Reproductive cycles and factors influencing reproductive cycle in shellfish and their application for aquaculture

Introduction

Shellfish, which comprise mollusks like clams, oysters, and mussels as well as crustaceans like shrimp and lobsters, have a variety of interesting reproductive techniques. Males and females of crustaceans, which include shrimp and lobsters, mate directly. The female receives sperm from the male during mating, and she stores it until eggs are ready for fertilization. The fertilized eggs are subsequently laid by the female,carries them attached to her abdomen until they hatch into larvae.

Reproductive cycle of Penaeus monodon (Black tiger shrimp)

1. Egg 0-3 mm in diameter-narrow Perivitelline space-Emrbyonic development fast-nauplius hatches out within 16-17hrs after fertilzation.

Nauplius

First hatched nauplius,pear shaped with3 pairs of appendages.Nauplius moults every 3-5 hrs.Passes through 6 naupliar substages. Nauplius swims actively; no mouth; Alimentary canal is inside the body.

Protozoea (Zoea)

After about 2 days nauplius metamorphose into protozoea -3 substages. Protozoea swims actively; strongly attracted towards light; mouth, Alimentary canal, feeding apparatus present; feeding on unicellular algae. Protozoea I Protozoea II Protozoea III 4. Mysis 3 substages, look very much alike except pleopod buds (swimming leg rudiments) in the later 2 stages. Mysis retain filtering mechanism, feeding on algal cells.1 st 3 walking legs not functional; don't swim actively like protozoea; less responsive to light; swim in the water column with head pointing downwards. Mysis I Mysis II Mysis II

Post larva

Filtering mechanism is lost/mandibles have cutting edge; walking legs have claws-unctional; capable of capturing/eating zooplankton; attraction to light lost. Mating: Take place between hard shelled males and mature soft shelled females which have just completed their premating moult.Time between moulting and mating varies from 1.2 to 21.8 hrs (avg. 9.1 hrs).Male gonopore brought close to sperm receptacle of female on thoracic region and transfer the sperm to receptacle as spermatophore. Spermatophore adheres to the sperm receptacle because of the glutinous nature of attachment matrix.After mating female may spend several hours in prespawning preening behavior.

Spawning

Spawning occurs after few hrs of mating. Eggs laid by female are fertilized by sperm attached to underside of thorax.The fertilized eggs are transferred to the ventral side of abdomen (brood chamber).Ovipository setae and attched to the ovigerous setae of 1st 4 pleopods and kept in place by a membrane.Movements of pleopods helps in aerating the eggs.Adult females carrying eggs on the ventral side of abdomen is called berried female.Fecundity varies from 20000-100000 eggs. However, their first broods, (i.e. those which are produced within their first year of life), are often not more than 5 000 to 20 000.

Incubation

Initially eggs are slightly elliptical (0.6-0.7 mm in long axis) and bright orange in color.Embryonic development proceeds.Incubation period average 19 days.Turns grey black 2-3 days before hatching.Freshly hatched larvae are called zoea.

Segmentation and embryonic development

Reproductive cycle of mussel Mating and Fertilization

Male freshwater mussels release sperm into the water. Females draw this sperm into their bodies through their siphons, where fertilization of the eggs occurs within specialized structures called marsupia, located in their gills.

Brooding

After fertilization, the eggs develop into glochidia, which are the larval stage of freshwater mussels. This brooding period lasts about 4 to 6 weeks.

Release of glochidia

Between late spring and early summer, typically from May to June, the glochidia are expelled into the water. At this stage, they are microscopic and approximately 0.2 mm in size.

Attachment to host fish

Glochidia must attach to the gills or fins of a host fish to continue their development. Some species release glochidia directly into the water, while others use specialized structures called mantle lures that mimic prey to attract fish.

Encystment and Development

Once attached, the glochidia encyst within the fish's tissues. The fish's immune response facilitates this process, leading to the formation of a cyst around the glochidia within 2 days.

Detachment and settlement

After completing their metamorphosis, juvenile mussels detach from the host fish and settle into the substrate of their aquatic environment. They continue to grow and develop into mature mussels over several years.

Biological factors

Genetic variability,stress,age and size.

Intrinsic physiological factors

neurohormonal and hormonal stimulation, genetic and nutritional factors.

Environmental factors Photoperiod

The length of a 24-hour cycle of light and dark.It is an organism's capacity to timing seasonal events in their life histories by using body length as an anticipatory cue. Many different kinds of animals utilize the length of the day or photoperiod as a preemptive cue to plan their seasonal activities. Photoperiod is most beneficial in forecasting future environmental conditions or situations at remote locations; it sends out a signal to begin physiological and developmental processes that lead to migration, dormancy, and reproduction.

Temperature

Major carps breed within a range of 24-31 °C. If an effective dose of pituitary, HCG, or ovaprim is given to fish, they will spawn successfully even if there is a significant increase or decrease in water temperature. Warm temperature plays a primary role in stimulating the maturation of gonads in many fish species. Temperature has a direct effect on gonads regulating their ability to respond to pituitary stimulation and effects on primary synthesis and release of gonadotropins.

Salinity:

Major carps breed within a range of 24-31 °C. If an effective dose of pituitary, HCG, or ovaprim is given to fish, they will spawn successfully even if there is a significant increase or decrease in water temperature. Warm temperature plays a primary role in stimulating the maturation of gonads in many fish species. Temperature has a direct effect on gonads regulating their ability to respond to pituitary stimulation and effects on primary synthesis and release of gonadotropins.

Water Depth

For effective reproduction, a number of fish species are dependent on particular substrate types and water depths.

Changes in the water level can modify the flow pattern, which can impact the movement of fish larvae and eggs.

