Mushroom corals (Scleractinia, Fungiidae) form a common element in the fauna of most Indo-Pacific coral reefs. Depending on the species, full-grown specimens are either attached or free-living, which are character states occurring in various evolutionary lineages and therefore do not necessarily reflect phylogenetic relationships among the Fungiidae (Cairns 1984, Hoeksema 1989, 1991b, 2009, Gittenberger et al. 2011, Benzoni et al. 2012). After settlement, each fungiid individual starts as a small attached coral (anthocaulus). While corals of attached species obtain a foliaceous or encrusting growth form (Hoeksema 1989, 1993a, 2009, Gittenberger et al. 2011, Benzoni et al. 2012), those of free-living species eventually become detached from their stalk, reaching the anthocyathus stage (Wells 1966, Hoeksema 1989, Hoeksema and Yeemin 2011, Hoeksema and Waheed 2012). Owing to their charismatic appearance, abundance and large polyp size, these free-living fungiids are usually easy to find. By growing large and occurring in high densities, they may form mono- or multi-species assemblages covering extensive reef areas (Littler et al. 1997, Hoeksema 2004, 2012a, Elahi 2008, Hoeksema and Koh 2009, Hoeksema and Gittenberger 2010, Hoeksema and Matthews 2011, Hoeksema and Benzoni 2013). They can live on various kinds of reef substrates, ranging from silt to solid rock, from nearshore to offshore and from shallow reef flats to deep reef bases (e.g., Hoeksema and Moka 1989, Hoeksema 1991a, Hoeksema 2012a, 2012b, Waheed and Hoeksema 2013, in press). Mushroom corals themselves may in turn act as habitat to associated faunas consisting of molluscs, crabs, shrimps, acoel flatworms, comb jellies, and fish, some of which are host-specific (Hoeksema and Kleemann 2002, Kleemann and Hoeksema 2002, Gittenberger and Gittenberger 2005, 2011, Hoeksema and Fransen 2011, Owada and Hoeksema 2011, Bos 2012, Hoeksema et al. 2012, 2013a, 2013b, Hoeksema and Farenzena 2012, Gittenberger and Hoeksema 2013, Van der Meij and Hoeksema 2013). The various mushroom coral species vary in size (Hoeksema 1989, 1991b, Gittenberger et al. 2011) and it is obvious that corals with large surface area and thick skeletons offer most habitat space for associated fauna in contrast to small species (Hoeksema et al. 2012).

The smallest species among free-living mushroom corals appear to be among the most difficult to identify because they show relatively few distinguishing characters and much ecophenotypic variation (Hoeksema and Moka 1989, Hoeksema 1993d). Döderlein (1901, 1902) classified them as the “patella group” within the genus Fungia Lamarck, 1801. He considered them to be the most “primitive” species owing to their small size, imperforate (solid) corallum wall, and rudimentary, hardly discernible costal spines and septal dentations (see Scholtz et al. 2012). The fossil record of this Fungia patella group could be traced back to the Cretaceous, while its distribution ranged from eastern Africa to the west coast of America and its maximum depth was known to be over 100 m (Döderlein 1901).

Previously, species of the Fungia patella group were considered to belong to the genera Cycloseris Milne Edwards & Haime, 1849, consisting of complete corals, and Diaseris Milne Edwards & Haime, 1849, representing radially fragmenting corals of the same species (see Hoeksema 1989). Döderlein (1902) already considered complete and fragmenting corals to be different forms that can be found within each species. He recognized the following six species and synonyms: 1. Fungia patella Ellis & Solander, 1786 (including Fungia patellaris Lamarck, 1801, Fungia tenuis Dana, 1846, Fungia hexagonalis Milne Edwards & Haime, 1849, Diaseris fragilis Alcock, 1893); 2. Fungia erosa Döderlein, 1901; 3. Fungia distorta Michelin, 1842; 4. Fungia cyclolites Lamarck, 1816; 5. Fungia elegans Verrill, 1868; 6. Fungia costulata Ortmann, 1889. Some authors still consider Diaseris a separate genus (e.g. Veron 2000, Claereboudt 2006) while there is morphological evidence for their synonymy (Hoeksema 1989, Hoeksema and Waheed 2011, 2012).

