INTRODUCTION
The subfamily Pontoniinae (Decapoda: Caridea: Palaemonidae) includes more than 500 species of predominantly symbiotic shrimps associated with different marine invertebrates such as sponges, coelenterates, molluscs, crustaceans, echinoderms and ascidians (e.g. Li, Reference Li2000). In the Indo-West Pacific region, eight genera of the subfamily have been found associated with bivalve molluscs: Anchistus Borradaile, Chernocaris Johnson, Conchodytes Peters, Neoanchistus Bruce, Paranchistus Holthuis, Platypontonia Bruce, Plesiopontonia Bruce and Pinnotherotonia Marin & Paulay. In most cases the shrimps are associated with bivalves that live largely exposed from soft substrates or attached to hard ones while the associations with deeply burrowing bivalves are rarer. After three hundred years of studies only three associations have been found. The first one found at Ambon Island (Rumphius, Reference Rumphius1705) was an unidentified shrimp, assumed to be a pontoniine, and bivalve host, Tapes literatus (Linnaeus, 1758) (Veneridae). Two other records were Neoanchistus cardiodytes Bruce, Reference Bruce1975 from Trachicardium ( = Vasticardium) pectiniforme (Born, 1780) (Cardiidae) in Madagascar (Bruce, Reference Bruce1975) and Pinnotherotonia rumphiusi Marin & Paulay, Reference Marin and Paulay2010 from Periglypta crispata (Deshayes, 1853) (Veneridae) from Palau (Marin & Paulay, Reference Marin and Paulay2010), and are known from type specimens only.
During studies of mangrove community structure in central Vietnam (Nhatrang Bay, Tre Island and Dam Bay) a heterosexual pair of N. cardiodytes was found in the mantle cavity of the deeply burrowing bivalve Mactra grandis Gmelin, 1791 (Mactridae). This is the fourth report of shrimps in symbiosis with burrowing bivalves, the second record of this species as well as its first record from Vietnam and the Pacific Ocean. Burrowing bivalves of the family Mactridae (Mollusca: Bivalvia) are reported for the first time as a host for pontoniine shrimps and M. grandis—as a new host for N. cardiodytes. Total body length (tl., mm) and postorbital carapace length (pcl., mm) are used as standard measurements. Examined specimens are deposited in the Laboratory of Ecology and Morphology of Marine Invertebrates (LEMMI) in the A.N. Severtzov Institute of Ecology and Evolution of the Russian Academy of Sciences.
SYSTEMATICS
Neoanchistus cardiodytes Bruce, Reference Bruce1975: 151–161, figures 1–7 (type locality: Nosi Be, Madagascar).
(A–B) Neoanchistus cardiodytes: (A) shrimps in the mantle cavity of host bivalve; (B) the same enlarged; smaller male is above female; (C–D) Mactra grandis: (C) exterior and interior of valves; (D) hinge details of the right and left valves. Scale bars: B, 3 mm; C, D, 10 mm.
MATERIAL
One non-ovigerous female and one male, Pacific Ocean, South China Sea, Vietnam, Nhatrang Bay, Tre Island, Dam Bay, low margin of tidal zone, silty sand bottom, inside mantle cavity of bivalve Mactra grandis, coll. T.A. Britayev, 2 March 2010 (LEMMI, catalogue no. 2010/01-02).
REMARKS
Morphology is in complete agreement with the original species description given by Bruce (Reference Bruce1975). The species clearly differs from the only congener, N. nasalis Holthuis, Reference Holthuis1986, by smaller size (known specimens of N. nasalis have carapace length of 10–15 mm), narrower rostrum reaching the distal margin of antennular peduncle (versus shorter broad and deep rostrum not reaching the distal margin of the second antennular peduncle in N. nasalis) and shorter reduced antennular flagellum as well as by association with different host species (specimens of N. nasalis were found inside Mimachlamys townsendi (Sowerby III, 1895), an endemic scallop from the Arabian Gulf and Gulf of Oman) (see Holthuis, Reference Holthuis1986).
COLORATION
Body semitransparent, covered dorsally with large rounded reddish spots, with margins darker than centre; appendages with similar irregular red markings (Figure 1A, B).
MEASUREMENTS
Female pcl., 3.6 mm, tl., 13 mm; male: pcl., 2.7 mm, tl., 9 mm.
HOST
The specimens were found inside the mantle cavity of Mactra grandis Gmelin, 1791 (Bivalvia: Mactridae) (Figure 1C, D). The shell was found intertidally in front of mangrove bushes, nearly completely burrowed in silty sandy bottom with only the upper part of the valves and siphon exposed. The length of the shell is 71 mm and height is 55 mm.
DISTRIBUTION
This is the second report of the species. It is known from the type locality, Nosi Be, Madagascar, and Nhatrang Bay, Vietnam.
DISCUSSION
Up until now, nine pontoniine shrimps species associated with bivalves have been reported from coastal waters of Vietnam (Britayev & Fakhrutdinov, Reference Britayev, Fakhrutdinov, Pavlov and Sbikin1994; Marin & Savinkin, Reference Marin, Savinkin, Britayev and Pavlov2007). The present record of Neoanchistus cardiodytes increases the number of bivalve-associated shrimp species in the Vietnamese fauna to ten.
All known associations between burrowing bivalves and shrimps, both specialized and accidental, have been considered recently (Marin & Paulay, Reference Marin and Paulay2010). The authors concluded that the associations involve large species of cardiid and venerid bivalves such as Vasticardium pectiniforme, Laevicardium elatum (Sowerby, 1831), Periglypta crispata and Megapitaria aurantiaca (Sowerby, 1831). Our finding adds to this list a new host species, Mactra grandis, belonging to large burrowing species of the family Mactridae.
It seems likely that shrimps associated with burrowing bivalves are extremely rare or rarely found. According to Marin & Paulay (Reference Marin and Paulay2010) dozens of Periglypta spp. shells across the Indo-West Pacific were opened and only a single shrimp was found. The only known association of unidentified shrimp with burrowing bivalve Tapes literatus was reported from Indonesia, over three hundred years ago (Rumphius, Reference Rumphius1705), and, apparently, has never been recorded since. Further specimens of Neoanchistus cardiodytes were re-discovered only after more than 50 years since its first collection in 1955 (see Bruce, Reference Bruce1975). At the same time, large shallow-water burrowing bivalves belonging to the families Mactridae, Cardiidae and Veneridae have been intensively harvested in South-East Asia and examined by researches over many years. Nevertheless extremely rare records of associated shrimps indicate their low population density (low prevalence) or they are living in a narrow environmental zone overlooked by fishermen and researchers.
ACKNOWLEDGEMENTS
We are thankful to Dr Richard Kilburn (Natal Museum, Pietermaritzburg, South Africa) and Dr Yuri Kantor (A.N. Severtzov Institute of Ecology and Evolution RAS, Moscow, Russia) for the identification of the bivalve host shell. Research work in Vietnam was supported by the coastal branch of the Russian–Vietnamese Tropical Centre. The study was partly supported by RFFI (grant no. 08-04-92244-GFEN_а and 09-05-00736_а); and the President Grant (grant no. МК-7747.2010.4.). Our manuscript benefited greatly from the comments of two anonymous referees.