Lunar Cycle

Many fish species, most of which live in coastal regions, have been found to exhibit apparent spawning cycles at intervals of 14.7 days during a specific lunar phase. These cycles are the explanation for lunar reproductive events. For instance, only at spring tide does a common coastal puffer species in Japan congregate at specific beachfront areas and spawn several hours before the morning or evening high tide. Every year and every two weeks during the spawning season, the grass puffer group spawns at the same spot. Advantages in reproductive techniques, like as timing and spawning success rates, may result from specific genetic features. Hermaphrodite animals, for instance, might gain from genetic diversity through outcrossing, which would increase their flexibility and resilience.

Stress

Environmental stressors that have an adverse effect on reproductive health include pollution and habitat deterioration. Stress can interfere with hormone pathways that are necessary for reproduction, which lowers fertility and the amount of offspring produced. It has been demonstrated that acute stress in fish inhibits ovarian sterogenesis, which impacts egg production.

Age and Size

An individual's age and size have a big impact on the success of their reproductive process. Bigger and more mature females frequently lay more eggs and better-quality progeny. Furthermore, maturation period varies amongst species, which affects overall population dynamics and reproduction.

Intrinsic physiological factors 3.1 Intrinsic physiological factors

Refer to internal biological processes that influence the growth, development, and reproduction of shellfish. These factors are primarily regulated by the nervous and endocrine systems and can be influenced by genetic,and nutritional factors.Intrinsic Physiological Factors involve:

Nervous system

Sensory Perception: Shellfish employ their chemoreceptors, antennae, and eyes to sense changes in their surroundings, including the presence of predators, food sources, and water quality. The brain receives this information and uses it to coordinate the right reactions.Neurohormones: The nervous system secretes neurohormones that have the ability to affect a number of physiological functions, such as stress reactions, growth, and reproduction.

Endocrine system

Hormones: Hormones, produced by endocrine glands are essential for controlling a wide range of physiological processes. As examples, consider:Growth hormones: Encourage expansion and maturation.Hormones involved in reproduction: regulate the life cycle.Stress hormones: Aid in the organism's ability to handle stress.Hormonal Regulation: Feedback mechanisms ensure that hormone levels stay within proper ranges, and the endocrine system is carefully regulated.

Genetic factors

Heredity: An individual's hereditary features, such as growth rate, capacity for reproduction, and illness resistance, are determined by genetic factors.Genetic variety: Distinct physiological traits and responses to external stimuli can result from genetic variety within a species.

Nutritional factors

Nutritional Factors: Shellfish necessitate particular nutrients for their growth and development. For the body to function at its best, an adequate intake of vital nutrients-such as proteins, carbs, fats, vitamins, and minerals-is required.Nutritional Deficiencies: Lack of some vital nutrients can cause stunted growth, decreased fertility, and heightened illness susceptibility.

Applications in aquaculture

Hormone treatments with LH, HCG or synthetic analogs are used to induce precocious spawning in many aquaculture species.

Manipulating photoperiod and temperature is used to control timing of reproduction and induce spawning out of season.

Providing appropriate spawning substrates helps control reproduction.

Inducing sterility using techniques like triploidy is used for genetic containment and improved production in aquaculture.

Conclusion

The success of finfish and shellfish in generating progeny is determined by a multitude of biological and environmental factors that are closely tied to their reproductive cycles. Effective management and conservation efforts depend on an understanding of these reproductive dynamics, especially in light of continuous environmental changes and human impacts on aquatic ecosystems. The complex interactions between reproductive cycles and environmental conditions require ongoing study in order to create management plans for fish and shellfish populations in a world that is changing quickly.

References

1. Dominguez-Castanedo O, Hernandez-Coronado C, Martínez-Espinosa D, Rosales-Torres A. Reproductive morphophysiology of an iteroparous batch spawner fish: from early follicle development to ovulation. Journal of Fish Biology. 2024;104:1-15. DOI:10.1111/jfb.15891.
2. Kovalskyi B, Zanko N, Holubnyk T, Pysanchyn N. Analysis of influencing factors on the quality of digital image reproduction. 2024. DOI:10.46489/gdtatione-0524-40.
3. Mañanós E, Duncan N, Mylonas C. Reproduction and control of ovulation, spermiation and spawning in cultured fish. In: Aquaculture reproduction. Boca Raton: CRC Press; 2009. p. 1-20. DOI:10.1201/9780849380549.sec1.
4. Mandal S, Mandal B. Climate change and its cumulative effect on fish reproduction. 2024; p. 28-42.
5. Mangi H. Water level fluctuation effect on fish reproduction success. International Journal of Ecology. 2024;2024:1-13. DOI:10.1155/2024/4876582.
6. Torsabo D, Ishak S, Mat Noordin N, Koh I, Yazed MA, Iber B, et al. Enhancing reproductive performance of freshwater finfish species through dietary lipids. Aquaculture Nutrition. 2022;2022:1-12. DOI:10.1155/2022/7138012.
7. Gosling E. Bivalve molluscs: biology and aquaculture. 2nd ed. Oxford: Wiley Blackwell; 2015.
8. Naik SG, Gowda HV. An overview of the life cycle, reproduction and factors influencing recruitment success of tropical oysters. Journal of Aquaculture Research and Development. 2015;6(5):1-10.
9. Maathuis RH, Van der Linden MJ, Sminia T. Reproductive biology of bivalves: a review. Aquaculture. 2020;524:736321.
10. Maneiro I, Pérez-Paralle ML, Villalba A. Reproductive biology of the European flat oyster (Ostrea edulis): a review. Aquaculture International. 2017;25(3):825-845.