Reproductive Cycle of Bolinus Brandaris (Gastropoda: Muricidae) in the Gulf of Gabès (Southern Tunisia)

Research Article Mediterranean Marine Science Indexed in WoS (Web of Science, ISI Thomson) and SCOPUS The journal is available on line at http://www.medit-mar-sc.net http://dx.doi.org/10.12681/mms.325 Reproductive cycle of Bolinus brandaris (Gastropoda: Muricidae) in the Gulf of Gabès (southern Tunisia) K. ELHASNI1, P. VASCONCELOS2,3, M. GHORBEL1 and O. JARBOUI1 1 Institut National des Sciences et Technologies de la Mer (INSTM), Centre de Sfax, B.P. 1035, Sfax 3018, Tunisie 2 Instituto Português do Mar e da Atmosfera (IPMA), Avenida 5 de Outubro s/n, 8700-305 Olhão, Portugal 3 Centro de Estudos do Ambiente e do Mar (CESAM), Departamento de Biologia, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal Corresponding author: [email protected] Received: 9 November 2011; Accepted: 22 October 2012; Published on line: 20 February 2013 Abstract The reproductive cycle of the purple dye murex (Bolinus brandaris) from the Gulf of Gabès was studied through gonad histology and calculation of bio-physiological indices (general condition index - K and gonadosomatic index - GSI). The shell length at first sexual maturity (SL50) of B. brandaris was also investigated using a macroscopic maturation scale. The population’s sex ratio was statistically skewed (M:F = 1:1.5), with a significant dominance of females over males. Monthly variations in the maturation stages of gonads, K and GSI showed that B. brandaris has an annual reproductive cycle, a long period of gonadal activity and a slight asynchronicity between the sexes. The spawning season comprised the period between April and July, with a clear spawning peak from May to June. The SL50 was reached at 56.4 mm in females and 54.6 mm in males. Although at present B. brandaris has no commercial value in Tunisia, it is predictable to become a fishery-exploited species in the near future. The present data, particularly the timing and duration of the spawning season and the size of individuals of both sexes at first sexual maturity, provide baseline information to propose the early adoption of fishery management measures, aimed at ensuring sustainable exploitation and long-term preservation of this alternative fishing resource, once commercial exploitation of this species commences. Keywords: Purple dye murex, Bolinus brandaris, reproductive cycle, size at first sexual maturity, fisheries management, Gulf of Gabès. Introduction al., 2009, 2011), constitutes an important component of the benthic communities in the Gulf of Gabès (southern Around the world, there are diverse fishery-exploited Tunisia), and is one of the most common shellfish species gastropod species, which number around 720 (Leiva & caught as by-catch by bottom trawlers. Recent fishing Castilla, 2002). Latest global fishery production statistics surveys in this area (dealing with stock assessment and in respect of miscellaneous marine gastropods (abalo- population structure of the fishing resources) showed that nes, winkles and conchs) total up to about 131 x 103 tons B. brandaris is mainly found in the 10−80 m depth range, whose approximate value is estimated in US$ 768 mil- with 45 and 60 m depth providing the highest catches per lion (FAO, 2011). The purple dye murex Bolinus branda- unit effort (CPUE = total biomass / hour) (K. Elhasni, ris (Linnaeus, 1758), a commercially valuable muricid unpublished data). species, is found on soft bottoms in the Mediterranean Many different types of studies have been conducted Sea and Atlantic Ocean, from the coast of Portugal to in regard to the purple dye murex, including the fishery Morocco (Poppe & Goto, 1991). This species has been and population structure in the Catalan coast (Martín et exploited since ancient times, not only as seafood but al., 1995) and in the Ria Formosa lagoon (Vasconcelos also for the purple dye used to decorate garments. Today, et al., 2008a). As regards B. brandaris, the most studied it is targeted by small-scale fisheries along the coasts of topic is imposex (development of male sexual characters Portugal (Vasconcelos et al., 2008a), Spain (Martín et onto females of prosobranch gastropods), both in the At- al., 1995), Italy (Ramón & Amor, 2001) and along the lantic Ocean, specifically in Portugal (e.g. Vasconcelos coastline of Greece (Katsanevakis et al.,2011). In Tuni- et al., 2010; 2011), and in the Mediterranean Sea, espe- sian waters, B. brandaris is found in the Bizerte lagoon cially in Spain (e.g. Solé et al., 1998; Morcillo & Porte, and in the Gulf of Tunis (northern Tunisia) (e.g. Abidli et 1999; Ramón & Amor, 2001) but also in Italy (Chiavarini 24 Medit. Mar. Sci., 14/1, 2013, 24-35 et al., 2003). In the southern Mediterranean, data on of B. brandaris from the Gulf of Gabès, including as- the incidence of imposex and the degree of severity in pects such as the sex ratio, gonad maturation and spawn- B. brandaris were gathered during monitoring surveys ing periods, and the size