Larger Gonads Research Articles

A comparison of breeding and moult cycles and life histories in two tropical starling species: the Blue‐eared Glossy Starling Lamprotornis chalybaeus and Rüppell's Long‐tailed Glossy Starling L. purpuropterus

The Blue‐eared Glossy Starling Lamprotornis chalybaeus and Rüppell's Long‐tailed Glossy Starling Lamprotornis purpuropterus were investigated in the field and in aviaries at Lake Nakuru National Park, Kenya for seasonality in reproductive activity and moult. The former species was found to be a seasonal breeder which nests after the onset of the heavy rains in April. Although some birds had large gonads prior to the rains in the dry season no nesting occurred. The rains were contemporary with increases in gonadal size and the plasma titres of LH, testosterone (T) in males and estradiol (E2) in females. These hormones are associated with the initiation of breeding activity. As breeding ceased in July and the moult began, the plasma titres again decreased. There was a bimodal breeding pattern which paralleled a change in biotope preference for nesting. Early nests, in the heavy rains, were on the open savanna and later nests were in the acacia forest. Late nesting birds also had delayed peaks in gonadal size, plasma titres of LH, T and E2 and a delayed moult onset. Data on individual captive birds demonstrate that these annual cycles have a distinctly individual character superimposed on the seasonal trends.In Rüppell's Long‐tailed Glossy Starlings no seasonality in breeding was found although the flight feather moult commenced and was completed in all individuals at about the same time. The moult extended over about ten months, so a great deal of breeding‐moult overlap was present. The absence of seasonality in field birds was reflected in the aviary birds, which had no pronounced cycles in the reproductive parameters measured (gonadal size, LH, T and E2 plasma titres). Breeding in field birds was regulated on a pair basis and correlated with increases in duetting. The striking differences in the seasonal organization between this species and Blue‐eared Glossy Starlings were presumably due to the different biotope preferences and social behaviour of the two species.

Photoperiodic Regulation of Gametogenesis in Sea Stars, with Evidence for an Annual Calendar Independent of Fixed Daylength

SYNOPSIS. Gametogenesis and gonadal growth in the west coast sea star Pisaster ochraceus normally begins in the fall and leads to large gonads full of gametes in the spring, when spawning occurs. The timing of gametogenesis can be shifted simply by maintaining the animals on a seasonally changing photoperiodic regime out of phase with ambient. When they are kept on a spring-summer photoperiodic regime during the fall and winter, gametogenesis proceeds 6 mo ahead of schedule in the following spring and summer. Gametogenesis can be shifted out of phase even when the eyespots are removed. Short daylengths that normally occur during the fall and winter are not required for gametogenesis to proceed, nor are even the long daylengths of spring and summer that precede the initiation of gametogenesis in the fall. The temporal program is insensitive to fixed daylengths (LD 15:9, 13:11, 12:12, 9:15) and appears to involve an endogenous calendar. Shifting the photoperiodic regime 6 mo out of phase also leads to a shift of the gametogenic temporal program in the sea stars Leptasterias sp. (a brooder) and Asterias vulgaris (from the New England coast), but not in the sea star Patiria miniata . Gametogenic timing also can be switched in the sea urchin Strongylocentrotus purpuratus but the mechanism of the photoperiodic response is fundamentally different; gametogenesis requires short daylengths; continues indefinitely under a repeated short day, fall-winter photoperiod regime, and apparently does not involve an endogenous calendar. As photoperiodic responses are investigated further in these and other marine invertebrates, the models developed primarily from studies on terrestrial organisms may need to be extensively modified or additional new models required.

Growth and sexual strategies in the fishThalassoma duperrey (Labridae), a protogynous hermaphrodite

Adults of the fishThalassoma duperrey, a protogynous hermaphrodite, were collected and growth observed in captivity to study the relationship between growth and reproduction among primary males, females, and secondary males. Sexual maturity is reached at about 60 mm standard length, probably less than 1 year after fertilization. Gonosomatic index in both males and females peaks at about 120 mm, nearly 2 years later. Shortly thereafter females typically change sex, and both primary and secondary males undergo color change. At the same time gonosomatic index falls abruptly and remains low in large fish. The above changes appear to reflect differences in reproductive effort over a lifetime and are interpreted as the optimum strategy given the social and mating system of this fish. Unless they cannot acquire enough food to develop large gonads, small individuals put a much greater proportion of energy into growth than reproduction apparently to minimize the period of low fitness. Intermediate-sized males and females generally invest heavily in gametes, though some retain small gonads. Large individuals (both primary and secondary males) greatly reduce their investment in gametes, probably trading the energy required to maintain reproductive territories for it. This kind of gonad ontogeny involving gonad regression, as inT. duperrey and other labrid fishes, is unique among vertebrates.

