Revista de Biología Tropical ISSN: 0034-7744 [email protected] Universidad de Costa Rica Costa Rica Vargas-Zamora, José A.; Sibaja-Cordero, Jeffrey A. Molluscan assemblage from a tropical intertidal estuarine sand-mud flat, Gulf of Nicoya, Pacific, Costa Rica (1984-1987) Revista de Biología Tropical, vol. 59, núm. 3, septiembre, 2011, pp. 1135-1148 Universidad de Costa Rica San Pedro de Montes de Oca, Costa Rica Available in: http://www.redalyc.org/articulo.oa?id=44922150016 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Molluscan assemblage from a tropical intertidal estuarine sand-mud flat, Gulf of Nicoya, Pacific, Costa Rica (1984-1987) José A. Vargas-Zamora1,2 & Jeffrey A. Sibaja-Cordero1,2 1. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Universidad de Costa Rica, 11501-2060, San José, Costa Rica. [email protected]; [email protected] 2. Escuela de Biología, Universidad de Costa Rica, 11501-2060, San José, Costa Rica. Received 08-II-2011. Corrected 20-III-2011. Accepted 05-IV-2011. Abstract: The availability of data sets covering more than a year is scarce for tropical environments. Advances in hardware and software speed-up the re-analysis of old data sets and facilitates the identification of hidden data patterns. From February 1984 to April 1987 (49 sampling dates), core samples (17.7cm2, 15cm deep) were collected at low tide at a sand-mud flat in the mid upper Gulf of Nicoya estuary, Pacific, Costa Rica. Predator exclusion experiments (cages 0.5x0.5x0.2m, galvanized wire, mesh size 5mm), were conducted at the site in 1985 (dry and rainy seasons sets). Samples were preserved with 5% buffered formalin in sea water stained with Rose Bengal, and washed after 24 hours on a 500 micron mesh sieve. The 1 120 cores yielded a total of 112 morphological species of which the mollusks were represented by 23 species, and included the bivalves Tellina rubescens, Tagelus bourgeoisae, Dosinia dunkeri and Leukoma asperrima, and the gastropods, Natica unifasciata, Nassarius luteostomus, Costoanachis rugosa and Turbonilla sp. The 23 species are indicative of a relatively rich sedimentary molluscan fauna. T. bourgeoisae had a seasonal oscillation, with higher abundances during the rainy seasons. T. rubescens was not seasonal, but presented an oscillation with peaks at about 1.5 year intervals. Many empty shells of Cosmioconcha modesta, lower number of N. luteostomus and a few of T. rubescens were found with boreholes by the predatory snail N. unifasciata. T. rubescens was not significantly more abundant inside or outside cages. T. bourgeoisae showed a significant increase within the caged areas. The numerical fluctuations of the mollusks became more important during the rainy season of 1985. Red tide outbreaks in the Gulf of Nicoya in 1985 may have had an impact on the molluscan populations. Rev. Biol. Trop. 59 (3): 1135-1148. Epub 2011 September 01. Key words: Natica, Nassarius, Costoanachis, Turbonilla, Tagelus, Tellina, Dosinia, Leukoma, benthos, estuary, tidal flat, cages, boreholes, Gulf of Nicoya, Costa Rica. From 1980 to 1983, an ecological survey The intertidal survey methods were not was conducted in the Gulf of Nicoya estuary specifically designed for the detection of the to provide baseline information on the Gulf in abundance pattern of any particular species support of management policies. Many papers (Vargas & Solano 2011). However, the gastro- covering the fields of physical, chemical and pods Natica unifasciata, Nassarius luteosto- biological oceanography were published (see mus, Turbonilla sp., and Costoanachis rugosa, references in Vargas & Mata 2004) making this and the bivalves, Tellina rubescens, Tagelus embayment one of the better known tropical bourgeoisae, Leukoma asperrima and Dosinia estuaries worldwide (Vargas & Solano 2011). dunkeri, appeared regularly in the samples over These studies were followed by a three year the study period. Information to date on most of survey of an intertidal 400m2 plot in the mid these species consists of entries in malacological upper estuary (Vargas 1989). collections, with scarce data on their abundance Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 59 (3): 1135-1148, September 2011 1135 fluctuations and ecology. The early work by pushed 0.1m into the sediment and left for Gonor (1965) on the predator-prey relationships periods of one (8 cages), two (4 cages) and of N. unifasciata on N. luteostomus at Golfito three (4 cages) months after which they were Bay (Pacific, Costa Rica), and the report by lifted and 14 cores taken from inside and 14 Palacios et al. (1986) on the reproduction of the outside (84 cores each) the caged areas. commercially important, Protothaca asperrima Samples were preserved with 5% buffered (now Leukoma asperrima) from the Gulf of Rose Bengal stained formalin in sea