Razor clam (Sinonovacula constricta) seafarming in China started almost 500 years ago mainly in the southeast region and particularly in the Fujian and Zhejiang coastal regions. In recent years, fishermen from Shandong Province have shown great interest in this bivalve species and have established culture sites. Expansion is however limited by the fact that the spat have to be brought in from the south. At present there are about 21,000 ha being cultured in China which produced, in 1988, over 140,000 metric tonnes.
The razor clam Sinonovacula constricta is distributed along the coastline of China and Japan, and typically inhabits intertidal areas with flat, muddy or sand-muddy bottoms (Fig. 5.1).
Figure 5.1. Shell of a razor clam, Sinonovacula constricta.
This species is deep-burrowing and sedentary, moving vertically in the upper substrate layer with the aid of a well developed foot (Fig. 5.2). During high tide the clam is usually found in the upper part of its vertical hole with its siphons extending beyond the sand/water interface. At low tide they retreat deep down their burrow in order to seek protection. The depth and diameter of the hole depends on the size of the clam, texture of the seabed and season. It has been observed that the depth is generally 5–8 times the length of the shell.
Razor clams are unlike other bivalves, such as the scallop, that frequently move over long distances; they tend to dwell in one spot throughout their life. Thus it is easy to culture them in open sea and no special facilities are required to prevent them from escaping.
Figure 5.2. Position of a razor clam during high (A) and low (B) tide.
Although razor clams are filter feeders, passively intaking food organisms from the surrounding seawater by means of their filter organs, they can, to some extent, mechanically select food particle. Observations have shown that the major food organisms in their stomach are benthic diatoms such as Nitzschia sp. Coscinodiscus sp. etc., while few spiny species have been recorded.
The “cheng” clam, as it is called in China, is a dioecious mollusc, however the two sexes can only be distinguished during the spawning season when the reproductive organs are ripe. The male gonad usually appears milky–white with a smooth surface, while the female organ has a rough surface, granular in appearance and beige in colour.
The spawning season of the razor clam is closely related to water temperature and therefore specimens from different localities tend to spawn at different times. In Fujian Province spawning season of razor clams lasts a long time from late September to the following January with the peak period occurring between mid–October and mid–November, while in Liaoning Province in the north spawning starts at the end of June. Adult and fully mature individuals can spawn 3–4 times in one spawning season, at intervals of about two weeks.
A razor clam lays an average of 193,000 eggs per spawning. Table 5.1 lists the fecundity of ten individuals ranging from 44–53 mm in shell length.
Table 5.1 Fecundity of razor clam.
|Shell length (mm)||Fecundity|
Fertilization of the razor clam gametes occurs externally in the surrounding seawater. The various developmental stages are shown in Table 5.2.
Table 5.2. Developmental time of the various embryological stages of the razor clam.
|First polar body||30 min||WT : 22–24 °C|
|Second polar body||60 min||pH : 8.5|
|2–cell||01 hr 52 min||SG : 1.006|
|4–cell||02 hrs 22 min|
|8–cell||03 hrs 02 min|
|Gastrula||06 hrs 50 min|
|Trochophore||08 hrs 50 min|
|Young spats||7–8 days|
Research observations have shown that the developmental rate of a fertilized egg is closely related to the seawater temperature. For example, a fertilized egg takes 5–6 days to develop into a young spat at 25–26 °C. However, at a seawater temperature of 18–20 °C, the time required would be between 9–10 days. The above information makes it possible to forecast the appropriate period for collecting natural spats for subsequent farming.
The growth rate of razor clams is closely related to the prevailing ambient conditions, such as stocking density, seabed texture, availability and type of food organisms, water temperature, salinity, etc. The optimum temperature and salinity for razor clam is 15–25 °C and 5–28 ppt, respectively. In Fujian Province, the highest shell growth rate occurs from May to July, while the fastest body growth rate, in terms of flesh, occurs between July to September. The reason for the above is that flesh accumulation is in inverse proportion to shell length during the reproductive period. One–year old clams usually attain a shell length ranging between 4–5 cm, while two–year specimens would be around 7 cm. After the second year shell growth tends to slow down, thus harvesting is usually carried out between one to two years after initial stocking.
At present, the production and availability of clam spats still depends on wild resources, simply because of the high fecundity of this clam species. If the preparation of the seedling bed is carried out properly, success in spats collection would be ensured.
Before seeding takes place, Chinese fishermen prepare the culture grounds to minimize spat losses. The size and shape of the culture grounds are determined by a number of environmental conditions prevailing on the site. The common practice is to make ditches between beds along the same direction of the tide current as well as constructing dikes (made of clay) on the side of open sea in order to minimize wave action. An important culture practice with regard to spat collection of this species is the plowing and subsequent flattening of the seedling beds. Following the plowing at depth of 20– 30 cm it is essential that the bed surface is level and smooth. Flattening of the beds is carried out 1–2 days before the forecasted date of spat settlement.
Observations have shown that early or late flattening produced poor spat recruitment, as shown in Table 5.3.
Table 5.3. Effects of the timing of bed flattening on spat recruitment.
|Time of flattening|
|No. of spat|
Suitable sites for farming the razor clam should have the following conditions:
intertidal areas, particularly in estuaries, where the substrate is exposed daily for 2–3 hours;
multi-layer substrate. Top 3–5 cm of thick soft mud, a sandy-mud middle layer 20–30 cm thick and a bottom layer of sand;
water temperature in the range of 15–20 °C; and
water salinity in the range of 8–20 ppt.
When the spats attain a shell length of 1.5 cm they are collected and used as “seedlings” in the culture grounds. The planting period in the provinces of Zhejiang and Fujian is usually from January to March. Similarly to rice or wheat seeds, the young clams are evenly broadcast or scattered over the field by hand.
The culture stocking density is related to the initial size of the seed. Seeds 10 mm in length are usually stocked at about 1,500 per m2. However, if the size is larger than 1 cm, the density is decreased proportionally. The number of young clams is generally determined by weight. The relationship between the size and weight of young clams is shown in Table 5.4.
Table 5.4 Relationship between size and weight of young clam.
|Shell length (mm)||5||10||15||20||25||30|
Besides collecting or extirpating predators, cleaning dikes, consolidating dams, etc. there are two other important routine tasks to be performed:
During the summer season, seawater temperature tends to rise sharply, particularly in shallow areas. In order to diffuse the heat which tends to be accumulated by the muddy surface, a layer of sand should be laid over the culture grounds to enhance sunlight reflection and heat dissipation.
During floods, a great deal of mud accumulates over the culture fields. The mud should be regularly removed to avoid suffocation and mortality.
Harvesting season of the razor clam in China is from late August to early September. Razor clams reared for one year usually attain a shell length of 4–5 cm, while two–year old specimens measure about 6 cm. After two years culture the growth rate decreases sharply and high mortality occurs.
Harvesting is carried out with simple hand tools, such as spades, hooks, etc. Usually one man can collect 30–40 kg a day. Most of the clams are sold fresh shell–on in local market, while only a small percentage is dried.
One major constraint to the expansion of razor clam culture is the limited supply of seed.