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Larval rearing and spat production of the razor clam Ensis siliqua (bivalvia: pharidae).


The pod razor clam Ensis siliqua (Linnaeus, 1758) inhabits fine sands and shows an extended latitudinal range from the Norwegian Sea and the Baltic, south to the Iberian Peninsula, into the Mediterranean, and along the Atlantic coast of Morocco (Tebble 1966). This species is most abundant between 3 m and 7 m, although it is found to depths of 10 m (Monteiro & Gaspar 1993). These clams are normally found burrowed with the long axis of the shell orientated roughly vertical (into the sediment). They are fast, deep burrowers.

E. siliqua is harvested by commercial fisheries in Spain, Portugal, and Ireland (Guerra & Lodeiros 2008). European production of razor clams has greatly increased, reaching 1,104 tons in 2003. However, captures vary from year to year, with a maximum of 2,670 tons in 1995 and low productions between 400 tons and 700 tons for 1997 and 2002, respectively (FAO 2005). These fluctuations could be the result of high fishing pressure and overexploitation of natural beds (Gaspar et al. 1998, Tuck et al. 2000, Fahy & Gaffney 2001). A remarkable feature of this fishery is its alternating yield year by year, even if this pattern is not uncommon among exploited bivalves (Tegelberg & Magoon 1969). These fluctuations are mainly the result of variations in recruitment strength, but fishing has been reported to amplify this phenomenon in other razor clam species, such as Ensis minor (Goriup et al. 1995).

To overcome these problems, aquaculture of razor clam species should be developed. To ensure the sustainability of razor clam fisheries, the production of seed for restocking throughout the year has become essential to alleviate the dependence on unreliable natural spatfall. An example can be seen along the Atlantic coast of France, where in the mid 1970s wild populations of juveniles of the scallop Pecten maximus collapsed, leading to the reliance on hatchery-produced juveniles to rebuild the fishery (Devauchelle & Mingant 1991).

The development of aquaculture systems for the production of the European razor clam has been scarce. The main difficulty in developing culture techniques for razor clams, such as the species Solen marginatus (da Costa & Martinez-Patino 2009), has been the requirement of a substrate to allow burrowing, thus preventing shell gaping resulting from the weak abductor muscle present in this species. For E. siliqua, it is essential to develop systems that will meet the requirements to enable the seed to reach a size suitable for transfer to on-growing sites. A major problem with hatchery cultures of the razor clam E. siliqua is the low survival during postlarval and seed cultures. Several factors should be considered when rearing E. siliqua seed for nursery systems--namely, the systems used to hold the seed as well as the period of time the seed can be held without substrate.

This study presents the results of the first investigations on larval culture and spat production of the razor clam E. siliqua. This study will define the adaptability of this species to culture conditions, thus assessing the potentiality of aquaculture development in this species or as a management tool for restocking or stock enhancement of depleted wild beds.


Broodstock Collection and Maintenance

One hundred individuals measuring 116.5 [+ or -] 10.9 mm in shell length (surpassing commercial size established at 100 mm) were hand gathered by divers from a natural bed in a subtidal area of Sardineiro, in Ria de Corcubion (Galicia, northwest Spain).

At bivalve hatchery facilities of Centro de Cultivos Marinos de Ribadeo-CIMA, the shells of the razor clams were cleaned to remove adhering debris and fouling organisms, thoroughly rinsed with filtered seawater, and then 100 individuals were placed in each of two 200-L rectangular fiberglass tanks. Individuals were cultured at 19 [+ or -] 1[degrees]C and 32 [+ or -] 2 ppt salinity in gently aerated sand-filtered seawater. The seawater was renewed daily by replacing approximately twice the tank volume. The individuals were fed daily with a mixed culture of Tetraselmis suecica, Isochrysis galbana, Pavlova lutheri, Chaetoceros calcitrans, Phaeodactylum tricornutum, and Skeletonema costatum in equal proportions at a ration of 6% mean dry meat weight of adults per day. Broodstocks were kept in these conditions for 1 wk and then induced to spawn. A subsample of 20 was used to determine the sex ratio. The proportion of males to females was established at 1:1.

Spawning Induction

Groups of 25 individuals selected at random from the conditioning tanks were exposed to different stimuli:

1. Thermal shock, with temperatures up to 25-27[degrees]C for 1 h, decreasing to 10-12[degrees]C for 30 min. A total of 3-4 cycles were performed. Additional stimulus was provided by adding gametes stripped from one of the conditioned bivalves.

2. Changing water levels by simulating tides, with brief dry periods as described in da Costa et al. (2008). The method consisted of leaving individuals for 30 min in emersion, and then for 1 h in seawater at ambient temperature (15 [+ or -] 1 [degrees]C). Thereafter, 3-4 cycles were repeated.

