Method for breeding robust oysters

Abstract

The application relates to a breeding method of robust oysters, belonging to the field of animal genetics and breeding, which comprises population selection, energy recovery of larvae and selection of robust oysters; the selection of the robust oysters is carried out by selecting larvae with strong phototaxis and strong swimming ability by using the phototaxis of the oyster larvae. The robust oysters with the advantages of fast growth speed, large individual, fast attachment rate, disease resistance and stress resistance can be bred by selecting the individuals with strong phototaxis.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of animal genetics and breeding, and particularly relates to a breeding method of robust oysters. BACKGROUND

[0002] Oysters belong to the family Ostreidae, class Bivalvia, order Pterioida, and are a kind of eurythermic and euryhaline shellfish distributed worldwide. They are widely favored by the public due to their high nutritional value and delicious taste, among which more than 20 species have commercial value.

[0003] In recent years, with the frequent occurrence of extreme weather and the expansion of the scale of aquaculture, the oyster aquaculture industry is facing serious environmental pressure and pathogenic infection threats. Carrying out selection breeding to improve the tolerance of oysters to environmental and pathogenic stress is a key means to solve the frequent large-scale death events in oyster aquaculture industry. At present, new oyster strains bred for disease resistance, high temperature tolerance and other traits have made significant progress. Although these strains perform well in dealing with single stress, comprehensive breeding needs to consider multiple economic traits to breed new varieties with disease resistance, stress tolerance and fast growth traits.

[0004] Robustness breeding refers to breeding new varieties of agricultural organisms with high production potential and tolerance to various stresses, which is a frontier breeding direction in agricultural breeding. The robustness of oysters is a very complex trait involving complex environmental and genetic interaction effects. In the face of the current severe breeding environment, selecting appropriate traits to measure the comprehensive robustness index of oysters to screen oysters with multiple high-quality traits is a problem that needs to be solved. SUMMARY

[0005] To solve the problems in the above background, the present application provides a breeding method of robust oysters. The present application finds that the robustness of oysters is closely related to the phototaxis performance of the larvae stage, and by selecting individuals with strong phototaxis ability, comprehensive robust oysters with fast growth rate, large individual size, fast attachment rate, and advantages of disease resistance and stress resistance can be bred.

[0006] The present application is realized by the following technical solutions:

[0007] A breeding method of robust oysters, the method comprising population selection, larval energy recovery and selection of robust oysters; the selection of robust oysters is carried out by selecting oyster larvae with strong phototaxis and strong swimming ability.

[0008] As one of the preferred embodiments, the population is selected by: selecting the D-shaped larvae of oysters; turning on an incandescent lamp, closing the air valve, standing still, and using siphon method to suck the upper water body to the screen gauze, and the oyster larvae are left in the screen gauze, and the bottom layer of weak larvae and un-hatched fertilized eggs are discharged.

[0009] As one of the preferred embodiments, the energy recovery of the larvae is: the selected larvae are re-placed into the breeding pool, the feed is fed, and the air valve is opened, so that the larvae are recovered from the disturbed state.

[0010] As one of the preferred embodiments, the selection of the robust oysters is: after the selected larvae are recovered for a period of time, the oyster larvae are placed into the selection device, and unilateral light irradiation is given, the oyster larvae will swim to the light due to phototaxis, the valve is closed after a period of light irradiation, and because of the different swimming speeds, the larvae will appear in different areas after a period of time, which are three areas of strong phototaxis, medium phototaxis and weak phototaxis, and the larvae in the strong phototaxis area have the strongest swimming ability, and the oysters in the area are considered to be robust oysters.

[0011] As one of the more preferred embodiments, the selection device is a tubular structure with a mouth-shaped structure, the device is composed of a first tubular body, a second tubular body, a third tubular body and a fourth tubular body, the first tubular body, the second tubular body, the third tubular body and the fourth tubular body are communicated with each other, the first tubular body and the second tubular body are at right angles and are provided with a first mirror at the inner wall of the right angle, the second tubular body and the third tubular body are at right angles and are provided with a second mirror at the inner wall of the right angle, the third tubular body and the fourth tubular body are at right angles and are provided with a third mirror at the inner wall of the right angle, the middle parts of the first tubular body, the second tubular body and the fourth tubular body are respectively provided with a first water outlet, a second water outlet and a third water outlet, the first water outlet, the second water outlet and the third water outlet are all provided with covers, the second tubular body and the third tubular body are provided with a first valve and a second valve, and the first tubular body and the fourth tubular body are provided with free ends.

