Identification of Reef-building Corals
The identification of reef-building corals is complicated by the large number of species, many poorly-defined and/or undescribed species, and ambiguous terminology. An overview of terminology and classification, and a basic phylogenetic tree, are given below to provide context for the identification of Indo-Pacific reef-building corals.
Coral terminology and classification: Corals are marine cnidarians that occur as polyps (sac-like body with a single tentacle-surrounded opening), usually forming colonies of many individuals. Click here for a more detailed definition, here for a general overview of corals, and here for coral-related terminology. Corals belong to Phylum Cnidaria and Class Anthozoa, which is made up of Subclasses Ceriantipatharia (black corals and tube anemones), Octocorallia (AKA Alcyonaria or the octocorals), and Hexacorallia (AKA Zoantharia). Octocorallia includes the soft corals, sea pens, blue coral, and other groups. Hexacorallia includes the sea anemones and the scleractinian corals (Order Scleractinia). Black corals, scleractinian corals, and a few octocorals consist of a thin layer of tissue growing over a hard skeleton, whereas most soft corals have no solid skeleton. Another important group that secretes hard skeletons is the fire corals, which belong to a different cnidarian class (Class Hydrozoa: Order Milleporina), and are taxonomically not included with the corals.
Many hard (AKA stony) corals are hermatypic (reef-building) or zooxanthellate, meaning that they contain symbiotic unicellular algae (zooxanthellae) and live in shallow, warm waters where they secrete massive skeletons that form the physical structure of coral reefs. Ahermatypic (non-reef-building) or azooxanthellate corals are not reef-building, and they can be either hard or soft corals; hard, ahermatypic corals lack zooxanthellae and have much smaller skeletons than hermatypic species. Most soft corals are ahermatypic because they don't secrete a skeleton that can become part of the reef. We are concerned mostly with hard, hermatypic corals in the Pacific because they are the major reef-builders where we work (but some non-corals are important reef-builders too, especially crustose coralline algae). Minor reef-building corals include hermatypic soft corals, such as Tubipora spp. (organ-pipe corals), which secrete red, tubular skeletons, and Sinularia spp., which secrete semi-solid skeletons made up of spicules. However, these soft corals generally contribute relatively little material to the reef structure, and thus are not included here as major reef-builders.
The major reef-building corals are mostly
scleractinians, but also include
the Hydrozoan fire
corals (Millepora spp.) and an octocoral, the blue
coral (in the order Helioporacea, which contains just one species, Heliopora
coerulea, found only in the Indo-Pacific). These three orders of hard, hermatypic corals
can all be major
reef-builders, depending on the locality, but they are only distantly related to
one another - see phylogenetic tree of major extant reef-building corals below
(does not include extinct or minor reef-building corals). Click here for more info on coral
classification, but note current challenges to traditional coral classification.
Identification of Indo-Pacific Reef-building Corals:
The non-scleractinian reef-building
corals are few in number (about 50 nominal species of fire corals, and just one blue
coral, Heliopora coerulea), but they can make up a substantial proportion of the reef-building colonies on
some Indo-Pacific reefs (e.g., both abundant in American Samoa, but neither
found in Hawai'i). However, commonly available reference guides to reef-building
corals are often limited to scleractinians (>100 genera and thousands of
species), so we provide a brief overview here
of how to distinguish fire corals, blue coral, and scleractinians: Fire corals have a smooth surface covered in small pores of
two different sizes (but both ≤
1 mm) that are not distributed evenly over the
surface. Fire corals get their name from the
stinging cells (nematocysts) contained in the tiny, hair-like tentacles (dactylozooids) that
can usually be seen 
extending from the pores. Blue
coral get its name from
the intense blue color of its skeleton, which is not visible on live colonies
unless a piece has been broken off. Some blue coral colonies have similar morphologies
and colors as some fire corals. The polyps of blue coral are small, and like
those of fire
corals, appear as pores, but blue coral pores are of uniform
size and distribution. Furthermore, blue coral polyps have feathery tentacles
with 8 tips, as opposed to the hair-like dactylozooids of fire corals (right).
Scleractinian coral polyps inhabit individual cup-like skeletons, called calyces (singular caylx) divided by radial partitions known as septa that show hexagonal symmetry (click here for an intro to Scleractinia). The calyces join to form colonies of many shapes and sizes. Identifying the Scleractinia requires an understanding of how caylx and colony morphological characteristics are used to classify the families and genera - see van Woesik's guide to scleractinian taxonomy. The Australian Institute of Marine Science (AIMS) produces several helpful books and other product for the identification of reef-building corals worldwide, including Veron's indispensable 3-volume set of books (covers all reef-building corals), the website AIMS CoralSearch (scleractinians only), and a sophisticated coral searching system called Coral ID that can be purchased from AIMS. Because of the diversity of similar genera and species, morphological plasticity, and incomplete classification, there is no easy way to learn how to reliably identify reef-building corals. A strong grounding in general coral morphology and classification is a necessary first step before attempting to identify taxa in the geographic area of interest. Following are some resources we use to help identify reef-building corals in Hawai'i and American Samoa.
The
American Samoa is part of the Samoan archipelago, which is less isolated than Hawai'i, and has greater diversity of marine species, including reef-building corals. In American Samoa, about 140 reef-building coral species have been recorded, including blue coral and several fire corals (the rest are scleractinians). See the NPSA coral inventory for photos of some of these species. The photos can be used in conjunction with the species descriptions in Birkeland's guide to coral ID in American Samoa (no photos).