at first sexual maturity. conducted along the Moroccan (Lemghich & Benajiba, 2007) and Tunisian coasts, specifically in the Bizerte la- Materials and Methods goon and in the Gulf of Tunis (Abidli et al., 2009; 2011). In contrast, the information available on the reproductive Study site and sampling cycle of the purple dye murex is relatively scarce despite Monthly sampling was carried out from January the fact that it is commercially exploited both in the Med- through December 2007 on board commercial bottom iterranean and in the Atlantic, especially in Spain (Martín trawlers operating out of the port of Sfax (southern Tuni- et al., 1995) and in Portugal (Vasconcelos et al., 2008a). sia). This fishing fleet essentially targets demersal fishes Indeed, the availability of data is confined to descriptions and crustaceans (mainly shrimps), besides the purple dye of the morphological and ultrastructural aspects of the murex caught as by-catch. 150−180 B. brandaris were reproductive system (e.g. Amor et al., 2004; 2007), while collected each month (an average of 160 individuals per two studies relate to the reproductive cycle of B. branda- month), from relatively shallow waters (ranging between ris harvested from the Spanish coasts of Catalonia (Ram- 40 and 50 m depth) of the Gulf of Gabès (Fig. 1). ón & Amor, 2002) and Andalusia (Tirado et al., 2002). In the laboratory, individuals were measured for Although the species is frequently found in the nets shell length (SL) using a digital calliper (precision 0.05 of bottom trawlers in the Gulf of Gabès, fishermen sort- mm) and weighed for total weight (TW) on a digital bal- ing the catches retain the commercially valuable target ance (precision 0.001g). The shell of B. brandaris be- species such as fishes, cephalopods and crustaceans but ing rather fragile (particularly the siphonal canal), it is usually discard the catches of B. brandaris. At present highly vulnerable to fishing damage caused by bottom this species has no commercial value in Tunisia and is not trawling. Therefore, in order to minimise bias in meas- found in local seafood markets. However, some fisher- urement, only those specimens whose shells were intact men retain small amounts of B. brandaris for their own were included in laboratory analyses. In addition to this, consumption, considering its meat to be tastier than that to reduce susceptibility for inaccurate weighment, indi- of Hexaplex trunculus, another muricid species common- viduals were first lightly blotted and dried, so as to allow ly found in the seafood markets of Tunisia. Therefore, as much water as possible to drain out from the mantle it is predictable that B. brandaris might soon become cavity. Since the purple dye murex does not exhibit exter- a commercially-exploited species. Moreover, the high nal sexual dimorphism, individuals were sexed after the value attributed to it in countries of southern Europe (es- sexual organs had been exposed by breaking and remov- pecially in Portugal and Spain, where first sale prices can ing the shell. Moreover, due to the occurrence of imposex reach 20 €/kg) (Vasconcelos et al., 2008a) indicates that (development of male sexual characters onto females of Tunisia might promote the exportation of the catches B. prosobranch gastropods) in B. brandaris collected in the brandaris to foreign countries in the near future. Never- study area (K. Elhasni, unpublished data), females were theless, lack of scientific knowledge persists on the spe- identified by the presence of a vulva and a capsule gland, cies’ biology, especially in this area, with particular refer- and males by the presence of a penis and the absence of ence to its reproductive cycle. a capsule gland. With exploitation of new gastropod species showing an increasing trend in international markets (Averbuj et Histology of the gonads al., 2010), and given the fishing potential ofB. brandaris For histological purposes, 20 individuals of both sexes in the Gulf of Gabès, it is of utmost importance to im- were sampled every month. Individuals were de-shelled prove the knowledge about this potentially valuable local and the central portion of the complex digestive gland- resource before the development of a commercial fishery. gonad was dissected. The collected tissues were fixed in A similar approach, i.e., gathering data on the species bi- Bouin’s fluid for 72 hours before being preserved in 70% ology before or at the early development of new fisheries, ethanol. These tissues were dehydrated afterwards in an was adopted earlier for other gastropod resources all over ethanol series (70-100%), cleared in toluene and em- the world (e.g. Valentinsson et al., 1999; Narvarte, 2006; bedded in paraffin wax blocks. Gonads were sliced into Averbuj et al., 2010). Similarly, within the plausible sce- sections of 4-7 µm thickness using a microtome, before nario of becoming a fishery-exploited species in the Gulf being stained with haematoxylin–eosin.

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