3. Stripping gonads of the conditioned razor clams.

4. Ultraviolet-treated seawater flow.

Individuals were placed into 200-L rectangular tanks without any substratum and held in row bars tied individually with rubber bands in open circuit. The specimens responding to the stimulus were separated into individual 1-L test tubes for the release of sperm or eggs, thus avoiding self-fertilization. A mixture of sperm from 3-4 males was used for fertilization at a ratio of 2 mL sperm/L egg suspension. Fertilization was conducted in a 5-L flask. After fertilization, the eggs were sieved through a 45-[micro]m screen to eliminate excess sperm.

Larval Rearing

The embryos were transferred to 500-L larval culture tanks with aerated water at 19 [+ or -] 1[degrees]C and 32 [+ or -] 2 ppt salinity. The water was changed every 2 days using 1-[micro]m sand-filtered, ultraviolet-sterilized seawater. Larval density in each container at the beginning of the experiment was 5 individuals/mL. D-stage veliger larvae were fed daily with T. suecica, I. galbana, P. lutheri, and C. calcitrans in equal proportions at 40 cells/[micro]L as the initial ration. Five different larval batches were studied during juvenile production. Larvae were collected from each tank through nylon screens, and shell length was measured for 100 individuals using a Nikon Laborphot-2 microscope connected to the image analyzer PC Image. All cultures were performed in duplicate.

Spat Production

Postlarvae were cultured in sieves with a thin layer of sand in 500-L larval rearing tanks with a down-welling system. Every postlarval batch was split into 2 tanks as replicates. Two different postlarval batches were studied during juvenile production for the on-growing experiment. The water was changed 3 times a week and the temperature was set at 19 [+ or -] 1[degrees]C and 32 [+ or -]] 2 ppt salinity. Feeding was prepared with P. lutheri, I. galbana, T. suecica, and C. calcitrans in equal proportions at an initial daily ration of 80 cells/[micro]L. Postlarval shell length for each tank was determined weekly for 50 individuals using a Nikon SMZ-10A stereozoom microscope connected to the image analyzer PC Image or a digital caliper for individuals with a length more than 3 mm.

When spat reached 10 mm in length, they were transferred to 200-L rectangular tanks in open circuit with a 10-cm layer of sand at the bottom for razor clam burrowing. The seed were fed with a mixed diet of T. suecica, I. galbana, P. lutheri, C. calcitrans, P. tricornutum, and S. costatum at the same proportion at a ration between 2% and 3% mean dry meat weight of the individuals per day. Samples for measuring seed were taken weekly. Razor clams were measured using a digital caliper and weighed individually on a GRAM Precision ST 3100 balance (SD, 0.01 g).

Substrate Trial during Seed Culture

Seed with an initial length of 2.54 [+ or -] 0.83 mm were cultured in sieves with inverted flow caused by airlifts in 1,000-L rectangular tanks in open circuit as described in the previous section. Growth and survival were compared during nursery culture experiments of E. siliqua using fine-to-middle-grain sand with low organic content and without substratum in 0.5-[m.sup.2] containers. Samples were collected every week to determine growth and survival.


Spawning Induction

Adult individuals of E. siliqua were spawned in the hatchery during May and June. Thermal shock was the only effective spawning method. Males released sperm as a dense cloud in the water column. Females released gametes discontinuously, for approximately 1 h, with the eggs gathering and appearing as whitish dots. The number of larvae reared from each of the 5 spawnings ranged from 2.25-5.08 million eggs (Table 1).

Larval rearing

Unfertilized eggs of E. siliqua were brownish and spherical, ranging from 76.9-99.3 [micro]m in diameter in the different larval batches (Fig. 1A). Typical straight-hinged D-shaped larvae developed from a trochophore by 24 hours-post-fertilization and these were 123.1-138.4 [micro]m long (Fig. 1B). At this time, D larvae have a digestive system consisting of a mouth, foregut, and digestive gland followed by an intestine and anus. The postanal tuft of a few simple cilia was dorsal to the anus. The mantle cavity was evident at this stage of development.

Five-day-old larvae, which become oval in shape, were slightly umbonate, with a length ranging from 201.6-221.8 [micro]m. Ten days after fertilization, they reached the pediveliger stage, with a size ranging from 318.4-366.8 [micro]m, crawling and swimming for short intervals, with the vellum still functional (Fig. 1C). Fifteen-day-old larvae settled with a size of 361.5-414.8 [micro]m (Fig. 1D). Larval survival ranged from 20.0-52.7%, with an average of 39.4% (Table 1).

