Carijoa riisei (Duchassaing & Michelotti, 1860) is an octocoral of the family Clavulariidae reported from different tropical locations worldwide, where it is considered an invasive species (^{Venkataraman et al., 2016}). The coral was first described from the Atlantic Ocean (Virgin Islands) and was later reported from Hawaii in the Pacific Ocean in 1972 (^{Kahng & Grigg, 2005}). Genetic evidence, however, suggests C. riisei is native to the Indo Pacific (^{Concepcion et al., 2010}). In the Eastern Tropical Pacific it has been reported from Mexico (^{Galván-Villa & Ríos-Jara, 2018}), El Salvador, Nicaragua (^{Cortés et al., 2017}), Panamá (^{Gómez et al., 2014}), Colombia (^{Sánchez & Ballesteros, 2014}; ^{Quintanilla et al., 2017}) and Ecuador (^{Cárdenas-Calle et al., 2021}). The only record for the Caribbean coast of Costa Rica was identified as Telesto riisei (later recognized as C. riisei, UCRMZ 93) collected in 1983 at the port of Limón (^{Guzmán & Cortés, 1985}). In Culebra Bay, on the northern Pacific coast of Costa Rica, Carijoa sp. was found to be the most abundant (more than 40%) community member at waters deeper than 20m (^{Cortés & Jiménez, 2003}). Museum specimens from the Gulf of Nicoya estuary identified as C. riisei were reported by ^{Vargas & Breedy (2021}). Carijoa riisei has also been reported from the oceanic (05o 32’N - 87o 03'W) Coco Island (Isla del Coco) by ^{Cortés et al. (2017}).

On December 7, 2021, at the Punta Morales Peninsula in the mid upper region (10o 04'N - 84o 58'W) of the Gulf of Nicoya, Pacific coast, during a very low tide (- 0,21m to + 2,93m), a solitary colony of soft coral was found growing on a rock exposed to sediment-loaded waters (Fig. 1 A, B). The objectives of this note are to identify the species, provide a description of the specimen, and list previous records from the Pacific coast of Costa Rica. A fragment of the colony was collected (Fig. 1 C) and kept in sea water where it was observed with the characteristic white polyps extended (Fig. 1 D, E). The fragment was later preserved in alcohol and a small section cut off to study the sclerites under a scanning electron microscope. A list of specimens of C. riisei deposited in the Museum of Zoology of the University of Costa Rica (UCRMZ) was assembled

Fig. 1. A. B  Solitary colony of Carijoa riisei (MZUCR 3317) on a rock at the low intertidal water edge, Punta Morales, mid upper Gulf of Nicoya estuary. C. Live fragment of the colony with retracted polyps and natural colour. D. Distribution of open polyps along the axis. E. Close view of open polyp. 

The colony found at Punta Morales consists of cylindrical conical polyps budding mostly laterally at opposite sides, at distances of 1-3mm apart distally and more widely dispersed basally, about 4mm apart. Branching is monopodial, with several single erect branches up to 13cm long. Polyps are joined at their bases by thin ribbon-like stolons. Apical polyps form clusters of 3 or 4 buds surrounding each axial polyp. The colour of the colony is light brown when alive (Fig. 1) and lighter when preserved in alcohol. An epizoic assemblage is present which includes hydroids, polychaete worms and micro-crustaceans.

Polyp sclerites are elongated rods, 0,27 - 0,46mm long, and around 0,017 - 0,047mm wide, with sparse, thorny tubercles; some with forked ends (Fig. 2A). Sclerites in stolons are tuberculate rods, with simple and sparse tubercles, some bifurcated or thorn-like, 0,11 - 0,32mm long, and 0,028 - 0,084mm wide (Fig. 2B), often fused into characteristic tangled clumps (Fig. 2C). Tentacles with flat, unornamented rods, 0,13 - 0,22mm long and around 0,012mm wide (Fig. 2D). All sclerites are colourless.

Fig.2 Carijoa riisei, MZUCR 3317: Scanning Electron Microscope views of sclerites from: A. Polyps. B. Stolons. C. Fused clumps of stolon sclerites. D. Tentacles. 

External morphology of the colony, polyps (Fig. 1) and sclerites (Fig. 2) are similar to descriptions of C. riisei provided by ^{Bayer (1961}, 1981), ^{Devictor and Morton (2010}), ^{Laackmann (1909}) and ^{Sánchez (1994}). Of particular relevance are the sclerites ornamented with prickles and thorns and sometimes fusing into clumps as described by ^{Bayer (1981}). However, a clear taxonomic status has not yet been defined for the species. Thus, the specimen collected in Punta Morales is provisionally identified as Carijoa riisei until integrative taxonomic studies including morphological and molecular analyses may contribute to clarify this issue in the future.

The collection of the Museum of Zoology of the University of Costa Rica has 50 records of specimens labelled as C. riisei for the Pacific coast of Costa Rica. The earlier records are from 1991 (Table 1). The coral was collected along the coast, from Salinas Bay on the North to the Golfo Dulce embayment on the South and at Coco Island (Table 1). Several collections come from the Gulf of Nicoya where 29 other octocoral species are reported by ^{Vargas and Breedy (2021}). The record 3323 (Table 1) is interesting as C. riisei was found at Culebra Bay on top of a black coral. A recent survey (^{Matamoros-Calderón et al., 2021}) of black coral forests (Myriopathes panamensis and Antipathes galapaguensis) in an area North of Culebra Bay reported no specimens of C. riisei on black coral branches.

Collections of C. riisei on the Pacific coast of Costa Rica have been occasional and most have ocurred more than a decade ago (Table 1). A comprehensive evaluation of its presence in intertidal and subtidal sites in both the Pacific and Caribbean coasts is needed to better assess its actual ecological role in coastal waters of Costa Rica.