[0012] As one of the further preferred embodiments, the upper side wall of the tubular body at one end of the preferred device is provided with a larva inlet.

[0013] Compared with the prior art, the present application has the following beneficial effects:

[0014] The robust breeding of oysters refers to: the oysters not only have the excellent traits of rapid growth, rapid attachment and low mortality rate in the larval stage, but also have faster growth rate and higher resistance to harsh environment and diseases in adulthood.

[0015] Oyster from the larva to the adult is a process of gradually changing from phototaxis to photophobism. Oyster has strong phototaxis in D-type larva period, can perceive light source and move to the light source. When the light source is unilateral, the phototaxis D-type larva will gather on one side. Through time limit, the oyster larvae with strong swimming ability are closer to the light source, and the oyster larvae with weak swimming ability are farther away from the light source. The oyster larvae with strong swimming ability are better in constitution, high in survival rate, large in individual, and stronger in adversity resistance to high temperature, bacteria and other adversity, which meets the concept of robust breeding and can better meet the current economic market. Therefore, the present application discards the traditional breeding method of selecting through adult phenotype, starts from the phototaxis of oyster larvae, selects the comprehensive robust oyster through phototaxis, and evaluates the comprehensive robust performance, and selects the comprehensive robust oyster which can resist various potential risks in the current breeding environment and also guarantees fast growth speed, large individual and fast attachment. The breeding method not only evaluates the phototaxis and planktonicity of larvae, but also indirectly selects the swimming ability, and the selection device used in the present application can accurately distinguish the oyster larvae of different robustness levels, improve the accuracy of selection, and has higher selection pressure on the larvae than the adult selection, shorten the time of larval culture and reduce the mortality of oyster in the later stage of culture, improve the growth speed of oyster, and can bring great economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The device for selecting robust oyster through phototaxis; 1, first tubular body, 2, second tubular body, 3, third tubular body, 4, fourth tubular body, 51, first reflector, 52 second reflector, 53 third reflector, 61, first water outlet, 62 second water outlet, 63, third water outlet, 71, first valve, 72, second valve, 8, larva inlet.

[0017] Figure 2 The growth curve of oyster larvae from the top of the shell to the completion of attachment in the phototaxis breeding.

[0018] Figure 3 The survival rate curve of oyster larvae from the top of the shell to the completion of attachment in the phototaxis breeding.

[0019] Figure 4 The number traits and survival rate column chart of three-month-old oysters with different phototaxis, wherein A is the shell height of oyster, B is the shell length of oyster, C is the shell width of oyster, and D is the survival rate of oyster.

[0020] Figure 5 The survival curve of three-month-old oysters with different phototaxis after infection of Vibrio alginolyticus.

[0021] Figure 6Survival curves of three-month-old oysters with different phototaxis under high temperature stress. DETAILED DESCRIPTION

[0022] The present application will be described in detail below by examples. It is necessary to point out here that the examples and test examples are only used to further illustrate the present application, but the content of the present application is not limited to the content involved in the examples, and should not be understood as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments according to the content of the present application described above. For example, the size, length adjustment and shape improvement of the device, the adjustment of the time of phototaxis breeding of robustness traits, and even the combination of selection and robustness breeding should all be within the scope of patent protection.

[0023] Example 1

[0024] A breeding method of robust oysters, the method comprising population selection, larval energy recovery and selection of robust oysters; the selection of robust oysters is carried out by selecting oyster larvae with strong phototaxis and strong swimming ability.

[0025] Individuals with large size and full gonads of one-year-old sexually mature oysters are selected as parents to perform artificial insemination and establish a population to be selected.

[0026] Population selection: the fertilized eggs after artificial insemination are hatched in a breeding pond for 24 hours, and selection is started when the larvae are D-type. Turn on the incandescent lamp and close the air valve. After standing for a period of time, due to the planktonic nature of oyster larvae, the larvae with good vitality will float on the water surface. The larvae are filtered into a 300-mesh silk screen by siphoning method. When a certain water level is reached, the larvae with weak vitality and un-hatched fertilized eggs at the bottom are discharged.

