Field and laboratory observations of the mass mortality of the yellow clam Mesodesma mactroides in South America: the case of Isla del Jabali, Argentina

Journal of Shellfisheries Research, August, 2004 by Sandra Fiori, Victor M. Vidal-Martinez, Raul Sima-Alvarez, Rossanna Rodriguez-Canul, Ma. Leopoldina Aguirre-Macedo, Omar Defeo

ABSTRACT Since the mid-1990s, the yellow clam Mesodesma maetroides (Deshayes, 1854) has experienced mass mortalities throughout its biogeographic range (23 [degrees]S to 41[degrees]S) along Atlantic exposed sandy beaches of South America. However, there is no information about the potential causes underlying these large-scale events. In the summer of 2002, a sudden massive mortality almost decimated the yellow clam population located to the south of Isla del Jabali (Argentina), at the southern edge of its geographical range. Field observations showed a drastic decrease in abundance front 2000 to 2002, without recovering since then. Mortality sequentially occurred at the beach in a north-south direction, following the same trend as in the large-scale event. Significant differences in individual size were found between dead and live clams with the largest individuals of the population being the most affected. Histologic analysis revealed the presence of a meront stage of an unidentified coccidian parasite in the epithelium of the middle intestine of 3 of the 14 clams examined. Necrosis was found in gills and stomach in 13 of the 14 clams examined. The results suggest that these parasites could play a role in the massive mortality events of the yellow clam populations all along South American sandy beaches since 1993. These findings provide an alternative explanation to the widely held notion that mass mortalities in sandy beach macrofauna are due to harmful algae blooms.

KEY WORDS: mass mortalities, bivalves. Mesodesma mactroides, coccidian parasite, sandy beaches, Argentina

INTRODUCTION

The yellow clam Mesodesma mactroides is an intertidal bivalve distributed along the warm-temperate Atlantic coast of South America, from San Paulo State, Brazil (24 [degrees]S) to the south of the Buenos Aires province, Argentina (41 [degrees]S; Fig. 1a). Yellow clam populations prosper primarily in the intertidal zone of microtidal dissipative beaches with gentle slope, fine sand, heavy wave action, and a wide surf zone often characterized by high primary production by surf diatoms (Defeo & Searabino 1990). The yellow clam has supported recreational and artisanal fisheries of high socio-economic importance (Defeo et al. 1993, Castilla & Defeo 2001).

[FIGURE 1 OMITTED]

Since 1993, episodic mass mortalities have decimated yellow clam populations throughout its entire geographic range. These events occurred mainly between late spring and early summer, following a sequential north-south direction (see Fig. 1a). Mortalities occurred first in Brazil in March 1993 (Odebrecht et al. 1995), then in Uruguay in December 1994 (Mendez 1995) and last in Argentina from September to November 1995 (Fiori 1996, Fiori & Cazzaniga 1999). These mass mortalities have prevented the rehabilitation of yellow clam populations throughout its range (Fiori 2002, Defeo 2003). These large-scale events have not been successfully explained, mainly because of the lack of sampling opportunities concurrent with the occurrence of these sudden phenomena. Odebrecht et al. (1995) conjectured that high abundance of dinoflagellates was a source of yellow clam mortality in Brazil, but when the mass mortality occurred in Uruguay, bioassays indicated that paralytic shellfish toxins were not present (Mendez 1995). The same held true for Argentinean coasts, where metal contamination, phytotoxins, abnormal phytoplankton composition, and protozoan tissue-parasites were discarded as causative factors (Fiori & Cazzaniga 1999).

In January 2002 (austral summer), a mass mortality occurred in the southernmost and isolated yellow clam population located at the south of Isla del Jabali, Argentina. During this event, living organisms were recovered for histologic study. Here we describe the mass mortality event at Isla del Jabali. First, we quantify the magnitude of the event in terms of population density and structure. Second, we perform a histologic examination of the visceral mass of clams, including gills, digestive gland and stomach. Finally, we discuss the large-scale implications of these findings.

MATERIALS AND METHODS

The study area located to the south of Isla del Jabali (Argentina: 40 [degrees]33'S; 62 [degrees]14'W) is a continuous sandy beach, ca. 15 km long, with gentle slope (1.64 [ or -] 1.16[degrees]), fine sand (mean grain size = 0.20 [ or -] 0.91 mm) strong wave action and high salinity (34.3 [ or -] 0.17 ppm) (Fiori 2002). The mean air temperature fluctuates between 6 [degrees]C (winter) and 19 [degrees]C (summer). During the mortality event, a stratified random sampling was carried out in 3 beach sites separated by 4 km and located in the north south direction, thereafter mentioned as sites 1, 2 and 3 (see Fig. 1b). A total of 18 random parcels were located in the mesolittoral area inhabited by the yellow clam, using 50 x 50 cm frames and digging up to a depth of 50 cm. The sediment from each parcel was sieved through a 1-cm mesh and all clams retained were counted, measured with calipers (0.1-mm precision) and discriminated as dead or alive. All clams collected were used to estimate the length frequency distribution (LFD) of the population discriminated by site and clam status: (1) dead clams, in life position into the sediment with the foot and siphons everted and the shell paired open, or with decomposed flesh; (2) alive clams, with closed shell and ability to burrow themselves in the substrate. We did not consider empty valves placed on the sand because we cannel attribute incontestably if these valves came from the mortality event. Two-way ANOVAs were used to test for differences between density and clam length, with status and beach site as main factors. A simple length based survival ratio SR was applied as SR = 1 - [D.sub.i]/([D.sub.I] [L.sub.i]), where D is the number of dead clams and L is the number of live clams for each length class i. The analysis was compared with density estimates obtained in summer of 2000, following the same stratified random sampling detailed above.