Coexistence of Sea Urchins in Seagreas Meadows: An Experimental Analysis of Competition and Predation

An experimental examination at two sites of the effects of competition and predation on Tripneustes ventricosus and Lytechinus variegatus, the common sea urchins in meadows of the Caribbean seagrass, Thalassia testudinum, suggests that intraspecific competition and predation on recruits influence their population dynamics. Grazing by Tripneustes strongly influenced the biomass of Thalassia. Density-dependent mortality caused the number of caged Tripneustes to converge on naturally occurring densities (about 1 urchin/M2), but Tripneustes mortality was not significantly affected by Lytechinus. Recruitment of Tripneustes was sparse at one site and did not increase when predators were excluded. However, recruitment at a more exposed site was an order of magnitude greater in predator exclusion cages than in an unmanipulated control plot. Tripneustes recruitment was inversely related to the density of adults at this site, but was not significantly affected by Lyte- chinus. L'techinus has a more generalized diet than Tripneustes, and had a moderate effect on the amount of Thalassia. Substantial mortality of Lytechinus occurred only at a density 16 times greater than its naturally occurring abundance; Lytechinus mortality was not significantly affected by Tripneustes. Recruitment of Lytechinus at a protected site decreased as the density of Tripneustes increased, but did not change significantly when predators were excluded. However, at a more exposed site predation reduced the density of recruits by an order of magnitude. Tripneustes had a higher growth rate than Lytechinus, and both urchins grew more rapidly at the exposed site. These differences were consistent with the sizes of the urchins; Tripneustes was larger than Lytechinus and both species were larger at the more exposed site. The growth and reproductive condition of Lytechinus were density dependent and differed between sexes; Lytechinus males grew to a larger size than females, but females had larger gonads. Naturally occurring densities of Tripneustes, which were close to carrying capacity, were some- what less variable than those of Lytechinus, although the differences were not significant. Coexistence of these urchins appears to stem from differences in their diets; there was no evidence of significant interspecific competitive effects on mortality, growth, and fecundity. Although predation substantially reduced urchin recruitment at one site, this effect appeared to be superimposed on, rather than necessary for, coexistence.

Reproductive cycles and fecundities of four common eastern Australian archaeogastropod limpets (Mollusca : Gastropoda)

Reproductive cycles of four species of limpets were determined using quantitative analyses of counts of eggs and weights of gonads. Montfortula rugosa has a breeding season extending for 8-9 months, but does not show marked peaks of reproductive activity. It spawns small numbers of eggs over a long period, and never has a very large gonad. Notoacmea petterdi has a very short spawning season in May or June. Prior to spawning the gonad may account for over half the wet weight of an individual. After spawning there is a long resting period, after which the size of the gonad and the number and proportion of mature eggs increases gradually over the next 7-8 months. Patelloida alticostata and P. latistrigata have strategies which are intermediate between the other two species. Spawning may occur over a 4-6 month period during which time there are two peaks of spawning activity. The production of mature oocytes recommences soon after spawning has ceased in July, and the gonad rapidly increases in size during the following 4 months. Estimates of fecundity are given for the three species of acmaeid limpet. In these species, fecundity increases with increasing size; yet individuals are capable of producing and spawning mature eggs at the end of their first year on the shore. The adaptive significance of such a strategy is discussed.

Parasite reproductive strategy and evolution of castration of hosts by parasites.

A modification of the Euler equation is used to describe a simple parasite life history in which survival decreases with age at a rate determined by mortality of the parasitized host. The advantages of castration of hosts by parasites are discussed using the modified equation in which castration is equivalent to reducing parasite virulence in the host to near zero or zero. It is suggested that castrating parasites can infest a wider range of hosts with higher mortality rates and that within parasite groups having castrating and noncastrating species, the former should infest hosts with higher mortality rates and relatively larger gonads.

