Diffs between the two Blasto's?

[Titus07]

What are the main differences? Do both come in a variety of colors?

 

[yellowslayer13]

one big one small one rarer one commoner it is that simple

 

[Hormigaquatica]

...it is that simple

No... no it isnt... lol

Taken from the AIMS database:

Blastomussa merletti: Characters: Colonies are phaceloid to plocoid, and consist of a few to large numbers of corallites. Corallites are less than 7 millimetres diameter. Septa are mostly in two cycles of which only the first reaches the columella. Septa have slightly serrated margins. Primary septa may be exsert. Columellae are poorly developed. Mantles, but not tentacles, are extended during the day and may form a continuous surface obscuring the underlying growth-form. Colour: Commonly dark red with conspicuous green oral discs. May also be pink, orange, brown or uniform dark grey with white margins to primary septa. Habitat: Reef environments, especially where the water is turbid. Abundance: Uncommon.

Blastomussa wellsi: Characters: Colonies are phaceloid, rarely subplocoid. Corallites are 9-14 millimetres diameter. Septa are not arranged in cycles and are numerous. They have small blunt teeth. Mantles, but not tentacles, are extended during the day and may form a continuous surface obscuring the underlying growth-form. Colour: Mantles are usually dark grey, but may be red or green. Oral discs are usually green but may be red or dark grey. Habitat: Lower reef slopes protected from wave action, and turbid environments. Abundance: Uncommon, rare in the Red Sea.
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B. merletti tends to be easier for hobbiests to keep- they seem to be more tolerant of differing tank conditions. Neither species really likes strong light or water flow (B. wellsi virtually always closes up or receeds when exposed to sudden intense light- keep in the shade if youre using halides). B. merletti also grows significantly faster than B. merletti. B. merletti should also be significantly cheaper (per polyp) than B. wellsi.