There is an artificial, albeit useful, convention amongst hobbyists distinguishing hard or stony corals (Order Scleractinia) on the basis of their apparent individual polyp size: Large and Small. Small Polyp Stony corals are made up of miniscule polyps generally borne on branching, encrusting or plate-like skeletons.
The artificiality of this arrangement is borne out in the membership of both SPS (Small Polyp Stony) corals and LPS (Large Polyp Stony) corals existence within the same families, sometimes genera. However arbitrary the division, there is general utility in the visual distinction of Small versus large Polyp corals. In a few words, SPS' prove harder to keep than LPS'. This is often cited as being due to the greater cleanliness and stability of environmental conditions where SPS' are found.
(Images #2,3: A LPS coral, Caulastrea furcata, Family Faviidae and a member of our family of SPS', Acropora cerealis. LPS' polyps are, well, larger than SPS')
Indeed, as many of the popular LPS' are collected in shallow back reef areas, the majority of SPS' hale from reef flats to deeper offshore environs where there is definitely less silt, and more homogeneous light and brisk circulation conditions.
For the hobbyist there are obvious practical implications. SPS require more stable and optimized environmental conditions; strong lighting, high water quality and even temperatures. Nonetheless the smaller polyped scleractinians have many endearing qualities; subtle to brilliant colors, fast growth and a propensity for culture through fragmentation that endear themselves to advanced reef hobbyists.
Amongst the stony corals, large, medium or small polyped, there are none larger, literally and figuratively than the Staghorns, Family Acroporidae ("Ackro-pore-id-ee"). This family contains the true corals number one and two most speciose genera (Acropora, Montipora). As reef builders the family rules supreme; their hermatypic colonies are the most important worldwide in terms of mass and volume.
Family Acroporidae, Classification:
Staghorn corals come in many shapes and all colors... and these traits can be highly variable per species. Most are typically branched, table-top shaped, or encrusting per type, but colors often ran the gamut of browns, whites to pinks, blues, yellows, greens, even purple, depending on growing conditions. As with other true or stony corals (Order Scleractinia) real determination to the species level rests on close examination of corallites (individual polyp skeletons), biochemical and genetic study. For aquarium identification, they are best labeled down to the genus ranking. All identifications here are tentative.
Genera, Species of Aquarium Interest: (To Do: Need to insert species by genera and images, links per Bklet section BLR)
All told the family Acroporidae contains four extant genera and several hundred species, of which dozens make their way into the aquarium interest.
Acropora, with a couple of hundred species, are what most people picture when they hear the word "coral". Most are branched tree-like or interwoven, with fast growing and often differently colored apical corallites (growing tips). These are the mass spawners, with their axial corallites releasing sex cells seasonally. Due to their fast growth (sometimes more than an inch a month), mix of available species and colors, this is a favorite genus of advanced reef aquarists. This genus is divided into fifteen 'groups' on the basis of gross physical appearance (See Fossa & Nilsen, Veron) with some correlation with aquarium survivability. (Images # . Acropora of the Caribbean, three spp.)