[0027] Larval energy recovery: the selected larvae are put back into the breeding pond, and appropriate feed is fed according to the density of the larvae and the size of the water body, and the air valve is opened to allow the larvae to recover from the state of disturbance and store energy for the subsequent selection process of robust oysters.

[0028] Selection of robust oysters by phototaxis: after the selected larvae recover for a period of time, the oyster larvae are put into the selection device, and unilateral light irradiation is given. The oyster larvae will swim towards the light due to phototaxis. After a period of light irradiation, the valve is closed. Because of different swimming speeds, the larvae will appear in different areas after a certain period of time. From the near light source end, there are three areas of strong phototaxis, medium phototaxis and weak phototaxis. The larvae in the strong phototaxis area have the strongest swimming ability, and the oysters in this area are considered to be robust oysters.

[0029] In this embodiment, one-year-old mature "Haida No. 1" Pacific oysters were selected as parent oysters, and a 35x35 population breeding parent was constructed to ensure good genetic background and rich genetic diversity of larvae.

[0030] In this embodiment, the selection time is 24 hours after the fertilized eggs are hatched, the selection starts 30 minutes after the air stone is turned off, and the remaining 12 cm of water level stops the selection. The remaining larvae are considered to be poor in vitality, and are discarded together with poor eggs. The phototaxis breeding is preliminarily screened to ensure the water environment.

[0031] In this embodiment, the larvae are recovered for 12 hours. For energy recovery of the larvae, the time should be as early as possible, and the stress of the larvae should be reduced to not affect the process of subsequent phototaxis selection of robustness. Sufficient feed should be fed during the period to increase the strength while reducing the stress.

[0032] In this embodiment, the method of selecting robustness of oysters by phototaxis is used for the first time. The properties of phototaxis, swimming and plankton are used, and the selection time is 2 hours. The specific time is calculated according to the swimming speed of mollusks of about 0.8-2.4 mm per second as described in the literature, combined with the length of the device.

[0033] In this embodiment, oysters with weak phototaxis have weak vitality, and the survival rate is too low during the larval stage, and a group completely fails to grow, completely losing economic value. The group with weak phototaxis can be considered as error individuals caused by rough selection. Therefore, in the subsequent robustness evaluation, the larvae with weak phototaxis are not evaluated, and are directly eliminated by default.

[0034] In further embodiments, different phototaxis and control groups of oysters are bred according to the normal breeding process after selection, to ensure that the feed is fed three times a day, the water is changed every two days, and a good living environment is provided. During the period, the growth and survival data of oyster larvae are measured, and the oysters are bred to completion of skinning and then hung in the breeding sea area for hanging breeding.

[0035] In further embodiments, the selection device is as follows Figure 1As shown in the tubular mouth-shaped structure, as a preferred embodiment, the selection device is a tubular structure of a mouth-shaped structure, which is composed of a first tubular body 1, a second tubular body 2, a third tubular body 3 and a fourth tubular body 4, the first tubular body 1, the second tubular body 2, the third tubular body 3 and the fourth tubular body 4 are in communication with each other, and the first tubular body 1 and the second tubular body 2 are at right angles and are provided with a first reflector 51 at the inner wall of the right angle, the second tubular body 2 and the third tubular body 3 are at right angles and are provided with a second reflector 52 at the inner wall of the right angle, the third tubular body 3 and the fourth tubular body 4 are at right angles and are provided with a third reflector 53 at the inner wall of the right angle, the middle part of the first tubular body 1, the second tubular body 2 and the fourth tubular body 4 is respectively provided with a first water outlet 61, a second water outlet 62 and a third water outlet 63, the first water outlet 61, the second water outlet 62 and the third water outlet 63 are all provided with covers, the second tubular body 2 and the third tubular body 3 are provided with a first valve 71 and a second valve 72, and the first tubular body 1 and the fourth tubular body 4 are provided with free ends.

[0036] The upper side wall of the tubular body at one end is provided with a larva inlet 8. The light source is a waterproof device that can be charged directly fixed when the device is made at the corresponding end of the larva inlet 8.

