This page links to photos of some of the hydrocorals we have seen in the Marshall Islands. Hydrocorals, although they create calcium carbonate skeletons similar to those of reef-building scleractinean corals, are actually in a different class, the Hydrozoa, which also includes the soft, feather-like hydroids. Some Hydrozoans, including the hydroids and fire corals (Millepora) possess potent nematocysts capable of delivering a painful sting to some of the more sensitive areas of human skin. Others, like the mostly pink Stylaster and multicolored Distichopora, do not have a sting powerful enough for a person to feel.
The World Register of Marine Species lists 26 species in the genus Distichopora, but it is not clear which of those our photos represent. Several different species or color forms are found in the Marshalls, but at times colors seem to blend together, suggesting that color may not play much of a role in differentiating species. It is possible that several or even all of our color forms are variations of one species. However, there are some unusual distribution patterns among the various colors in places we have visited in the Marshalls. In the southern part of the Kwajalein, only the purple variety is present. Roughly 25-30km up the reef northwest of Kwajalein, a bright yellow variety begins to show up on the seaward reef and on lagoon pinnacles, particularly those swept by tidal currents near reef passes. About 55km south of Kwajalein at Namu Atoll, the seaward reef is adorned with a bright red variety that forms larger colonies than the purple and yellow at Kwajalein, where the red variety is not present at all. About 680km northwest of Kwajalein at Enewetak Atoll, the red form is present, but smaller and more delicate than the robust colonies at Namu. Enewetak also commonly has what is probably a more orange variety of the Kwajalein yellow one, particularly on the western tip of the seaward reef, an area called the West Spit.
These are the fire corals, so called because their powerful nematocysts are capable of delivering a painful and long lasting sting to bare skin. Some of the colonies form interesting and photogenic castle-like structures.The World Register of Marine Species lists about 15 species of Millepora worldwide, but it is not entirely clear which of those our photos represent. The paddle-like Millepora platyphylla and columnar M. exaesa seem to blend together in some colonies, and we have seen some of M. platyphylla develop branching tips at the tops of their colonies that look like M. tenera. Unfortunately, all of these fire corals at Kwajalein are under severe stress from warming ocean water, to which they are particularly sensitive. They are among the first coral species to bleach out when the water reaches 30°C (86°F), which is happening ever more frequently as climate change progresses. Despite diving at Kwajalein during periods starting in 1965, we first saw coral bleaching in the fall of 2009, which was also the first time we saw the water temperature reach 30°C. Most lagoon Millepora colonies bleached and many died. The temperature hit 30°C again in the falls of 2013, 2014 and 2016, this time killing off most of the remaining lagoon Millepora as well as the seaward reef colonies that had largely survived 2009, as well as affecting and often killing many other species of reef corals. By the time we left island in 2017, I'm not sure I could have found a living colony of Millepora. Bleaching happened again in 2017 and very seriously in 2018, which may have caused this genus to become local extinct at Kwajalein.
Stylasters are often called the pink corals since many, although not all, are basically pink in color. They are common, sometimes abundant, in ledges and caves, usually out of direct sunlight, from very shallow water down as deep as we have gone in the lagoon and on the seaward reefs. Like the other hydrocorals, species are neither well known nor easy to identify, so we refer to all of them here as Stylaster sp. There are likely other species or forms we have not yet recognized.
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