Oddly, both Fungia patella and Fungia patellaris are junior synonyms of Fungia fungites (Linnaeus, 1756), the type species of Fungia (see Hoeksema 1989). However, Döderlein's (1902) photographs of his Fungia patella corals show that he referred to complete and fragmenting specimens of the free-living species Cycloseris fragilis (Alcock, 1893) and Cycloseris sinensis Milne Edwards & Haime, 1851 (see Hoeksema 1989). The outlines of Döderlein's (1902: pls. 1–2) smallest specimens are either circular or hexagonal.

Van der Horst (1921) mentioned six specimens of Fungia patella (sensu Döderlein), which he obtained from the Siboga Expedition collections: “The 3 Cycloseris-forms of Stat. 315 have both surfaces flat. Two of these specimens have the ribs obviously only at the edge of the corallum, the centrum of the under surface being irregularly covered by fine granulations. The septa of these two specimens are equal in height and very much grained. They have the appearance of Fungia patella var. filigrana Död. In the third specimen (dimensions 52 × 52 m.M.) the ribs reach the centrum, the scar of the attachment is here obvious. The edge of the corallum is slightly undulating.”

Boschma (1923a) had access to only the first two of these specimens and also to mushroom corals collected during the Danish Expedition to the Kei Islands in the Banda Sea, eastern Indonesia, and to other corals from Hawaii, among which were small specimens that according to him belonged to “Döderlein's, patella-group”. He recognized eight separate species besides Fungia pattella (sensu Döderlein 1902), which previously were considered synonyms: 1. Fungia patelliformis Boschma, 1923; 2. Fungia fragilis (Alcock, 1893); 3. Fungia marginata Boschma, 1923; 4. Fungia distorta (Michelin, 1842); 5. Fungia sp.; 6. another Fungia sp.; 7. Stephanophyllia neglecta Boschma, 1923; 8. Fungia vaughani Boschma, 1923. Several species were represented by complete and fragmenting shapes. Among the complete corals, several had a hexagonal outline, such as juvenile specimens of Fungia tenuis (Boschma 1923a: pl. 9), which actually belong to Fungia hexagonalis (see Boschma 1925, Hoeksema 1989). He had many specimens available to him that he identified as Fungia marginata; some of these were illustrated together with the original description (Boschma 1923a) and other ones in a subsequent publication (Boschma 1925). In the diagnosis of Fungia marginata, Boschma (1923a) refers to a circular corallum outline, although he also states that young stages are hexagonal in shape. According to him this species could be distinguished by its thick corallum margin, which might be true if compared with for instance his Fungia patelliformis (a synonym of Cycloseris fragilis (see Hoeksema 1989) but not with some other species, such as Cycloseris costulata (see Hoeksema 1989).

The two specimens (ZMA Coel. collection) earlier described by Van der Horst (1921) as Fungia patella from Siboga Expedition Station 315 in the Paternoster Islands, Indonesia were considered syntypes (Van Soest 1979). One of these syntypes is a coral with a circular outline and a diameter of 48.5 mm (ZMA Coel. 604, herein designated lectotype; Figure 1), whereas the other is a smaller specimen with hexagonal outline, a diameter of 13 mm, and relatively thick primary costae (ZMA Coel. 723, herein designated paralectotype; Figure 2). Based on these two types, Fungia marginata is considered a junior synonym of Fungia costulata (Hoeksema 1989). Because of its doubtful identity, the juvenile specimen is not useful as type. Boschma (1923a) assumed that Fungia costulata does not have a solid corallum wall and has more or less equal costae and therefore described Fungia marginata as a species with a solid corallum wall, unequal costae, and thick corallum margin as distinguishing characters. Actually, the type of Fungia costulata does have a solid wall, whereas costae of all mushroom coral species may be dissimilar in size, including Fungia costulata (Hoeksema 1989, Gittenberger and Hoeksema 2006). Furthermore, the identity of Fungia marginata has never been really clear, because most subsequent authors confused it with other species (see Hoeksema 1989).