The relationship between larval shell length and height was linear (Fig. 2) and described as

log H = 1.043L - 0.195; [r.sup.2] = 0.959, P < 0.05,

where L and H are the length and height (measured in micrometers) of the larvae, respectively.


Spat Production

Newly settled postlarvae measured 304.9 [+ or -] 28.9 [micro]m, whereas 1-mo-olds reached 652.3 [+ or -] 79.6 [micro]m in length. During this phase, the shape of the razor shell became apparent. The shell was still translucent and internal postlarval organs were visible (Fig. 3A). Mortality was very high from settlement until 1 mo of cultivation, with survival of 1-mo-old spat as low as 5.2%. Progressively, the shell became opaque and colored, and took on the appearance of the adult shells (Fig. 3B). Three-month-old spat measured 20.9 [+ or -] 2.7 mm in length, reaching 39.0 [+ or -] 4.0 mm after 180 days of culture (2.2% survival from settlement). The growth of postset razor clams can be described as

L = 0.001 [.sup.2.034],

where L is length in millimeters and x is the number of days of culture (Fig. 4).

Weight was 0.01 g/individual after 60 days of culture, increasing to 0.13 g at day 90, and reaching 0.87 g after 180 days of culture.


Substrate Trial during Seed Culture

Growth of individuals held in containers with substratum was faster than when the experiments were carried out without sand. Seed reared with substratum reached 5.28 [+ or -] 1.03 mm in length after 14 days of culture, whereas without substrate they only reached 4.17 [+ or -] 1.30 mm. In contrast, survival was higher without substratum (70.36%) than with (33.03%).


Thermal shock successfully induces spawning in some bivalves. E. siliqua broodstocks easily released gametes when subjected to thermal stimulation from l0-25[degrees]C. Similarly, spawning was obtained by exposing Ensis directus to an initial increase in temperature from 13-25[degrees]C, and Ensis macha when the temperature was raised from 11[degrees]C to 17-18[degrees]C (Loosanoff & Davies 1963, Lepez et al. 2008). However, not all razor clam species respond to thermal stimulation. S. marginatus broodstocks failed to spawn when subjected to thermal stimulation (da Costa & Martinez-Patino 2009). When returned to tanks containing seawater (19-20[degrees]C), spawning occurred the next day. In Ensis arcuatus, changing water levels by simulating tides caused spawning (da Costa et al. 2008). Larval production of E. siliqua is guaranteed by thermal shock stimulation. One constraint in larval production is that the spawning season, which occurs during May and June (Darriba et al. 2005), in natural populations is short. Therefore, conditioning methodologies should be developed in an attempt to extend the spawning season and to secure larval availability for seed production.

Although the oocyte size in E. siliqua varied greatly, ranging from 76.9-99.3 [micro]m in diameter, settlement of all larval batches occurred after 14-15 days of rearing. Martinez (2002), using similar culture conditions (diet and temperature), observed larval settlement after 19 days of culture. In E. arcuatus, egg diameter was approximately 75 [micro]m and metamorphosis took place at day 20 (da Costa et al. 2008). Loosanoff and Davies (1963) found that E. directus eggs varied in diameter from about 64-73 [micro]m, most being 71 [micro]m, and settlement occurred at 24[degrees]C only about 10 days after fertilization. Breese and Robinson (1981) reported that average size of Siliqua patula eggs was 90 [micro]m, with larval development lasting 20 days. Settlement in most of the razor clam species occurred within 15-20 days of rearing, except for S. marginatus, with larvae that settle at day 8-9 because of the large size of the eggs (156 [micro]m in diameter) (da Costa & Martinez-Patino 2009).


Average larval survival in E. siliqua was 39.4%, thus not limiting seed production. Comparisons with other razor clam species show that this average survival is slightly lower than in S. marginatus (53%) but higher than in E. arcuatus (da Costa 2009, da Costa & Martinez-Patino 2009). Lepez et al. (2005) reported larval survival in E. macha of 25-50% in premetamorphic larvae.

After 1 mo of culture, E. siliqua reached 0.6 mm, slightly less than the 1 mm that was reached by E. arcuatus at this time (Darriba 2001). S. marginatus postlarvae grew faster than these two Ensis species, reaching 2.1 mm length 1 mo after fertilization (da Costa & Martinez-Patino 2009). A faster development in this critical phase may reduce mortality in the nursery. Three-month-old E. siliqua juveniles reached 21 mm in length, whereas E. arcuatus and S. marginatus reached 11 and 15 mm, respectively (Darriba 200l, da Costa & Martinez-Patino 2009). Kenchington et al. (1998) reported a size of 7-9 mm in 3-mo-old E. directus juveniles, and Lepez et al. (2008) cited 10 mm in length at the same time for E. macha.