water, and Nicoya, are notable exceptions. washed with freshwater on a 500 micron mesh Caging experiments were conducted by sieve. Sorted (30X dissecting scope) specimens Vargas (1988, 1996) at the site to test for the were stored in 70% ethanol. A collection of impact of crab, fish and bird predation on the morphological species was assembled and a benthic community. There were changes in code asigned to each one. Molluscan species the abundances of several of the numerically identification was based on Keen (1971), and dominant species, and in the composition of later updated with Skoglund (1992), Cruz-Soto the community, as revealed by multivariate & Jiménez-Ramón (1994), Valdés & Cama- statistics. However, the relatively lower abun- cho-García (2004), Rodríguez-Sevilla et al. dance mollusks were not analyzed further. The (2009), Magaña & Espinosa (2009), and www. advances in hardware and software during the marinespecies.org (March 2011). last decades speed-up the re-analysis of old Seasonality and temporal trends for the 49 data sets and facilitates the identification of dates were analyzed with the Generalized Addi- hidden data patterns. tive Model (GAM) of the free mgcv package in The objective of this paper is to make The R Project for Statistical Computing (Wood accesible complementary data on the abundance 2006), with abundance data log10 (x+1) trans- fluctuations and ecology of this molluscan assem- formed to homogenize variances. The GAM blage (an artificial group of interacting species). was carried out with the family quasi (Cha- loupka et al. 2008). Species abundances inside and outside cages were compared with a paired MATERIALS AND METHODS Student’s t test on the log transformed monthly From February 1984 to April 1987 (total: totals (Normality and equality of variances 49 sampling dates) core samples (17.7cm2, were tested previously). Chi-square tests were 15cm deep) were collected by Vargas (1987, used to determine significant season (dry vs 1988) in lower intertidal at the Punta Mora- rainy) differences. A SIMPER analysis (Clarke les (mean tidal range: 3m) sand-mud flat in & Warwick 1994, Hammer et al. 2001) was the mid upper Gulf of Nicoya (10ºN, 85ºW), applied to the data for each condition (caged vs Pacific coast of Costa Rica. The study was uncaged). The percentage of change attributable conducted on a 20x20m area of the flat, about to mollusks and other assemblages was obtai- 20m from the edge of a white sand beach (see ned using PAST (Hammer et al. 2001). map in Vargas 1987). In 1984 and 1985, 28 2 cores (0.05m ) were collected (12 to 18 days RESULTS intervals, 25 dates). A monthly sampling sche- me was followed (14 cores, 0.02m2) from 1985 A total of 1 120 sediment cores were pro- to 1987 (24 dates). cessed, representing an area of 1.98m2. On Macropredator (birds, fish and crabs) average, the sediments at the flat were 65% exclusion experiments (cages 0.5x0.5x0.2m, sand and about 35% mud. Other information galvanized wire, mesh size 5mm), were con- on environmental data is included in Vargas ducted by Vargas (1988,1996) at the site in (1987,1988,1989) and Vargas & Solano (2011). 1985 (dry and rainy seasons sets, eigth cages The 1 120 cores yielded a total of 112 spe- each). He deployed sixteen cages that were cies. The polychaete worms were represented 1136 Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 59 (3): 1135-1148, September 2011 by 45 species and the crustaceans by 29. lined track. A group of the scavenger snail, A group of 15 species included flatworms, Nassarius luteostomus was observed (Dec. nemerteans, two sipunculans, oligochaetes, two 26th, 1984) feeding on a dead Callinectes brachiopods, two echinoderms, hemichordates, arcuatus crab, and a few gaping T. rubescens a lancelet and a gobiid fish (Vargas 1989). were found (Aug. 19th, 1985) lying on the sedi- The mollusks were represented by 23 ment surface. The abundances of the bivalves, species and their abundances are included in T. rubescens, T. bourgeoisae, L. asperrima and Table 1. The deposit feeding pink bivalve, D. dunkeri, are included in Figs. 1 and 2, for Tellina rubescens was most abundant and ran- 1984-1985 and 1984-1987, respectively. The ged in length from 5mm to 40mm. Many small abundances of the gastropods, N. unifasciata, bivalves of various shapes and less than 5mm Turbonilla sp., N. luteostomus and C. rugosa were pooled as unidentified juveniles (Table collected from 1984 to 1987 are included 1). Among the gastropods, Natica unifascia- in Fig. 3. C. rugosa was found only from ta was observed frequently crawling on the August 1985 to March 1986 (Fig. 3B). Only sediment surface and leaving behind a mucous the GAModels applied to the abundances of, TABLE 1 List of mollusks collected by coring at a 400m2 plot at the Punta Morales intertidal sand-mud flat.