Boschma (1925) had access to many more Fungia marginata specimens according to the accompanying identification labels, but he did not mention them in the original species description. These specimens are still available in the collections of the Zoological Museum – University of Copenhagen (UZMK) and Naturalis Biodiversity Center (RMNH Coel.). Among these corals, several specimens are slightly larger than the syntypes and they actually do show enlarged lower order costae, although this was not clearly illustrated in his plates (Boschma 1925: pls. 5–6). These specimens are re-examined in the present study along with many small mushroom corals collected during various recent field surveys (1983–2013). The last ones were striking because of their outstanding long and thick costae and their colorful appearance in comparison with the usual brown hues found in individuals of related species (Hoeksema 1989, Gittenberger and Hoeksema 2006). Based on this unique set of characters consisting of a relatively small adult size, enlarged primary order costae and variable coloration, these corals are considered to belong to a new species, herein described as Cycloseris boschmai sp. n.

All species of the former Fungia patella-group belong to Cycloseris, previously considered a subgenus of Fungia (see Hoeksema 1989). As a consequence of recent molecular phylogenetic studies, so far 13 species have been recognized in the genus Cycloseris, three of which are attached (Gittenberger et al. 2011, Benzoni et al. 2012) and not part of the old Fungia patella-group. The new species, here described as the free-living Cycloseris boschmai sp. n., is the fourteenth. Specimens were either available in museum collections or were photographed and sampled during SCUBA diving in the field. In addition to a detailed description, many illustrations are presented to show the phenotypic variation of this small Cycloseris species, following in the foot steps of Döderlein (1902) and Boschma (1923a, 1925). For the purpose of specimen identification, a key to the presently known free-living extant Cycloseris species is given.

Although some material of Cycloseris boschmai sp. n. was already available in museum collections (RMNH, UZMK), the species could only be discovered because of much fieldwork (1983–2013) with proportionate opportunities for observations and sampling to enable separation of the new species from resembling ones. Boschma (1923a, 1925) might have had the same species in mind when he described and studied Fungia marginata, but his selection of type specimens from the Paternoster Islands and the unclear comparison with other species of Döderlein's, Fungia patella group are not convincing. He described this species because of its supposedly thick corallum margin as compared to the other species in the Fungia patella group, but this character is not useful when applied several other Cycloseris species.

The Fungia marginata material from Banda is suitable as type material of the new species. Because the Banda specimens were wrongly identified by him, this does not concern a new name for an existing species but an entirely new species (Hoeksema 1993b). These old museum specimens are not just useful as type material but also because they supply information about habitat (field data) and asexual reproduction by budding. They also constitute the oldest material of Cycloseris boschmai, which gives them potential historical value as baseline material in studies on changing coral faunas (Hoeksema and Koh 2009, Van der Meij et al. 2010, Hoeksema et al. 2011, Van der Meij and Visser 2011, Hoeksema and Wirtz 2013).

Cycloseris boschmai sp. n. is the smallest mushroom coral known so far (see key). Superficially, it resembles Cycloseris costulata, which has less prominent costae, a more even corallum margin (not undulating), a larger maximum size and less colourful appearance (see Hoeksema 1989, Hoeksema and Van Ofwegen 2004, Gittenberger and Hoeksema 2006). Both species can be found on reef slopes and sandy reef bases. Cycloseris costulata is common and wide-spread (Hoeksema 1989, 2012a) and has a variable growth form (Hoeksema and Moka 1989, Gittenberger and Hoeksema 2006), which is why much material had to be examined to find sufficient consistency in the diagnostic characters of the new species.

Cycloseris boschmai sp. n. also resembles Cycloseris tenuis, which is more oval, less corlourful (see Gittenberger and Hoeksema 2006), and with lower order costae that are rougher and more irregularly ornamented. Cycloseris tenuis is most common on upper reef slopes (Hoeksema 2012a), whereas the rarer Cycloseris boschmai shows a deeper depth range. The new species also resembles Cycloseris vaughani, which has sharper lower order costae, a brown colouration (Hoeksema 1989, Hoeksema and Van Ofwegen 2004) and a much deeper depth range (Hoeksema 2012a).

With the inclusion of Cycloseris boschmai sp. n., 11 free-living Cycloseris species are distinguished. Since the taxonomic revision of the Fungiidae by Hoeksema (1989) various other mushroom coral species were reported as new to science (Veron 1990, 2000, 2002, Hoeksema and Dai 1991, Hoeksema 1993a, 1993c, 2009, 2012c, Latypov 1995, 2006, Ditlev 2003, Mondal and Raghunathan 2013). Two of these were originally classified as Cycloseris but they appear to be synonyms of previously described species and one of these is not a Cycloseris.

