Fire Corals (Millepora spp)

Still Drafting

From D&J Greenberg Guide to Corals and Fishes of Florida, the Bahamas and the Caribbean, 1977. Click on image for larger version

Overview: In the past I have relied on the descriptions of D&J Greenberg (1977) to distinguish 3 species of Millepora.

However, studying my photos and looking at more specimens in the field and referencing some of the more recent  scientific lit., I find I cannot definitively say that any are M. squarrosa, rather I see a continuous gradation/variation in forms between those that are, apparently definitively M. complanata, and those I have been calling M. squarrosa but now think are not.

To date (2015-2024),  I have not seen any of the thick walled types characteristic of M.squarrosa within the MPA (Marine Protected Area). On the other hand,  M. alcicornis is readily distinguishable, and abundant in some areas.

In some of the historical lit for Barbados (see below), M. squarrosa is  cited as present and abundant on some reefs. Lewis 2006 presents a photo of the typical thick-walled type which I can’t seem to find now – but he doesn’t give any quantitative estimates of abundance of M. squarrosa versus M. complanta and in earlier papers, he cited only one species of Millepora (M. alcicornis). Other authors give quantitive data but there are not any photos of specimens, so they could have been using the same, rather loose criteria I was using to distinguish M. squarrosa. 


The identification/taxonomy of Millepora species in the Caribbean (and elsewhere) has been problematical. Bellairs founder and its first Director, John Lewis, published several papers on Millepores. His reports of which species occur in Barbados changed over time, reflecting new knowledge more observations etc.

In his paper on the fringing reefs of Barbados, Lewis (1960) mentions only one species, Milleora alcicornis; he cites it as common in three of his 6 zones:

– Reef Flat: not cited
Diploria-Palythoa :  “The hydroid Millepora alcicornis is often abundant on the rock surface. The colonies are short, stout, and encrusting, or form short blade-like structures.
Reef Crest: “Of the other [than true corals] invertebrates found in the reef crest zone, the hydroid Millepora alcicornis is the most common (Fig. 8). It forms clumps of blade-like structures on the surface and grows up to the water level of low water spring tides.
– Seaward Slope: not cited
– Reef Front: not cited
Deep Water Communities: Millepora alcicornis is abundant and forms delicately branching structures well as well knotted encrustations on the branches of dead corals

In the early 1970s, Two McGill geologists, CW Stearn & R. Riding published this paper:
Stearn, C. W. & Riding, R. : Forms of the hydrozoan Millepora on a Recent Cora reef. Lethaiu, Vol. 6, pp. 187-200. Oslo, 15th April, 1973. available as PDF here. They examined the forms of Millepora on the “fringing reef immediately offshore from the Bellairs Research Institute of McCill University at Holetown and to the top of the First Ridge, 600 m to the southwest” Their perspective is that of geologists who use knowledge of current-day reefs to interpret ancient reefs.

ABSTRACT Distribution of the calcareous hydrozoan Millepora in the shallow water off the west coast of Barbados supports the view that morphologic variation in this genus is not primarily the response of a single species to environmental factors. Four forms are present on the fringing reef (1 to 6 m depth) and on an offshore ridge (1 1 to 17 m depth). Branching, bladed, and boxwork forms co-exist at depths below 2 m and are thought to be genetically distinct (biospecies). They are equivalent to M.
alcicornis, M. complanata, and M. squarrosa respectively. With decreasing depth they disappear in the order: branching, bladed, boxwork. This sequence reflects increasing colony strength and appears to be governed by increasing water agitation. The encrusting form is commonest in very shallow water, but also occurs in deeper water growing on gorgonians. It appears to be an environmental growth form of the
other species

Stearn & Riding provide detailed descriptions of the 4 forms.

Effects of eutrophication on reef building corals. Part II. Structure of scleractinian coral communities on inshore fringing reefs, Barbados, WI.
Tomas (Tom) Tomascik & Finn Sander Marine Biology 94(1):53. Three hydrozoans, Millepora alcicornis Linnaeus, M. squarrosa Lamarck, M. complanata Lamarck, and a colonial zoanthid Palythoa mamillosa (Ellis and Solander), were included in the study, since they were considered to be an important component of the fringing reef community
Frm Table 2: Average relative coverage (RCC % per transect) for 7 reefs:
M. squarrosa 2.3 to 8.3% cover
M. complanata 0.9 to 17.5
M. alcicornis 0.1 to 1.6

Lewis, J. B. (1996). Spatial distributions of the calcareous hydrozoans Millepora
complanata and Millepora squarrosa on coral reefs
. Bulletin of Marine Science 59,
188–195. PDF available here His description of the two species:

Both species, MilLepora complanata and M. squarrosa are common on the fringing reefs of Barbados and occur within the same reef zones (Lewis, 1960; Stearn and Riding, 1973). However, whereas M. complanata colonies are amorphous and ill-defined in shape with irregular, encrusting bases which may cover several m2 and prominent, upright, bladelike branches (Fig. 1a), M. squarrosa 190 BULLETIN OF MARINE SCIENCE. VOL. 59, NO. I. 1996 forms rounded, discrete colonies up to 25 em in diameter with thick, united plates (Fig. ]b).

This seems to be the first time he mentions M. squarrosa in Barbados

In 2006, Lewis published a comprehensive review on “Biology and Ecology of the Hydrocoral Millepora on Coral Reefs (Advances in Marine Biology Volume 50, 2006, Pages 1-55). Under a section on Systematics and History he notes

There still remain taxonomic problems in the genus Millepora (de Weerdt, 1981, 1984; Estalella, 1982). Although some 50 species of millepores have been described (Veron, 2000), 11 extant species of Millepora from the Pacific and six from the Atlantic are recognised (de Weerdt and Glynn, 1991; Cairns et al., 1999). A diYculty lies in the variable phenotypic forms of colonies of each species with no apparent stable morphological characteristics of taxonomic value as well as a lack of unambiguous morphological diagnostic characteristics between species… it is likely that studies involving molecular systematics may be required to confirm species identifications.”

In that paper in Fig 2. he presents a colour photo of  “Densely packed colonies of Millepora complanata on a fringing reef at Barbados, depth 2 m.”, evidently, the equivalent of what he had described in 1960 as M. alcicornis in the Reef Crest zone. In the same (2006)article he provides descriptions/photos of M. alcicornis; these are distinctly branching types. His Fig. 3  is a photo of  “Millepora squarrosa on a fringing reef at Barbados, depth 1 m. Colony is 15 cm in width”

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs
Caroline E. Dubé et al., 2019. Ecology, biology and genetics of Millepora hydrocorals on coral reefs. Ch 2 in Invertebrates-Ecophysiology and Management, p.15ff

Nunes, F. & Sweet, M. 2022. Millepora squarrosa. The IUCN Red List of Threatened Species 2022: e.T133143A179619810. Accessed on 26 February 2024. “Crustal Fire Coral Millepora squarrosa has most recently been assessed for The IUCN Red List of Threatened Species in 2021. Millepora squarrosa is listed as Critically Endangered under criteria A3c….This coral species has a relatively small range and can be locally common. Global level, species-specific population data are limited; however, coral reefs have declined globally and are expected to continue rapidly declining due to increasing severe bleaching conditions under temperature stress caused by climate change as well as a variety of other threats…”

Amaral, Brazil paper