The free-living mushroom coral Polyphyllia novaehiberniae (Lesson, 1831) is known to occur in the West Pacific and is distinguished from its only congener, P. talpina (Lamarck, 1801), by a thinner and more fragile coral skeleton, and longer, more parallel arranged septa (Hoeksema 1989). During a survey in Northern New Caledonia in 2019, a 12-m2 aggregation of P. novaehiberniae was observed on a sandy reef flat 40 m away from the shoreline (20° 17′ 55.55″ S; 164° 5′ 0.73″ E) at 1.5-m depth at low tide (Fig.1a). This dense aggregation consisted of individuals of various sizes and growth forms, partly overtopping each other. No other individuals were observed within a radius of at least 100 m.

Fig 1
figure 1

aPolyphyllia novaehiberniae aggregation in New Caledonia (scale bar 10 cm) representing a wide range of growth forms; b Elongated form derived from sexual reproduction (i) and star-shaped form (ii) (scale bar 10 cm); c Round corals originating from regenerated fragments (i) and fused individuals regenerated from an empty stalk (ii) (scale bar 5 cm)

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While this type of mushroom coral field is usually exclusively derived from either asexual reproduction (Hoeksema and Gittenberger 2010) or sexual reproduction (Hoeksema and Benzoni 2013), this aggregation resulted from a combination of reproduction mechanisms. Typical elongated individuals showed no regenerated fractures and were derived from sexual reproduction (Fig.1b). Among the asexually reproduced individuals, round ones were formed around regenerating fragments (Fig.1c), while star-shaped specimens (Fig.1b, c) were derived from several polyps that had fused together when they were still asexually reproduced buds regenerating from a shared attached stalk, as previously observed in other mushroom coral species (Hoeksema 1989, Hoeksema and Yeemin 2011). This situation is exceptional since monospecific aggregations of free-living fungiids have never been reported as a result of a mix of three reproduction mechanisms: (1) sexual production through planulae settlement and coral detachment, (2) asexual production by fragmentation, and (3) budding of juveniles that had regenerated from previously vacated stalks (Hoeksema 1989: Fig. 42a–f, m, n, h–j). This situation is even more special, since monospecific aggregations of mushroom corals are relatively rare compared with those consisting of multiple species (Hoeksema and Matthews 2011, Hoeksema and Benzoni 2013).