
Certainly not worth pushing further. We all have them.
However,
Unlike those that visited areas and possibly study them whenever they can on "pleasure" trips. Logging in an unknown amount of hours diving each year... I've taken facts from people that are not collecting anything for sale. But studying. Real Biologists interested in conservation, not opening the flood gates for people to ruin areas they are struggling to protect. Numbers and conditions of things are not always reported accurately by various parties. For various reasons.
The Black Tang was discovered over 135 years ago. (It's lived way longer before that as well)
I don't see population decline (the math documented since then) and since it was found in a time that was challenging to dive and troll for fish with the technology back then.. it seems unlikely the species level has changed that much since. Unless you want to give man credit for affecting it. Yet since the area is so remote it's hard to see that as well. If Gunthers men found it back then, try talking to someone that stays in the area regularly. oh, thats difficult though, since a lot of people don't go there. The fact is believe it or not, even in Large numbers, people don't always know where to look for something all the time. The ocean has it's secrets that are hard to unfold. If he found and studied it back then, it seems unlikely that population has declined so, to: you hardly ever see one currently. Bit of a short time span. Even though 135 years may seem long to us humans. Undisturbed by us... for quite a long time.. It's a hard sell to buy. Especially from people who want to sell it and those pleasure diving not living with the fish at all times.
Basslets:
To date there have been no successful captive breedings of Basslets.
Yes, this certainly would help the hobby distribute them eventually for a lower cost.
However, the efforts since the '60s by people have not been enough to make it happen. The fish is attains a high price because no one is interested enough to go after it more aggressively. 'An why would they? They could make the cost go down for the market. But how much would they gain from a fish, that after it happened would still not be interesting enough to purchase enough to justify the long costs. It could be done. But it's not that lucrative with the effort of an undertaking to make it attractive.
This still says nothing about it's true numbers in the wild. It's not as rare as you think it is. This has to do with collection "efforts". Not how many are truly out there. If you based rarity on "fact" of what is there and capable of being there, you would know reality in the habitat is quite different than what the "hobby" that has a hard time acquiring enough of it, wants you to believe.
There are independant people with Billions of dollars, some want to build spacecrafts. Others ARE building Subs. Not to mention the things we have already accomplished in this world and out of it to date. Almost anything is possible to pull off. But unless it has something to do with "making money" it's not explored far enough as far accomplishing it. Thus there are many things we have not found, don't know the answers to. As well as things you are told are "rare", but we have barely begun to even open the door to knowing the real answers of what's out there and how much there is. Rare is sometimes to "our knowledge" as the public knows it. Not neccessarily the actual case with all things.
Prehistoric sharks are swimming the ocean at depths that we really can't go much to explore enough or fast enough for evidence of more of them because of the difficulties in doing so. Uh.. they were "exstinct"... maybe it's just "rare" now.. or maybe we are full of bologna because we really don't know the facts as they truly are... Just because you don't see enough(supposedly) of something.(ex: Tangs) Doesn't mean they are rare. There could could untold numbers living and thriving. With some species of fish and animals, possibly for millions of years.
The ocean covers most of the globe. Has anyone ever tried going from place to place to another and spotting something before it moves..? LOL you guys make me laugh.

The best equipment in the world may not get you to see that school of black Tang that's no longer there. However, somebody else can get a different experience when they go down sometime or somewhere else, just by chance.