Andrea Marconato

Sperm Economy in a Coral Reef Fish, Thalassoma Bifasciatum

In many coral reef fishes, males face the evolutionary problems of how to allocate sperm among frequent, daily mating in order to maximize the number of eggs they fertilize. A method is developed and tested for collecting and counting the number of sperm and eggs released during separate spawns of the coral reef fish Thalassoma bifasciatum. The method was used to examine the pattern of sperm allocation for pair— and group—spawning males. The number of sperm released by pair—spawning males varied positively with the number of eggs released by their female mates and with female body size. The data suggest that males economize on sperm release by providing the minimum amount of sperm needed to fertilize the egg clutch of the female partner. Fertilization efficiency (the number of eggs fertilized by a given number of sperm) was higher with large females than it was with small females. Males differed significantly among themselves in sperm output per spawn, with some males consistently releasing more sperm than other males with same—sized females. Male—male differences were not due to differences in male body size, quality of the spawning site, or the apparent degree of water movement. The number of sperm released per spawn did not decline throughout the daily spawning period, a pattern that disproved one but not all possible patterns of sperm depletion. In group spawns, the total number of sperm released and the number released per male were respectively 50 and 6 times the number released in pair spawns, on average. Both of these measures increased significantly with the clutch size of the spawning female. Overall, sperm production is probably sufficiently costly that males have been selected to allocate sperm carefully among their frequent daily spawns.

Endocrinology of teleost reproduction: A testicular steroid pheromone in the black goby, Gobius jozo L.

Abstract The male black goby, Gobius jozo, is known to possess a large mesorchial gland which synthesizes large amounts of 5β-reduced androgens conjugates, especially etiocholanolone glucuronide. When gravid females releasing eggs on stripping were exposed singly to this steroid in a tank monitored through a closed television circuit, most of them manifested appetitive behaviour with negative kinesis towards the steroid source and were stimulated to lay their eggs. The steroid stimulus was effective in low concentration (below 2μM), in still water, and in the absence of sensorial reinforcements by a male partner. Females were little or not responsive outside in the interval between ovulation and oviposition. The pheromonal system of the black goby resembles that of the pig where the smell of the testicular 16-androstenes in the boars breath induces the mating stance in the estrous sow. Both species are characterized by overdevelopment of the Leydighian compartment, chemical divergence between hormonal an...

Female mate choice, male-male competition and parental care in the river bullhead, Cottus gobio L. (Pisces, Cottidae)

Male body size was tested for its influence on female mate choice, male-male competition and ability to defend broods in the river bullhead, Cottus gobio L., a polygynous fish with paternal care. Females presented with two potential mates of different sizes significantly preferred to spawn with the larger male. Males smaller than, or 1·5 times longer than, the female were rarely selected as mates. Larger males were more successful in defending their brood from conspecifics, which may explain female preference for them. Unmated large males displaced smaller guarding males from their nests and retained the acquired egg masses. Competition between males for nest sites with eggs can be accounted for by the preference of females for males already guarding eggs: by seizing a nest containing egg masses, a male will increase his chance of being chosen.

Male competition and female choice in Padogobius martensi (Pisces, Gobiidae)

Abstract In natural populations of Padogobius martensi, a small freshwater goby, the number of eggs per nest increases with the size of the guarding male. The present study is a laboratory investigation of factors determining this size-related reproductive success of males. Females mated randomly with respect to male size but showed a marked preference for spawning in large nest sites, which were always occupied by the larger males. Courtship interference, nest take-over and egg cannibalism reduced the reproductive success of smaller males, although the progeny of some small males were guarded by the larger males that took over their nest.

Sperm allocation, sperm production and fertilization rates in the bucktooth parrotfish

Abstract Males and females of pelagically spawning marine fishes face different problems in maximizing reproductive success. Males must allocate sperm over frequent, successive spawns. Females must ensure successful fertilization of their egg clutches. This study examined sperm allocation between individual spawns, the daily ‘standing crop’ of sperm just prior to spawning, gonadal investment (testis weight/body weight) in males and fertilization rates of females of the coral reef fish Sparisoma radians . Males released a median of 8.94×10 6 sperm per pair spawn, with the number increasing with increasing clutch size and body size of the female mate. Standing crop of sperm correlated with male spawning frequency, and testicular investment correlated with degree of sperm competition by streaking males. Fertilization efficiency (number of eggs fertilized by a given number of sperm) increased with the body size of the female partner. Fertilization rate, i.e. the proportion of the clutch successfully fertilized, was over 90% in most spawns, but significantly increased with the number of sperm released by the male. The probability that a spawn was joined by a second, competing male (‘streaker’) increased with the body size of the spawning female. Overall, the sperm allocation pattern can be explained both as a form of sperm economy in which males release the minimum number of sperm needed to fertilize most of the eggs in clutches of different size, and as a response to an increased probability of sperm competition with females of increasing size.

On the mechanism of sperm release in three gobiid fishes (Teleostei: Gobiidae)

SynopsisIn fish, gamete release is commonly assumed to be synchronous in externally inseminating fishes. By collecting and counting the number of sperm and eggs released during separate matings in three demersal spawners, the mediterranean gobies, Zosterisessor ophiocephalus, Gobius niger, and Knipowitschia panizzae, we observed that gametes are released asynchronously. Males release sperm before females start laying their eggs. Sperm is released in the form of sperm trails laid on the nest surface; subsequently active spermatozoa leave the trails and move in the water for several minutes. Sperm trails consist of bands of viscous material in which sperm is embedded. In most cases eggs are not laid directly over the sperm trail, suggesting that sperm may contact the eggs after the latter are released in the water. Male sperm duct glands, seminal vesicles, known to secrete mucosubstances, are likely involved in the production of sperm trails. The possible influence of this mode of insemination on the mating style of marine gobies is discussed.

The biology of a population of spined loach, Cobitis taenia L.

Abstract A spined loach (Cobitis taenia L.) population inhabiting a small river in northern Italy was studied. At each age, females were larger than males and both sexes showed a marked seasonal pattern of growth with a high rate from May through November. Total population density averaged 2.38 ± 1.6 ind/m2 and the annual production in the study area was 3.3 g/m2. The first year class during the summer accounts for more than half the production. The sex‐ratio did not significantly depart from unity. Most individuals achieved sexual maturity in their second spring after hatching and spawning occurred in late May and June. The relationship between female total length and fecundity was lnF=l.07+0.065L. Males showing three different liveries were found throughout the year; the seasonal variation in their frequency suggested that males change livery in relation to their reproductive conditions.

Mating preferences of the female river bullhead, Cottus gobio (Cottidae, Teleostei)

Abstract Field observations on the influence of site and male quality on choice by the female river bullhead (Colitis gobio) are reported. Females appeared to choose males of large size, and apparently they preferred to spawn with males whose nests contained eggs.