The annelid polychaete worms of the family Sabellariidae build reefs by cementing together sand grains with a biocement produced by the worms themselves (^{Fournier et al., 2010}). These reefs serve as a refuge for a diverse fauna and as such the worms act as ecosystem engineers increasing habitat heterogeneity and species diversity (^{Bruschetti, 2019}). ^{Fauchald (1977}), reported the presence of seven species of sabellariid worms for the Pacific coast of Panama including Phragmatopoma attenuata Hartman, 1944. P. attenuata, was reported from several locations on the Pacific coast of Costa Rica, such as the Gulf of Nicoya estuary (^{Dean, 2009}; ^{Sibaja-Cordero & Vargas-Zamora, 2006}) and further North at Culebra Bay by ^{Sibaja-Cordero & García-Méndez (2014)}. Phragmatopoma sp. was reported from Culebra Bay and Junquillal Bay by ^{Sibaja-Cordero et al., (2014)}. P. villalobosi has been described from the North Pacific of Costa Rica by ^{Chávez-López (2020}).

There are no reports on the associated fauna of Phragmatopoma spp. reefs from Costa Rica except the recent description of several species of tardigrades from worm tubes of P. attenuata (^{Bartels et al., 2021}). Thus, the objective of this report is to list the species of crustaceans found in sabellariid reef samples from the Pacific coast of Costa Rica.

At low tide (Tidal range: 3m) we dislodged fragments of worm colonies (Fig. 1A) with a chisel, stored them in plastic bags and later disaggregated the material by hand. We collected occasionally from 2012 to 2021 on the North coast of Costa Rica at two relatively pristine sites: Ostional Beach- Ostional Wildlife Refuge (9.993420° N - 85.701841° W) and Junquillal Bay-Santa Elena Gulf (10.965081° N - 85.696202° W), Table 1. Additional samples, collected on July 20, 2023, came from worm reefs found on rock groins at the tip of the port city of Puntarenas in the mid upper Gulf of Nicoya estuary (9.976638° N - 84.851288° W). This site receives pollutants from the city, river discharges, tourist and fishing boat operations (^{Sibaja-Cordero & Gómez-Ramírez, 2022}). We sorted the samples under a stereoscope and placed the crustaceans in 70% ethanol for further identification and storage. We deposited a reference collection at the Museum of Zoology of the University of Costa Rica (MZUCR).

As mentioned by ^{Chávez-López (2020}) the separation of P. attenuata from P. villalobosi is based on the length of the filaments of the median plume and we are uncertain of the presence of this feature in the worms collected. We refer to the worm reefs sampled in this study as build by Phragmatopoma sp. (Fig. 1B) until further taxonomic work allows a clear identification of the species involved. Voucher specimens of Phragmatopoma spp. are deposited at the Museum of Zoology, University of Costa Rica, Polychaete collection (MZUCR-2506-01, MZUCR-2501-01, MZUCR-2505- 01). For the identification of the crustaceans, ^{Rathbun (1930}), ^{Haig (1960}), ^{Brusca (1980}), Brusca & Iverson (1985), ^{Kim & Abele (1988}), ^{Anker et al., (2009}), and ^{Salgado-Barragan & Hendrickx (2010}) were consulted. We included photographs of selected species in Fig. 1 C-L.

A total of 26 species of macro-crustaceans belonging to Anomura (16 species), Brachyura (5), Caridea (4), and Stomatopoda (1), were found in samples from the North coast. The family Porcellanidae was represented by 12 species (Table 1). Eight species belonged to the genus Petrolisthes, but Neopisosoma spp. were more abundant. Micro-crustaceans were also present, including unidentified amphipods (MZUCR 3089-01). Two isopods: Aphantolana costaricensis (^{Brusca & Iverson, 1985}) (MZUCR 3609-07) and Cirolana browni Van Name, 1936 (MZUCR 3369-10) were identified but several were not (MZUCR 3089-02, 04, 05; 3369-08, 09, 10).

Samples from the Puntarenas worm reefs yielded 183 individuals and five identified species: Neopisosoma mexicanum (MZUCR 3803-01, Fig.1C) was the most abundant (127 ind., 69%, 35 ovigerous females, Fig.1I), followed by Pachygrapsus transversus (42 ind., 23%, Fig.1F, MZUCR 3803- 02); Alpheus javieri (6 ind., 3%, Fig.1E, MZUCR 3803-03); Acantholobulus sp. (5 ind., 2,6%, Fig.1H, MZUCR 3803-04), and the isopod Ligia baudiniana (3 ind., 1,6%, MZUCR 3803-05). Thus, the reef samples collected in this study yielded a total of 30 identified species including decapods, the stomatopod Neogonodactylus festae (Fig.1D) and three isopods.

A two-year quantitative survey of crustaceans from Phragmatopoma lapidosa reefs on the coast of Florida found 51 species and pointed out the dominance of porcellanid, xanthid and grapsid crabs such as P. transversus (^{Gore et al., 1978}).^{Moran (1984}) found that in El Salvador two porcellanids (N. mexicanum and Petrolisthes artifrons), P. transversus, and the xanthid Eriphia squamata (Fig.1K) were the more abundant species in sabellariid reefs. P. lapidosa reefs from Brazil yielded five species of porcellanids (^{Micheletti-Flores & Negreiros-Fransozo, 1999}). In Panama reef colonies of Idanthyrsus cretus had a diverse fauna of 27 species of crustaceans (^{Barrios et al., 2009}), while the fauna associated to P. moerchi reefs in Chile included only five species (^{Sepúlveda et al., 2003}). In Venezuela, Phragmatopoma sp. reefs provided habitat for 27 species of decapods among 1 527 individuals of which 928 (60%) were porcellanids (^{Velásquez et al., 2017}). P. caudata reefs in Brazil yielded 3 390 individuals and 39 species of macro-crustaceans were found of which the most abundant was the porcellanid Pachycheles greeleyi with 351 (10%) individuals (^{Lane-Medeiros et al., 2021}).

The diversity of crustaceans found in sabellariid worm reef studies is highly variable and is influenced by the species of worm, sampling effort, and qualitative vs quantitative collection methods thus comparisons are difficult. The number of crustacean species found in Phragmatopoma sp. reefs from pristine sites on the North coast of Costa Rica appears relatively high when compared to other studies cited above. The relative lower diversity of Puntarenas reefs deserves further study to evaluate the roles of pollution and estuarine conditions. As found in other studies, porcellanid crabs seem to prefer these habitats. The grapsid P. transversus is a frequent inhabitant of worm reefs at both sides of the American continent.

Fig. 1 A. Cluster of sand tubes of Phragmatopoma sp. B. A group of whole worms. C. Neopisosoma mexicanum. D. Neogonodactylus festae. E. Alpheus javieri. F. Pachygrapsus transversus. G. Clibanarius albidigitus. H. Acantholobulus sp. I. N.mexicanum, with eggs. J. Megalobrachium pacificum. K. Eriphia squamata. L. Williamstimpsonia stimpsoni. B-L. Scale in mm.