A study of reproductive biology of the ocean pout (Macrozoarces americanus L.) in captivity

Yao, Zuxu (1994) A study of reproductive biology of the ocean pout (Macrozoarces americanus L.) in captivity. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The reproductive biology of the ocean pout (Macrozoarces americanus L.) was studied. This included (1) the biochemistry of vitellogenin (egg yolk protein precursor), (2) the seasonal reproductive cycle and the associated endocrine changes, (3) the regulation of fish reproduction and (4) the biology of egg fertilization. -- Synthesis of vitellogenin in the liver of ocean pout was induced by administration of the ovarian steroid 17β-estradiol. Vitellogenin was isolated from the plasma by gel filtration and characterized biochemically. It is a glycolipophosphoprotein, 577.8 KD in molecular weight, and contains 17.92% lipid and 3.56% total phosphorus, of which 80.34% (or 2.86% of total vitellogenin) is in the form of lipid-bound phosphorus and the rest (19.1%, equivalent to 0.68% of vitellogenin) is protein bound-phosphorus. The vitellogenin protein contains high levels of the essential amino acids, including arginine, lysine, isoleucine and leucine, valine, threonine and phenylalanine, and some non-essential amino acids, such as aspartic acid, serine, glutamic acid and alanine. With the isolated vitellogenin, antisera were prepared in rabbits and a vitellogenin radioimmunoassay was developed and used for sex determination and for studies of the seasonal changes in plasma vitellogenin. -- The seasonal reproductive cycle of female ocean pout was found to be organized into 4 successive phases of reproduction: 1) a post-spawning quiescent phase (August- September to February), in which the small ovaries (GSI 1.21 ± 0.25) are inactive and predominated by small previtellogenic oocytes (ca. 1.0 mm diameter); a low level of both sex steroids (testosterone and 17β-estradiol) and vitellogenin occurs in the plasma; 2) the reproductive preparatory phase (March - May), in which the ovaries develop slowly (GSI 4.61 ± 2.34) and consist of both previtellogenic and small vitellogenic oocytes (4-5 mm diameter); plasma levels of sex steroids and vitellogenin begin rising slowly; 3) the rapid ovarian growth phase (June - August), in which both the diameter of the vitellogenic oocytes (7 - 8 mm); the GSI (16.76 + 6.54) and levels of sex steroids in the plasma rapidly increase and 4) the ovulation and spawning phase (August - September), when fish spawn and the plasma sex steroids return to basal levels. The females spawn once per year, producing a single batch of 1200 - 1700 eggs (8-9 mm diameter). The seasonal reproductive cycle in males was found to be composed of two phases, (1) a quiescent phase (October - May) and (2) the reproductive phase (June - September). The spawning season of males (occurrence of spermiation) starts with rapid testicular development, following a dramatic increase in both testosterone and 11-ketotestosterone in the plasma. -- Increasing water temperature in the winter stimulated feeding activity but did not alter the timing of the reproductive cycle of the ocean pout. However, lengthening the photoperiod in the winter stimulated steroidogenesis and vitellogenesis and advanced the spawning season. Administration of gonadotropic hormone releasing hormone analog to mature females advanced and synchronized spawning through accelerating the preovulatory decline of plasma testosterone and 17β-estradiol. -- Sperm motility, pH, osmolarity and ionic composition of the ocean pout seminal plasma were studied. Milt collected from the reproductive tract (sperm duct) of mature males contained highly motile spermatozoa (sperm) in a very low concentration. Motility of the sperm was enhanced in ovarian slime collected from the ovaries of prcspawning females but was immobilized instantly upon dilution with seawater, suggesting that the ocean pout is an internal fertilizer. pH and osmolarity of the seminal plasma were in a range of 7.2-7.5 and 365-406 mOsM, respectively. Various ions including Na+, K+, Ca1+, Mg++ and Cl- as well as glucose were detected and quantified in the seminal plasma. Fertilized eggs were produced by artificial insemination, both in vivo (injecting sperm directly into the ovary of mature females) and in vitro. The in vitro insemination necessitated a contact period of 5 hours between the eggs and sperm for fertilization of the eggs before the eggs could be transferred into seawater for incubation. Larvae hatched from in vivo artificially inseminated eggs. -- Studies of copulation and spawning behaviours of the ocean pout showed that males developed a papilla (protrusion of the genital pore) and fish copulated through direct genital contact for internal fertilization of eggs. Females spawned spontaneously in captivity, 6-17 hours after copulation. A complete spawning process was determined to consist of 4 successive steps, including (1) oviposition (spawning), (2) wiping the eggs with skin mucus, (3) wrapping herself around the eggs and (4) guarding the egg-mass. Wiping the eggs with skin mucus could be an effective means for preventing leech and fungal infection of the eggs. Parental care and fanning increased water flow to the eggs, which is important for egg survival during a lengthy period (3 months) of incubation.

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