[0037] In further embodiments, it is necessary to ensure that the cultivation density of oyster larvae is low enough during the cultivation of oyster larvae to avoid the difference in growth and survival of oyster larvae caused by uncontrollable cultivation density.

[0038] In further embodiments, after the selection of phototaxis, the larvae are not subjected to special light treatment, and the light only plays a role in selection.

[0039] In further embodiments, phototaxis is used for the first time to measure robustness, and phototaxis and robustness are measured by swimming ability. The degree of phototaxis will vary between different oyster larvae, and larvae with the same degree of phototaxis will reach different areas within a limited time due to different swimming abilities. Phototaxis reflects vitality, and long swimming races reflect physical strength. Larvae with good physical strength and vitality will be selected. The growth rate, survival rate, and ability of selected larvae to resist different adversity and disease are more prominent. Therefore, after the selection of phototaxis, the robustness of oysters with different phototaxis is evaluated, which includes the growth rate and survival rate of oyster larvae ( Figure 2 ), the size and survival rate of three-month-old oysters ( Figure 3 ), and the survival curve of three-month-old oysters after high temperature and Vibrio alginolyticus stress ( Figure 4 ).

[0040] The following will be described with reference to specific examples:

[0041] (1) Construction of a candidate population of Crassostrea gigas

[0042] A number of first instar individuals of "Haidai No. 1" with selective full gonad and large size were selected, and each individual was further examined for gonad development under a microscope. 35 female and 35 male individuals were selected as parents, and a candidate breeding population was constructed. The sperm and eggs of the female and male individuals were dissected into a 15-liter bucket, and after 40 minutes of maturation of the egg cells, the sperm and eggs were mixed at a ratio of 1:5 to 1:10, and then stirred with a glass rod to promote fertilization. During this process, microscopic examination was performed every 10 minutes, and when the second polar body was observed, water was added and left for 1 hour, and then the excess sperm was poured out, and the fertilized eggs were transferred to a cement nursery pond for cultivation, and waited for the hatching of the fertilized eggs.

[0043] (2) Population selection

[0044] After the fertilized eggs in the nursery pond hatched for 24 hours, the air stone was closed for 30 minutes, the incandescent lamp was turned on, the water pipe was placed on the surface of the water body, and the upper layer of larvae was sucked until the water body was left with 12 centimeters of water, and then the remaining larvae were considered to be weak in vitality, and were removed together with the inferior eggs that had not hatched at the bottom. After cleaning the cement pond, the larvae were returned to the nursery pond.

[0045] (3) Energy recovery of larvae

[0046] After the larvae were returned to the nursery pond, the air stone was turned on, and golden algae were fed to recover the strength and reduce the stress. During this process, the vitality of the larvae was observed at irregular intervals, and when the vitality of the larvae was fully recovered, phototaxis selection was started.

[0047] (4) Selection of healthy oysters using phototaxis

[0048] The oyster larvae with recovered vitality were raised to the shell top larvae for phototaxis selection. The selection device as shown in Figure 1 was filled with seawater containing feed, and then the oyster larvae in the nursery pond were collected with a 300-mesh silk net, and the collected oyster larvae were transferred to the selection device. Figure 1The larvae enter the device through the entrance 8, the incandescent lamp on the other side of the device is turned on, and the light source reaches the entrance 8 of the larvae through the reflection of the reflector. After the oyster larvae feel the light source, they will swim in the direction of the light. After 2 hours, the oysters will be distributed in different areas due to different swimming speeds. Finally, the first valve 71 and the second valve 72 are closed, the first water outlet 61, the second water outlet 62 and the third water outlet 63 are opened respectively, and the larvae in different areas are filtered by a silk screen. The larvae discharged from the first water outlet 61 have weak phototaxis and swimming ability, the larvae discharged from the second water outlet 62 have moderate phototaxis and swimming ability, and the larvae discharged from the third water outlet 63 have strong phototaxis and swimming ability. Part of the larvae are taken out to keep the density of the larvae at no more than 50 individuals per 100 milliliters, and are cultured in different culture barrels respectively. The chlorella is fed at 9 o'clock in the morning, 3 o'clock in the afternoon and 9 o'clock in the evening every day, the water is changed every two days, and after attachment, the oysters are transferred to a culture pond for cultivation. Concentrated chlorella is fed three times a day, and after a period of temporary cultivation, the oysters are hung in the Sanggouwan sea area of Rongcheng for cultivation. The population without robustness selection is used as a control, and the same feeding, water changing, attachment and natural cultivation processes are carried out as the selected population.

