This study investigates the reproductive cycle, size at first sexual maturity and biochemical composition of Hexaplex trunculus from the Bizerte lagoon (northern Tunisia). Overall, females predominated over males, resulting in an unbalanced sex ratio of 1.2:1. Males dominated in the smaller size classes (< 40 mm shell length, SL), sex ratios were balanced at the 40 to 50 mm SL range, and females dominated in the larger size classes (> 50 mm SL). The gonad developmental stages and the gonadosomatic index (GSI) indicated that H. trunculus apparently has an annual reproductive cycle with extended periods of gonadal activity. In both sexes, examination of the appearance of the gonads showed that gamete release occurred mainly from March to May (with an apparent spawning between March and April), followed by a period of empty gonads between June and August. Sexual maturity occurred at smaller sizes in males (SL 50 = 41.02 mm SL) than in females (SL 50 = 50.47 mm SL). The contents of protein, lipids and carbohydrates were invariably greater in the gonads than in the foot, confirming the key role of the reproductive tissues in energetic storage. The monthly variation in protein, lipids and carbohydrates in the gonads followed the oscillation in the relative proportion of developed gonads and in the GSI, reflecting energetic mobilisation during maturation and spawning. Overall, the data gathered in this study constitutes valuable baseline information for making preliminary recommendations to the fishing community targeting H. trunculus in the Bizerte lagoon. Further studies and more solid data on the reproductive cycle of the species are required before definitive management measures for this locally important artisanal fishery can be proposed. KEY WORDS: Reproductive cycle · Spawning season · Size at sexual maturity · Biochemical composition · Fisheries management · Hexaplex trunculus · Muricidae · Bizerte lagoon Resale or republication not permitted without written consent of the publisherAquat Biol 10: 155-166, 2010 156 the Atlantic is mainly restricted to the coasts of Portugal and Morocco, and the Madeira and Canary archipelagos (Poppe & Goto 1991, Houart 2001). This species is harvested for human consumption and has commercial value in several Mediterranean countries such as Spain, Italy, Croatia, Cyprus and Turkey (Fischer et al. 1987), as well as along the Atlantic coasts of Spain (mainly in Andalusia) (Anon 2001) and Portugal (mainly in Algarve) (Vasconcelos et al. 2008a). In the Bizerte lagoon (northern Tunisia), the banded murex is the target species of an artisanal fishery using baited traps, but is also a bycatch species in fishing nets (Gharsallah et al. 2004). The banded murex constitutes a locally important fishing resource in the Bizerte lagoon, with estimated monthly catches of ~1.8 tons between 1999 (I. H. Gharsallah unpubl. data), but the fishery lacks specific regulations (e.g. minimum landing size, closed seasons in fishing activity).While some studies on the reproductive biology an...

The grooved carpet shell Venerupis decussata (=Ruditapes decussatus) (Linnaeus, 1758) is a Mediterranean autochthonous species that in Tunisia is heavily harvested from natural populations. It represents one of the important economic issues, especially in terms of employment and the trade balances of the country via its exportation. Despite its economic value, no studies have been carried out about the spread of this mollusc. Thus the status of this species is still poorly known and there is a knowledge gap about its stocks assessment. This study aimed to investigate the distribution, abundance and population structure of the carpet shell clam in Sfax coastal areas. In terms of geographic occupation, maps of the population distribution were drawn. The results showed a scattered distribution pattern of the species according to location ranging between 0 and 152 ind m−2. The consequence was a remarkable biomass which represented 891 tonnes of total fresh weight and high abundance levels reaching over 261.7 ± 84.2 million individuals estimated in the area of 4182 hectares. The distribution of the species has also been investigated in consideration with the size which varied markedly according to location, the shell length ranged between 2 and 41 mm. The overall sex ratio (F:M) was 1:1.17, which was significantly different from parity (1:1). The size at which 50% of the population reached maturity was 24.08 and 24.76 mm shell length for males and females, respectively. Findings of the current study suggest that V. decussata proliferates along southern Tunisian coasts.

Detailed description of the operculum of Hexaplex trunculusComprehensive knowledge on the gastropod operculum is essential to elucidate the underlying principle of using this structure for ageing purposes. In this context, the following paragraphs present a detailed description of the operculum of the banded murex (Hexaplex trunculus), including information on the type, function, morphology, structure and composition.The operculum of H. trunculus is brownish (light-brown in smaller specimens and darkbrown in older individuals), roughly oval-shaped and moderately slim (Figures 1A,B). The operculum is corneous or horny, i.e. the bulk of the opercular material is composed mainly by a proteinaceous substance known as conchiolin, more or less impregnated with salts of calcium (Fretter & Graham 1994;Checa & Jiménez-Jiménez 1998).As typically found in siphonostomatous shells, H. trunculus has a rigiclaudent operculum (aperture fitting) (Figure 1A) that is shaped to conform perfectly to the shell aperture and almost does not flex when the organism retracts and seals the opening (Checa & Jiménez-Jiménez 1998). Nevertheless, because conchiolin is not rigid, the edges of the operculum are somewhat flexible (the edges flex and bend outwards as the animal withdraws into the shell, except for the columellar edge which is buried in the opercular groove) (Fretter & Graham 1994).Like most neogastropods, H. trunculus has a concentric operculum (Checa & Jiménez-Jiménez 1998) with a nucleus in the left side of the siphonal edge of the operculum (siphonal nucleus, representing the beginning of its formation) (Figure 2B). In species with a relatively long and wide siphonal canal (such as H. trunculus), concentric opercula with siphonal nucleus allow for a more precise fitting of the opercular edge to the shell aperture (Fretter & Graham 1994;Checa & Jiménez-Jiménez 1998). Indeed, concentric opercula have a band in the internal surface (close to the labrum), which wedges outward and becomes concave towards the edge of the operculum (Figure 2A), providing an exact fit to the interior of the shell aperture (Checa & Jiménez-Jiménez 1998).The underside of the operculum is attached to the dorsal surface of the foot by the operculigerous disc (metapodium) (Fretter & Graham 1994), which is a conspicuous disc-like epithelium quite distinct from the surrounding foot (Checa & Jiménez-Jiménez 1998). The operculigerous disc is never fully adhered to the internal surface of the operculum, being restricted mainly to the labial half of the operculum (Checa & Jiménez-Jiménez 1998). Most part of the operculum underside is roughened and with a fingerprint-shape (coinciding with the area of attachment of the columellar muscle) (Figure 2A) (Fretter & Graham 1994). In horny opercula, a shiny material (known as varnish or gloss) secreted by the foot epithelium is applied underneath the conchiolin (Fretter & Graham 1994;Checa & Jiménez-Jiménez 1998). This varnish layer is limited to a strip around the labial and siphonal edges of the operculum, where the con...

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