E. siliqua seed culture is feasible: however, survival during the first stages of postlarval culture still hinders high yields during this culture phase. Postlarval and seed survivals were quite low, being 5.2% and 2.2% after 30 days and 180 days from settlement, respectively. Mortality was high from settlement until 1-mo-old spat. After this developmental stage, survival levels off. This behavior was also observed during postlarval culture of the razor clam S. marginatus (da Costa & Martinez-Patino 2009). Postlarval and seed survival were quite low in S. marginatus, being around 5-10% after 30 days and 2-5% after 90 days from settlement (da Costa & Martinez-Patino 2009). Lepez et al. (2008) reported seed survival of 5% after 50 days of nursery culture of E. macha.


The culture of the razor clam is constrained by the fact that they need to be buried and that they are sensitive to manipulation. The use of substrate during early postlarval stages may not allow adequate cleaning of the containers and removal of dead individuals. This may allow proliferation of bacteria and fungi during this culture stage, thus promoting infections driving this sensitive phase of culture. For large-scale seed production, it is important to reduce the volume of the sand needed or even to avoid using substratum. Rearing E. arcuatus spat with a length of 4 mm showed 50% survival without substratum, whereas animals grown in two different types of sand exhibited survival ranging from 80-90% (da Costa et al. 2010). Growth of E. arcuatus spat was similar under the different conditions studied. In contrast, S. marginatus seed reared under the same experimental conditions exhibited higher survival when cultured without substrate (81.9%), compared with 36.1% and 53.3% for the other two treatments with two types of sand (da Costa & Martinez-Patino 2009). Similar results were achieved in the current study with E. siliqua, which revealed better survival in seed grown without substratum (70%) than when grown in sand (33%). One explanation for these results could be that high stocking density may reduce the energy necessary to maintain the shells closed as a result of the weight of neighbors pushing one another. More research is needed to determine what size should be reached for culture without substrate, because of the short time frame of this substrate experiment. High stocking density during nursery culture may be effective for individuals measuring less than 10 mm. Larger than this, stocking density should be reduced to ensure optimal feeding supply. This could lead to increasing energy losses preventing shell gaping, and thus reducing growth rate. Furthermore, it was observed that after a few months of seed rearing without substrate, individuals exhibited deformities in their shells (da Costa, unpublished). Substrate has to be provided progressively to acclimate seed for on-growing conditions in natural beds. Brousseau & Baglivo (1987) found that faster growing Mya arenaria inhabit finer sediments, whereas those that live in coarser sediments expend more energy in repairing damage to their shell margin. Selection of sediment grain size may, therefore, be a factor to consider when aiming to increase growth rate.

Seed availability is one of the major constraints for replenishing the resource or to initiate farming. In this context, the current study provides useful information for hatchery and nursery culture of E. siliqua. These animals are easily induced to spawn and are fast growing; however, low survival exhibited during early postlarval and seed culture still hinders razor clam aquaculture development. The current state-of-the-art precludes this species from being a good candidate for aquaculture. However, this species presents potential for restocking or stock enhancement of natural beds.

The future research needs for the aquaculture production of E. siliqua should focus on postlarval and seed culture. Mortality was high from settlement until 1-mo-old spat in this razor clam. More research is needed to ascertain the specific dietary and environmental (in terms of rearing temperature) requirements of E. siliqua during nursery culture. The improvement of postlarval survival could lead to a large-scale development of this razor clam aquaculture.


The authors are grateful to the staff of Centro de Cultivos Marinos de Ribadeo-CIMA (Xunta de Galicia). Fiz da Costa Gonzalez was supported in part by a Xunta de Galicia fellowship. Special recognition goes to Dr. Miguel Gaspar, Instituto de Investigacao das Pescas e do Mar, for very helpful comments and English corrections. The authors thank J. L. Catoira for the samples and collaboration. This research was funded in part by the SHARE-90 and TIMES projects (Interreg IIIB).


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(1) Centro de Investigacions Marinas, Conselleria do Mar, Xunta de Galicia, 27700 Ribadeo, Lugo, Spain,

(2) Dpto. de Bioloxia Celular e Molecular, Universidade da Coruna, A Zapateira s/n, 15071 A Coruna, Spain

* Corresponding author. E-mail:
Details of larval culture in E. siliqua.

                    No. of Eggs      Percent
Date of Spawning    (in millions)   Settlement

May 14, 2004            5.08          20.02
May 19, 2004            2.83          50.66
May 24, 2005            3.10          28.02
June 1, 2005            2.25          52.69
June 2, 2005            4.67          45.72
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Article Details
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Author:Da Costa, Fiz; Martinez-Patino, Dorotea; Ojea, Justa; Novoa, Susana
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:4EUSP
Date:Aug 1, 2010
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