Overlap of Breeding and Molting Schedules in a Collection of African Birds

Many birds in northern temperate regions are known to have distinct and separate times of the year for breeding and for molt, but little information is available for tropical birds. Miller (1961) found a separation of molt and breeding in tropical Zonotrichia capensis, and he also described an inverse correlation in time between nesting activity and molt in an equatorial South American cloud forest (Miller 1963). Moreau (1936, 1950) reported overlap of molt and gonadal activity in less than 10 per cent of a sample of birds collected in equatorial Africa. Exceptions noted by Moreau were Colius striatus among the nonpasserines and Turdoides jardinei among the songbirds. These species had large gonads both in the breeding season and at the time of molt. In none of these studies was each of the instances of overlapping schedules discussed.

Sex differentiation and gonadogenesis in lampreys (Parts I and II)

The development of the gonads has been studied in ammocoetes of the landlocked sea lamprey, Petromyzon marinus. Up to lengths of about 70 mm, the gonads remain undifferentiated: the germ cells proliferating to form first small groups and finally large cysts of germ cells. At lengths of 70 to 80 mm, over 70% of all gonads examined were of the cystic type. Measurements made on large numbers of animals at this stage indicate the presence of two distinct groups with smaller and larger gonads. These represent the future male and female gonads respectively. These size differences are thought to correspond to differences in germ cell numbers in the two sexes. In a few cases the sex of the gonads can be distinguished at lengths of 71 to 80 mm, but differentiation is not generally complete below lengths of 100 mm. Differentiation is preceded or accompanied in both sexes by the appearance of growing oocytes. The frequency distribution for oocyte counts in bimodal, the lower numbers representing the future male gonads. During auxocytosis and differentiation large numbers of germ cells undergo degeneration; the majority at the zygotene or pachytene stage. In definitive females, the surviving oocytes represent only 20 to 30% of the total number of germ cells in the cystic gonad. Oogenesis begins within the cysts which are broken up by the invasion of somatic elements. The presence of large cysts is therefore without relevance to the future sex of the gonad.The extensive regression of germ cells in the future male gonads results in an actual decrease in size during differentiation. The somatic characters that distinguish the earlier male gonads of Lampetra planeri are less developed in marinus. In size and histological structure these often resemble the undifferentiated gonads of earlier larval stages. The male germ cells are derived from residual undifferentiated elements which survive the extensive regression at the cystic stage. At first the male gonads contain only a few isolated germ cells which proliferate slowly throughout larval life. At metamorphosis there is a very marked acceleration in the division of the male germ cells. Male germ cells retain their undifferentiated character throughout the larval period. In regard to the mode,of sex differentiation marinus is more differentiated than planeri. This is supported by the existence of a sex dimorphism in gonad size (and probably germ cell numbers) preceding morphological and histological differentiation, and by the evidence of bimodality in oocyte numbers. The vastly reduced fecundity of the brook lamprey compared with that of the sea lamprey is associated with significant differences in the pattern and phasing of gonad development in the two species. In planeri, germ cell proliferation begins earlier than in marinus, but is shorter in duration, terminating in auxocytosis and differentiation. This results in an enormous reduction in the total numbers of germ cells. Expressed as a percentage of the total numbers, the proportion undergoing regression during sex differentiation is thought to be similar in both species. The difference in the number of germ cells which survive as oocytes in the female gonads of the two species is thus a direct consequence of precocious oocyte development and the earlier curtailment of the gonial divisions in the brook lamprey ammocoete.

The biology of Asterias rubens L. II. Parasitization of the gonads by the ciliate Orchitophrya stellarum Cépède

The parasitic astomatous holotrich ciliate, Orchitophrya stellarum Cépède, has been found in the testes of starfishes (Asterias rubens) in the Plymouth area. It was found only in starfishes from numerically rich and well-fed populations, which also showed large gonad growth. In such populations it occurred in up to 28% of the males during March-April 1947, and in lower percentages during the spring of the three following years. The parasite was never found in a geographically adjacent population containing smaller numbers of poorly fed starfishes. The parasite was found to be relatively more common in the testes of medium-sized starfishes than in those of large and small starfishes.Presence of the parasite causes a complete breakdown of all the germinal tissue of the testes, so that most of the infected starfishes suffer complete castration. This involution of the testicular tissue has been studied, and photomicrographs are given which show a number of stages in the process.