[0049] (5) Robustness evaluation of oysters with different phototaxis

[0050] ① Evaluation of larval survival rate

[0051] During the cultivation in the culture barrel, 50 milliliters of culture seawater were randomly taken from a 100-milliliter beaker at 1, 2, 6, 10, 12, 14, 16, 20 and 24 days after the larvae hatched, dyed with Lugol's iodine solution, and the oyster larvae were counted. The survival rate was calculated by repeating three times. Figure 2 The results showed that the survival rate of the larvae with strong phototaxis was the highest, the survival rate of the larvae with weak phototaxis was significantly lower, and a large number of larvae died. There was a significant difference between each group (P<0.01).

[0052] ② Evaluation of larval growth rate

[0053] During the cultivation in the culture barrel, 50 milliliters of culture seawater were randomly taken from a 100-milliliter beaker at 1, 2, 6, 10, 12, 14, 16, 20 and 24 days after the larvae hatched, dyed with Lugol's iodine solution, and the oyster larvae were counted. The survival rate was calculated by repeating three times. Figure 2 The results showed that the larvae with strong phototaxis grew the fastest and attached the earliest, the larvae with weak phototaxis grew the slowest and attached the slowest, the larvae with moderate phototaxis and the control group were basically the same, and there was a significant difference between the larvae with strong phototaxis and the larvae with weak phototaxis (P<0.01).

[0054] ③ Evaluation of three-month-old juvenile growth rate

[0055] Three-month-old oyster spat were retrieved from the Sanggou Bay in Rongcheng. Thirty oyster spat were randomly selected from different phototaxis groups and the control group. The length, width and height of the oysters were measured to evaluate their growth rate ( Figure 3 The results showed that the oysters with strong phototaxis grew the fastest and were the largest, and the oysters with strong phototaxis outperformed those in the control group, with significant differences (P<0.01).

[0056] ④ Assessment of the survival rate of three-month-old spat

[0057] The number of spat of each rope of three-month-old oysters with different phototactic characteristics was recorded, and the number of dead individuals was recorded. This was repeated three times to obtain the survival rate of Taiping Ocean oyster spat ( Figure 3 The results in D) show that the survival rate of oyster juveniles with strong phototaxis at sea is higher than that of oysters with weak phototaxis, and there is a significant difference (P<0.01). The survival rates of oyster juveniles in the neutral and control groups are basically the same.

[0058] ⑤ Evaluation of the survival rate of three-month-old juvenile spat under high temperature stress

[0059] The retrieved three-month-old spat were temporarily reared for 3 days. Thirty spat from each of the strong phototactic, neutral phototactic and control groups were placed in 30 liters of seawater with a salinity of 30 parts per thousand and subjected to a high temperature treatment of 33 degrees Celsius. During this period, Chlorella was fed every day, and the water was changed once a day. The spat were observed three times a day, and the number of deaths was recorded. The survival curves of the three-month-old spat in different groups after 7 days of high temperature stress were drawn ( Figure 4 The results in A) show that among the oyster spat under high temperature stress, the spat with strong phototaxis are better than the control group and the spat with medium phototaxis, and there is a significant difference (P<0.01). The survival rates of the phototactic and control groups are basically the same.

[0060] ⑥ Evaluation of the survival rate of three-month-old juvenile shellfish under Vibrio alginolyticus stress

[0061] Take 30 three-month-old spats that have been temporarily cultured for 3 days and put them into 30 liters of seawater. 7 CFU / ml of the bacterial solution was used for an 8-day immersion infection experiment with Vibrio alginolyticus. The cells were then placed in normal seawater for observation for another 4 days. The temperature was 24 degrees Celsius and the salinity was 30 parts per thousand. Mixed Chlorella was fed daily, and fresh water and fresh Vibrio alginolyticus solution were replaced daily. The number of deaths was recorded three times a day, and a survival curve was drawn. Figure 4 The results showed that the resistance of Vibrio with strong phototaxis was better than that of Vibrio with medium phototaxis, and the resistance of Vibrio with medium phototaxis was better than that of Vibrio with weak phototaxis, and there were significant differences (P<0.01).

