Calathiscus, Claereboudt, Michel R. & Al-Amri, Issa S., 2004

Claereboudt, Michel R. & Al-Amri, Issa S., 2004, coral (Scleractinia, Poritidae) from the Gulf of Oman, Zootaxa 532, pp. 1-8 : 2-8

Calathiscus new genus

Type species: Calathiscus tantillus

Diagnosis: see under only known species, Calathiscus tantillus Calathiscus tantillus new species ( Figs. 1 –6).

Material examined

Holotype: Sultan Qaboos University, SQU040001, Cemetery Bay­ Pillar Rock, 8m, Muscat ( Oman). Collector: Michel Claereboudt. The holotype consists of a whole colony (1. 7 x 3.4 cm) ( Fig. 1 , 4 ).

Paratypes:

1­Florida Museum of Natural History, UF535, Masirah Island, 3–4 m ( Oman). Collector: Gustav Paulay ( Fig. 2 ,A).

2 Institut Royal des Sciences Naturelles de Belgique, IG30190, Cemetery Bay, Muscat, 6 m. Collector: Michel Claereboudt ( Fig. 2 B).

Diagnosis

Corallites are 1.7 mm in diameter with calices small, rarely exceeding 1.5 mm There are two complete and one incomplete, variable, cycles of septa (6+6+3—10). A columella is absent. Living colonies have large polyps, with an elongated column and lanceolate tentacles that are normally extended during the day. Tentacles vary in number from 15 to 22 corresponding to the number of septa.

Previous records

This species has not been previously recorded.

Description

Colonies are encrusting or hemispherical, very small, rarely more than 30 mm across. Corallites are rounded and very small (1.7 mm in diameter on average) with calices rarely exceeding 1.5 mm. The wall is thin and consists of rows of irregular, granulated denticles that correspond to septal insertions, and are connected with synapticular links leaving few pores between adjacent corallites, particularly in lightly calcified colonies from deeper water. Calices are shallow. Septa form 2 subequal cycles with a third cycle expressed to varying degrees in different corallites and colonies. Only the first 2 cycles are complete, and show an irregular pattern of septal fusion. There are between 15 and 22 septa, some reduced to a single row of denticles along the corallite wall. A ventral trident (or triplet) is present in some calices and defines with a dorsal septum an axis of bilateral symmetry. The lateral septa do not form regular pairs as in Porites , but are occasionally fused. In most specimens, a single synapticular ring is clearly visible surrounding a central fossa in which there is no columella. Pali formed by large granulated denticles are present on the margin of 6 septa although in some lightly calcified specimens, the pali are poorly defined. There is an additional denticle on some septa between wall and palus.

Living colonies have large polyps, with an elongated column and lanceolate tentacles, that are normally extended during the day. All observed colonies were light green with radiating brown marks on the oral cones and a darker ring on the tentacles. Tentacles vary in number from 15 to 22 and correspond to the septal arrangement. Colonies are very sensitive to even slight changes in water pressure or movement and respond readily to the slightest disturbance by retracting the polyps.

Remarks

The series of corallites studied showed considerable variation in the development of the various skeletal features, particularly the number of septa and size of the pali. The number of septa varied from 15 to 22. The development of a ring of synapticulae linking the septa in the center of the corallites and the fusion patterns of the septa (ventral triplets and lateral pairs) was also variable. The appearance of the living polyps on the other hand was very stable, and the species was readily identifiable underwater by its color, number of tentacles and small size.

Etymology

Calathiscus is named after a small woven wicker basket (C alathiscus in Latin) to reflect simultaneously the “porous” and delicate nature of the calices of this genus and the small size of the colonies. Gender masculine.

The specific epithet tantillus , literally “ever so small” in Latin, was chosen to suggest the small size of the calices and the colonies of this species.

Habitat and distribution

All colonies were found attached to hard substrate: fragments of coral skeleton, large flagstone of limestone and Ophiolite bedrock. Colonies of Calathiscus tantillus have been found in several reef communities between 4 and 22 m in depth in mixed communities dominated by several species of Cyphastrea , Leptastrea , Favites and small colonies of Porites and Goniopora . In the Gulf of Oman, these depths correspond to the lower part of the reefs, as reefs rarely extend below 12– 15 m. The species has been found in communities established on ancient limestone but also, more commonly, in coral communities growing directly on ophiolite. Although never common and always cryptic, colonies of Calathiscus tantillus can be relatively abundant but never exceed 2 or 3 cm 2 in size. Some specimens observed at the deepest location (Fahl Island, 22 m) where partially discolored with the disk and tentacles taking brown­purple color.

Calathiscus tantillus was observed in most coral rich communities in the Muscat Area (Daymaniyat Islands, Cemetery Bay, Bandar Khayran) and one specimen (Paratype 2, FLMNH UF535) was collected in the Arabian Sea near the Island of Masirah (20º0’40”N, 58º38’0”E).

Discussion

Calathiscus tantillus is easy to differentiate from species of Porites by the large size of its extended polyps and the lighter, more porous structure of its skeleton. The new species is distinguished from Goniopora by the small size of the calices (almost 1/2 the diameter found in species with small corallites such as G. tenuidens Quelch, 1886 , G. eclipsensis Veron and Pichon, 1982 , and G. minor Crossland, 1952 ), fewer septa, as well as by the fusion patterns of the septa and the arrangement of pali and denticles: particularly the presence of a ventral triplet in many well calcified colonies and the pali present only on the first cycle of septa. Although fully extended polyps with long columns are also found in other poritids, the number of tentacles (15–22) in this species separates it from Goniopora which has 24 tentacles and Alveopora which has only 12 ( Veron 2000).

The mixture of characteristics from Porites and Goniopora found in the present series of specimens confirms its affinities with the Poritidae : Goniopora , Porites , Stylaraea and to a lesser extent Alveopora . The most complex calicular structures in the Poritidae are found in Goniopora and Poritipora ( Veron 2002) with 3 cycles of septa present in most species ( Veron and Pichon 1982). Stylaraea , Porites and Alveopora have only two cycles of septa ( Veron 2000). Bernard (1903) argued for a complete separation of Alveopora from the Poritidae , and although morphologically there were few arguments against the idea that Alveopora is derived from Goniopora by a further reduction of complexity and development of the skeletal features ( Veron and Pichon 1982), recent genetic studies have shown that Alveopora may be more closely related to acroporids than to other Poritidae ( Romano and Cairns 2000) . The reduction in the number of septa is also visible in the polyp number of tentacles: 24 in Goniopora , and Poritipora ( Veron 2002) , typically 15–22 in Calathiscus and 12 in Alveopora , Stylaraea and Porites .

Several species of Poritidae are also characterized by very small colonies. The largest colonies of Stylaraea for instance are only a few cm across and several small Porites not exceeding a few cm in diameter are being described (J.E.N. Veron, Pers. Comm.).

Figures

FIGURE 1. Calathiscus tantillus, n. gen., n. sp. Holotype: A. View of the corallum. B. Details of the corallites.

FIGURE 2. Calathiscus tantillus, n. gen., n. sp. Details of the corallites A. Paratype (FLMNH, UF 535). B. Paratype (IRSNB, IG 30190).

FIGURE 4. Calathiscus tantillus, n. gen., n. sp. A. Holotype. Underwater micrograph of the live colony. B. Details of a colony showing the typical aspects of the expanded polyps.