The fragile mushroom coral Halomitra clavator Hoeksema, 1989 (Scleractinia: Fungiidae) is known from only a few localities in the central Indo-Pacific (Hoeksema 1989, 1993). It can be distinguished from its more common and widespread congener, H. pileus (Linnaeus, 1758), by its thinner corallum, club-shaped septal spines (opposed to sharp), and cream-coloured coral margin (opposed to violet, Fig. 1).

Fig. 1
figure 1

Halomitra clavator at West Halmahera. a Small (2.5 cm) anthocaulus showing primary mouth at its centre. b Anthocaulus (9 cm) with three secondary mouths (small arrows) around the primary mouth (large arrow). c, d Patch of small (10–15 cm) regenerating fragments. e Cluster of large (25–40 cm) regenerated corals

Full size image

During a biodiversity survey off West Halmahera (northern Moluccas) in 2009, H. clavator was observed at four out of 39 reef sites (10%). This is nearly as common as in Madang Lagoon (Papua New Guinea), where it was found at three sites out of 24 (12.5%), with specimens up to 1 m in diameter (Hoeksema 1993).

At a submerged reef in Halmahera’s Dodinga Bay (0°51.152′ N, 127°35.325′ E), several complete corals (2.5–25 cm diameter) were found at 18–20 m depth. These included the smallest H. clavator specimens in attached anthocaulus stage ever recorded (Fig. 1a–b). At another site (Guraici I., 0°1.283′ S, 127°14.287′ E; 17–22 m deep), monospecific patches (3–5 m wide) of regenerating mushroom coral fragments were found (Fig. 1c–e). Many were positioned on top of others, resulting in layers of up to three corals and densities of >100 m−2. This situation is exceptional since previous reports on high concentrations of asexually reproducing mushroom corals concerned less rare species: i.e., H. pileus (Linnaeus, 1758), Zoopilus echinatus Dana, 1846, and Fungia fralinae Nemenzo, 1955 (Pichon 1978; Littler et al. 1997; Hoeksema 2004).