Comparison of biochemical composition and muscle hypertrophy associated with the reproductive cycle of diploid and triploid scallops, Argopecten ventricosus

Journal of Shellfisheries Research, August, 2004 by E. Palacios, I.S. Racotta, A.M. Ibarra, J.L. Ramirez, A. Millan, S. Avila

ABSTRACT Triploid organisms have a limited capacity to develop gonads, and thus are considered sterile or partially sterile. The objective of this study is to compare triploid and diploid organisms of the same age, and grown under similar conditions during a 1-year period, to determine to what extent sterility affects size and number of adductor muscle cells, and accumulation of biochemical reserves at first maturation. Adult catarina scallops (Argopecten ventricosus) were matured, and eggs were treated with cytochalasin-B to induce triploidy. Untreated diploid controls and triploids were grown in Bahia Magdalena, Mexico in Nestier trays at 3 m depth from April 2000 to March 2001. Scallops were sampled monthly from June 2000. The gonadosomatic index (GSI) was significantly larger in diploid organisms, whereas the muscle index was larger in triploid organisms. A small proportion (40%) of diploid scallops exhibited reproductive activity during the first year. In spite of high GSI, the gonads of triploid scallops were mostly immature, except for some individuals that formed mature oocytes by March 2001. Triploids had 123% higher adductor muscle weight than diploid scallops at the end of the sampling period, and most of this increased gain was a result of adductor muscle cell size (hypertrophy), and not cell number (hyperplasia). Total lipids and proteins concentrations were significantly larger in the gonads of diploid scallops. Muscle carbohydrates were significantly larger in triploid scallops, which suggests decreased transference of carbohydrates form muscle to gonad in triploid sterile organisms.

KEY WORDS: energy-of-allocation, gonad, maturation, muscle, oocytes. Pectinidae, reproduction, Argopecten

INTRODUCTION

During the reproductive period of pectinids, there is an important accumulation of lipids and proteins in the gonads (Barber & Blake 1981, Barber & Blake 1985, Martinez 1991, Pazos et al. 1996), mostly provided by food or mobilization of endogenous reserves when food is not readily available (Barber et al. 1991). The adductor muscle of pectinids is the site of one of the largest reserves of glycogen and protein mobilized during maturation (Barber & Blake 1981, Barber & Blake 1985, Epp et al. 1988). When reproductive activity begins, there is a large transfer of biochemical components to the gonad, and the weight of the muscle decreases. Also, water content in the adductor muscle is modified, and this can affect its taste and texture and thus its market price (Allen & Downing 1991). Triploid organisms have limited capacity to mature, and in most species where triploidy is attained, individuals do not produce gametes or viable larvae, and are considered sterile. An advantage of sterile organisms is that they transfer less biochemical energy to the gonad, so other tissues can gain more weight over time. For this reason, triploidy has been induced in pectinids (Tabarini 1984, Beaumont 1986, Komaru & Wada 1989, Ruiz-Verdugo et al. 2000, Yang et al. 2000, Maldonado-Amparo et al. 2004) and other mollusks of commercial interest (for review, see Beaumont & Fairbrothers 1991). The adductor muscle weight of triploid catarina scallops can be 182% that of diploids after 280 days of grow-out (Ruiz-Verdugo et al. 2000). In catarina scallop, reproduction has a significant impact on meat (muscle), because commercial scallops are harvested at 1 year (6-cm length), and first maturation age can be as early as 4 months or 2-cm length (Cruz et al. 2000). Although triploid muscle weight gain is assumed to result from less transfer of biochemical energy from the muscle to the gonads (Ruiz-Verdugo et al. 2001b), no studies have analyzed the effect of increased muscle energy reserves on muscle fiber growth (hypertrophy) or number (hyperplasia). An increase in muscle fiber numbers or size can have different effects on the texture of the muscle. The objective of this study is to compare triploid and diploid organisms of the same age and grown under the same conditions during a 1-year period, to determine to what extent sterility affects the biochemical reserves accumulated in muscle, and the size of adductor muscle cells.

MATERIAL AND METHODS

Adult catarina scallops (Argopecten ventricosus) were matured under laboratory conditions at CIBNOR, as described by Ramirez et al. (1999). For triploid induction, eggs were treated with 0.5 mg/L cytochalasin-B (CB) during 15 min, after 50% of eggs were in the first polar division, as described by Ruiz-Verdugo et al. (2000). Untreated diploid controls and treated organisms (containing triploids and diploids) were transported to Laguna Rancho Bueno (Bahia Magdalena, B.C.S., Mexico) where they were grown separately in Nestier trays at 3 m depth (Ruiz-Verdugo et al. 2000).

A total of 30 scallops (from both CB-untreated and treated groups) were collected at approximately monthly intervals from the grow-out area at Laguna Rancho Bueno from June 2000 to March 2001 and transferred to the laboratory at CIBNOR. Upon arrival, total live animal weights, and shell-free tissue weights (biomass) of all samples were recorded. Adductor muscle and gonad weight were recorded from July 2000 to the end of the study in March 2001. Organ indices for the two tissues were calculated as the proportion of organ wet weight to total tissue weight minus gonad weight, to eliminate any effect of on-going maturation.