Shellfish Breeding and Hatchery Management (2+1)

17.1.2 Eye stalk ablation and its role in gonadal development

Introduction :

Shrimp seeds were collected from natural wild waters in the past. Following the development of hatchery technology and larval production in captivity, seeds have been produced in the hatcheries and supplied to the farms. However, there has been a difficulty in getting the mother shrimps for the seed production. Matured mother shrimps have been collected from wild waters for copious seeds and production of good quality seeds.

It is a fact that all species of shrimps do not attain sexual maturity in confined waters. It has been found that hormones secreted by the X-organ sinus gland complex are responsible for the inhibition of the gonad maturation and breeding. Therefore, removal of eyestalk in the matured shrimps is believed to result in the inducement for the maturation and spawning. Ablation or removal of the eyestalk for induction of maturation in crustacean is an approved practice in the crustacean seed production as an essential prerequisite for the maturation of the shrimps in captivity. In female penaeid shrimp and female crabs the eyestalk has the gland (a complex) for the production and storage of the gonad inhibiting hormone (GIH) that inhibits the maturation of the ovary.

Eye stalk ablation

It is the removal or extirpation or elimination of one or both eyestalks in order to at least reduce the level of inhibitory hormone or to fully prevent their secretion in the body for full and accelerated maturation of the ovaries.

The eye stalk ablation can be either unilateral or bilateral ablation based on the removal of single or both eye stalks. If both eye stalks are removed, then it is called as bilateral eye stalk ablation.

Background of eye stalk ablation

The stimulating effect of eyestalk ablation on reproduction of decapod crustacean was first evaluated for Penaeid culture in the early 1970’s when French researchers attempted bilateral eyestalk ablation (both eyes). Ablated females suffered high mortality (Probably due to hormone non-synchronization) and ova were typically reabsorbed with out subsequent spawning.

These problems were alleviated by the ablation of only one eye stalk (unilateral eyestalk ablation) which provided moderate hormonal stimulus without re-absorption of ova or excessive mortality. Consequently, unilateral eyestalk ablation rapidly emerged worldwide as a simple procedure for inducing reproduction of numerous species of Penaeid shrimp reared in captivity.

The ablation process

It is more essential to perform the ablation at appropriate stage in order to avoid loss of mother shrimps. For all practical reasons, it is emphasized that only inter molt shrimp is to be used for ablation. Post-molt (stage) female shrimps are not recommended for ablation, due to increased risk of handling mortality associated with softened exoskeleton and weakness of the animal Pre-molt (Stage IV) individuals are also not recommended since there may be immediate molting during recovery from the ablation process leading to loss or delay in spawning.

It was observed that ablation under taken between 8-20 days post-molt resulted in significantly greater egg production than that of ablation at 13-15 days post-molt indicating the inter moult period is the best one for the ablation process.

Methods of eyestalk ablation:

Several methods are available for the removal of eyestalk. They are,

(i) Cutting the eye stalk near the base with a pair of scissors followed by sealing with a pencil type soldering iron. This method is called electrocautrization.

(ii) Pinching of eyestalk and squeezing of eye ball contents out

(iii) Incision of eyeball followed by eunucleation of contents.

Enucleation, cauterization and ligation are the methods that have been effective in removing (or) destroying the X organ / sinus gland complex. Never the less, the stress that will be arising due to the ablation has its own ill effects on the animals.

Stress can be reduced and losses minimized if shrimps are held in chilled water before and after ablation. To minimize stress, the ablation should be performed as quickly as possible under chilled water. Further the stress to the mother shrimps can be reduced by doing the ablation in the early morning hours, when the temperatures is the lowest. Care should be taken to see that female mortality due to ablation is very low.

a) Enucleation

Grasp the eyestalk just behind the eyeball using the thump and index finger. Squeeze hard and roll the thump and finger outwards away from the body, thus crushing the eyestalk and squeezing out the contents of the eye. The objective is to squeeze the contents outwards and not let them follow the eyestalk back in to the head region. Enucleation has the advantages of simplicity and rapid clotting of hemolymph within the empty eyestalk.

b) Ligation

A string is tied around the base of the eyestalk as close to the carapace as possible. The string should be drawn fairly tight causing the eyestalk to fall off in a few days. The procedure does not leave the shrimp with open wound. However, successful ablation is often unpredictable and this process will give the result only after two or three days.

c) Cautery: (Electrocauterisation)

This method is the severing eyestalk followed by sealing of the wound through the process called electrocauterization. Heated forceps, or the application of a silver nitrate bar on the wound will prevent the secondary infection at the wound site.

Ø Pinching method of ablation required one person and it is without application of antibiotics.

Ø Ligation requires two persons, one to hold the shrimp and the other to tie the eyestalk.

Ø Cautery requires either a cautrizer (or) silver nitrate bar.

Effects of eye stalk ablation

The effects of eyestalk ablation vary with season and stage in the molt cycle. Ablation is performed on either left or right compound eyestalk.The damaged eye should be ablated in order to leave the shrimp with one unablated functional eye.

Fecundity and possibilities for spawning:

Ovarian development in sexually matured female can commence within 3 days of ablation, followed by a first spawn within one week. If ablated during the inter molt stage, the females will mature and spawn immediately. If ablated during early molt, they will molt before maturing. There have been varied observations and results due to ablation.

- The fecundity and viability of larvae that are obtained from abated females are observed to be inferior to larvae from females matured in the wild indicating the superiority of the larvae from the natural spawning.

- In the captive maturation there might be poor or low embryonic development that may influence juvenile’s survival and growth.

- If the females are smaller, there will be higher lipid in the body and variations in the distribution of yolk among oocytes. These were identified from the crab (Paratelphusa hydrodromus) during the pre-spawning.

- Those differences presumably are consequences of hormonal imbalances due to ablation. They are physiological (or) embryological limitation such as improper oocyte differentiation, nutrient storage food supply (or) temperature.