Cycloseris colini Veron, 2000 is a synonym of Lithophyllon spinifer (Claereboudt and Hoeksema 1987). The central dome and upward margins, combined with the large corallum size as described by Veron (2002) are characters commonly found in Lithophyllon spinifer (see Hoeksema 1993a: fig. 14, Veron 2000, Hoeksema and Van Ofwegen 2004, Hoeksema 2008: fig. 9). A specimen from Palau (MTQ G55817) was designated holotype by Veron in 2002, but since the species was described in 2000 this designation was invalid (ICZN 2011). Hence, this specimen is hereby designated lectotype.

Cycloseris densicolummelus Latypov, 2006 has not been described in an official publication but in an electronic document that was distributed via a CD-ROM. This work should have contained a clear publication date and a statement naming at least five major publicly accessible libraries in which copies of the optical disc were to have been deposited (ICZN 2012). Since only the year of translation has been mentioned in the introduction and no names of libraries were given, this name is not valid. Although the publication by Latypov (2006) is said to be an English translation of an original book in Russian, the latter does not mention Cycloseris densicolummelus but “Cycloseris sp. 1” (Latypov 1995: 95, pl. 28 fig. 2). The specimen indicated as “holotype”, spec. 1/95158 deposited in the Museum of Institute Marine Biology, Vladivostok 69041, Russia, is from Mai Rut Island, Gulf of Thailand. The illustration with the species description (Latypov 2006: figs 46–8) shows that it is a specimen of Cycloseris costulata. The well developed tentacular lobes, the intensive granulation of the lateral septal surfaces and the densely packed trabeculae of columella, which are indicated as diagnostic characters, do not really distinguish Cycloseris densicolummelus from Cycloseris costulata (see Hoeksema 1989, Gittenberger and Hoeksema 2006).

Because Cycloseris boschmai is a rare species (considering that most material was gathered during fieldwork in a time span of 30 years) and its geographic distribution range is restricted to the Coral Triangle, not much can be said about its ecology. Specimens are difficult to find, owing to their small body size compared to other mushroom coral species (Hoeksema 1991b, Gittenberger et al. 2011), which may be restrictive to the settlement of associated fauna and therefore none of its symbionts was reported previously (Hoeksema et al. 2012: “Cycloseris sp.”). In the present study, one of the photographed specimens shows a coral barnacle (Figure 11e), which is now the only known associated animal.

Small-sized free-living mushroom corals have been reported to show much mobility (Hoeksema 1988, Yamashiro and Nishihira 1995), which may help them to escape from competition for space with other organisms (Chadwick 1988, Hoeksema and De Voogd 2012). The distinctive large lower order costae may be useful as ridges in stabilizing the corals in order to prevent them to slide too rapidly downslope to deeper reef zones with sandy substrate (Hoeksema 1988). The enlarged ridge-like costae can also be seen in some other Cycloseris species (see key) and in the small-sized free-living deep-sea coral Deltocyathus rotulus (Alcock, 1898) (see Cairns and Kitahara 2012: fig 18Q).

Although free-living Cycloseris species were previously considered primitive, this is not the case according to their phylogeny reconstruction (Gittenberger et al. 2011). Their predominant habitat of deeper sandy substrates can also be considered an advanced trait (Hoeksema 2012b). The sand in the mouth of nearly dead specimens from Banda (Figure 10) suggests that they were collected from a sandy substrate, which may not be their preferred habitat. During the author's, fieldwork (1983–2013), no specimens were observed on sandy reef bases. Cycloseris boschmai corals are small but not thin, which may not facilitate mobility and sediment-shedding as seen in large-polyped corals (Bongaerts et al. 2012, Erftemeijer et al. 2012). The presence of buds in several of the specimens from Banda, in addition to the sand in their stomata, (Boschma 1925; Figure 10) may also indicate that specimens have been buried (Gilmour 2002). Specimens of Cycloseris hexagonalis have been observed to show a similar abundance of buds on a sandy slope in eastern Sabah (BWH personal observation 2009). Budding may be a mushroom coral's, last resort of survival when its mouth is clogged and not capable of food intake anymore (Boschma 1922, 1923b, 1923c).