Reproductive cycle of the stout razor clam, Tagelus plebeius , in the Mar Chiquita Coastal Lagoon, Argentina

Journal of Shellfisheries Research, August, 2004 by M. Cledon, A.C. Peralta Brichtova, J.L. Gutierrez, P.E. Penchaszadeh

ABSTRACT A population of the dioecious euryhaline bivalve, Tagelus plebeius, living in the Mar Chiquita Lagoon (37[degrees]40'S, 57[degrees]26'W) exhibits an annual gonad cycle. Spawning occurs during the austral spring and summer (October to March) and is dependent on water temperature. Two reproductive modes were observed during consecutive years: partial spawning events in December 1999 to January 2000 and at the end of March 2000, and one continuous spawning event in October 2000 to March 2001. Mature oocytes measured 75-95 [micro]m. At the end of the spawning events there was a sharp drop in body weight, indicating that body mass is determined by gonad status.

KEY WORDS: razor clam, reproduction, Tagelus

INTRODUCTION

The Psammobiid euryhaline bivalve Tagelus plebeius (Lightfoot 1786) inhabits tidal flats on the Atlantic coast of the Americas (Osorio Ruiz 2002) from 34[degrees]N to 41[degrees]S, contributing more to the total biomass of these areas than any other bivalve. It is such a common inhabitant of the estuaries and coastal lagoons of Argentina, that shell beds of this species modify the environment (Gutierrez & Iribarne 1999). Individuals live buried in sandy substrates, and can tolerate a wide range of salinities (<10 [per thousand] to >30 [per thousand]) and temperatures (Chanley & Castagna 1971).

Most Tagelus species, including T. plebeius, are economically important as a source of food in Chile, China, Spain, and other countries, with the harvest of more than 4000 tons per year in Chile alone (Osorio Ruiz 2002). Some information is available on the biology of other Tagelus species such as, T. dombeii (Urban 1996) and T. divisus (Fraser 1967). Blommer (1907) described the anatomy of T. gibbus and T. divisus, and T. dombeii has been used as an indicator of contamination in marine environments (De Gregori et al. 1994). Despite its economic importance, there are some large gaps in our knowledge of the biology of T. plebeius. Here we present a study of the reproductive cycle of a population in Mar Chiquita Lagoon (Buenos Aires, Argentina).

MATERIAL AND METHODS

From September 1999 to August 2001, 20 adult T. plebeius (more than 40 mm in shell length) were collected every month with a shovel. The sampled population is located in the middle zone of the Mar Chiquita Lagoon. The maximum length of each specimen was measured to the nearest mm with a digital vernier calliper. The edible part was removed from the shell, weighed to the nearest mg, fixed in Bouin's solution for 72 h, preserved in 70% alcohol, dehydrated in ethanol series and infiltrated with paraffin. For histologic analysis, gonad sections of 5 [micro]m thickness were stained with Mayer's hematoxyline and eosin. About 10 females of each sampled month were analyzed under a Zeiss microscope. Of each female the diameter of 10 unbroken oocytes, showing nucleoli were measured from 5 sections. These sections were anatomically distant from each other to avoid repetition of measurements.

Surface water temperature was measured with a mercury bulb thermometer four times a day on 3 consecutive days of each month to calculate the daily mean temperature of the month. The mean water temperature and its standard deviation (SD) were plotted together with the gonadic stage and wet weight of the individuals.

RESULTS

No Hermaphroditism or Sex Reversal Was Encountered

Sex can be distinguished only histologically and presents a ratio of male to female not significantly different from 1:([chi square] = 0.52, P > 0.05).

The diameter of the oocytes (Fig. 1) and mean body mass (Fig. 2) were correlated with water temperature (Fig. 3), with significant decreases at the beginning of spawning and reabsorption from October 1999 to January 2000 (ANOVA, P < 0.05), May to July 2000 (ANOVA, P < 0.05) and November 2000 to March 2001 (ANOVA, P < 0.05).

[FIGURES 1-3 OMITTED]

No Asynchronous Gametogenesis Was Observed

Figure 1 shows how mean oocyte diameter increases, whereas the SD remains more or less constant, during winter maturation. Before spawning begins, all oocytes are mature, mean oocyte diameter is at its highest with a small SD. Once spawning begins, the mean oocyte diameter decreases and the SD increases, because a new cohort of oocytes simultaneously starts to grow. As water temperature rises in the early spring, body mass increases and gametes start developing (Fig. 4), reaching their maturity (Fig. 5) in October. During 1999, the main reproductive event occurred between December and January, when modal oocyte diameters were 75-95 [micro]m and water temperature in the lagoon was about 20[degrees]C. This event was characterized by a nearly complete evacuation of the gonads (Fig. 6) and a sharp drop in shell free wet mass (SFWM) to the annual minimum. After this event, new gametes developed and body mass increased again rapidly. Another spawning event occurred at the end of March 2000 with a similar body mass fluctuation.

[FIGURES 4-6 OMITTED]

During the period of sexual activity, non-released gametes coexist with developing ones. When the water temperature dropped below 20[degrees]C, gonad reabsorption started (Fig. 7) and SFWM decreased. By winter, gonads were completely reabsorbed.