[0062] ⑦Robustness assessment

[0063] The determination of growth and survival rate through the oyster larvae period, and the determination of three-month-old juvenile growth and survival, and high temperature stress and Vibrio stress, the oysters with strong phototaxis are superior to the oysters of other groups in all aspects, and the difference is very significant, indicating that the oysters selected by phototaxis have the comprehensive trait of robustness.

[0064] The present application establishes a new breeding method for selecting and breeding comprehensive robustness based on the phototaxis of oyster larvae. For the first time, the phototaxis of oysters is used to associate with the robustness of oysters to achieve the purpose of robustness selection. Compared with the traditional method of selecting high-quality oyster seedlings in the upper layer of water body by phototaxis, this technology is more accurate in screening, avoiding the differences in performance of screened larvae caused by different siphon positions, and the disadvantages of some larvae not being screened due to being far away from the siphon position. At the same time, the swimming performance difference of larvae with different phototaxis is associated with robustness, and the comprehensive robust oysters with high survival rate, fast growth rate and stronger resistance to adversity are screened. In addition, compared with adult selection, selecting larvae has higher selection pressure, avoiding the limitations caused by adult selection bias and sample size selection. This selection method shortens the breeding time, reduces the death in the sea area, and has outstanding economic advantages such as large individual size of the breeding product, which can better meet the market demand.

[0065] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for breeding a robust Crassostrea gigas, characterized in that, The method comprises population selection, larva energy recovery and selection of healthy oysters; the selection of healthy oysters is carried out by using the phototaxis of oyster larvae to select the larvae with strong phototaxis and swimming ability; the selection of healthy oysters is carried out as follows: after the selected larvae are recovered for a period of time, the larvae are put into a selection device and irradiated with unilateral light; the larvae swim to the light due to the phototaxis; after a period of light irradiation, the valve is closed; because of the different swimming speeds, the larvae appear in different areas after a period of time, i.e., three areas of strong phototaxis, medium phototaxis and weak phototaxis; the larvae in the area of strong phototaxis have the strongest swimming ability, and the oysters in the area are considered to be healthy oysters; The population selection is carried out on the D-shaped larvae of oysters as follows: an incandescent lamp is turned on, an air valve is closed, and the system is kept still; the upper water body is sucked into a screen gauze by using a siphon method, and the larvae of oysters are left in the screen gauze; and the larvae with weak vitality and un-hatched fertilized eggs are discharged; The larvae energy recovery is carried out as follows: the selected larvae are put back into a breeding tank, feed is fed, and the air valve is opened to make the larvae recover from the state of being disturbed; The selection device is a tubular structure with a mouth-shaped structure, and comprises a first tubular body (1), a second tubular body (2), a third tubular body (3) and a fourth tubular body (4); the first tubular body (1), the second tubular body (2), the third tubular body (3) and the fourth tubular body (4) are connected to each other; the first tubular body (1) and the second tubular body (2) are at right angles and are provided with a first reflector (51) at the inner wall of the right angle; the second tubular body (2) and the third tubular body (3) are at right angles and are provided with a second reflector (52) at the inner wall of the right angle; the third tubular body (3) and the fourth tubular body (4) are at right angles and are provided with a third reflector (53) at the inner wall of the right angle; the middle parts of the first tubular body (1), the second tubular body (2) and the fourth tubular body (4) are respectively provided with a first water outlet (61), a second water outlet (62) and a third water outlet (63); the first water outlet (61), the second water outlet (62) and the third water outlet (63) are respectively provided with covers; the second tubular body (2) and the third tubular body (3) are provided with a first valve (71) and a second valve (72); and the first tubular body (1) and the fourth tubular body (4) are provided with free ends.

2. The method for breeding a robust oyster according to claim 1, wherein The upper side wall of the tubular body at one end of the selection device is provided with a larva inlet (8).

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