Alternative reproductive tactics are associated with sperm performance in invasive round goby from two different salinity environments

During male–male competition, evolution can favor alternative reproductive tactics. This often results in a dominant morph that holds a resource, such as a nest for egg laying, which competes with a smaller sneaker morph that reproduces by stealing fertilizations. The salinity environment can influence male growth rates, for example, via osmoregulatory costs, which in turn may influence the use of sneaker tactics for small males competing for mating opportunities. Salinity can also affect sperm directly; however, little is known of how salinity influences sneaker tactics through sperm performance. We sampled males of the invasive round goby (Neogobius melanostomus) from two environments, a freshwater river and a brackish estuary. This fish has two male morphs: nest‐holding dark males and non‐nest‐holding light males. We examined the role of water salinity of 0, 8, and 16 on sperm performance and found that for estuarine males, a salinity of 0 reduced sperm velocity compared to a salinity of 8 and 16. Riverine males had low velocity in all salinities. Sperm viability also decreased by over 30% in 0 salinity, compared to 8 and 16, for fish from both environments. Gobies produce ejaculate contents in specialized glands that could in theory shield sperm in an adverse environment. However, gland contents did not improve sperm performance in our tests. Body mass and age estimates indicate that riverine males invested more in somatic growth compared to estuarine males. Estuarine light morph males had a high enough gonadosomatic index to indicate sneaker tactics. We propose that when sperm performance is low, such as for the riverine males, sneaker tactics are ineffective and will be selected against or phenotypically suppressed. Instead, we interpret the increased investment in somatic growth found in riverine males as a life‐history decision that is advantageous when defending a nest in the next reproductive season.     

Sperm adaptation in relation to salinity in three goby species

In externally fertilizing species, the gametes of both males and females are exposed to the influences of the environment into which they are released. Sperm are sensitive to abiotic factors such as salinity, but they are also affected by biotic factors such as sperm competition. In this study, the authors compared the performance of sperm of three goby species, the painted goby, Pomatoschistus pictus, the two-spotted goby, Pomatoschistus flavescens, and the sand goby, Pomatoschistus minutus. These species differ in their distributions, with painted goby having the narrowest salinity range and sand goby the widest. Moreover, data from paternity show that the two-spotted goby experiences the least sperm competition, whereas in the sand goby sperm competition is ubiquitous. The authors took sperm samples from dissected males and exposed them to high salinity water (31 PSU) representing the North Sea and low salinity water (6 PSU) representing the brackish Baltic Sea Proper. They then used computer-assisted sperm analysis to measure the proportion of motile sperm and sperm swimming speed 10 min and 20 h after sperm activation. The authors found that sperm performance depended on salinity, but there seemed to be no relationship to the species' geographical distribution in relation to salinity range. The species differed in the proportion of motile sperm, but there was no significant decrease in sperm motility during 20 h. The sand goby was the only species with motile sperm after 72 h.     

Female mate choice is not affected by mate condition in a fish with male care

Several studies have shown that mate choice based on condition leads to higher reproductive success of the choosing individual. Yet, a growing body of literature has failed to find support for mate choice based on mate condition, even when the choosing individual would clearly benefit from such a choice. This indicates that animals’ mate choice is often more complex than currently appreciated and that even well-founded expected preferences cannot be taken for granted. Using the broad-nosed pipefish, Syngnathus typhle, we manipulated male condition experimentally to explore whether it affects female mate choice. In this sex-role-reversed species, males care for the offspring in a specialised brood pouch. Males are the choosier sex, but given the opportunity, females are selective as well. During brooding, males can both provide embryos with nutrients and take up nutrients that originate from eggs deposited in the pouch, and embryo survival correlates positively with male condition. Together, this suggests that it would be beneficial for females to mate with males in high condition. However, we found no female preference for males in better condition. Thus, this study adds to the literature of mate choice that is unaffected by mate condition. Possible reasons for our result are discussed.     

Effects of territorial damselfish on cryptic bioeroding organisms on dead Acropora formosa

Many damselfishes exclude other grazers from their territories and “farm” filamentous algae within their territories. In this study the indirect effect of damselfish territories on faunal composition and abundance of internal bioeroders of dead Acropora formosa (Dana, 1846) was investigated in territories of two damselfish species, Stegastes nigricans (Lacepède, 1802) and Plectroglyphidodon lacrymatus (Quoy and Gaimard, 1825). S. nigricans tends to be more protective and defend their territories more aggressively than P. lacrymatus. Newly killed branches of A. formosa were placed inside and outside damselfish territories, for 1 or 2 years, at a coral reef near Zanzibar, Tanzania. As predicted, the coral branches became covered with more filamentous algae in the S. nigricans territories than in the controls, with intermediate levels in the P. lacrymatus territories. Among the internal bioeroding fauna, polychaetes were by far the most common group. In total, there were significantly more borers in the first year than the second, which was mainly due to a high abundance of sabellids. Furthermore, sabellids were significantly more abundant in control areas and in the P. lacrymatus territories compared to the S. nigricans territories. However, many other genera showed the opposite pattern, with more polychaetes in the fish territories compared to the controls. There was also a clear difference in assemblage structure between S. nigricans territories and controls. Thus, we found strong effects of whether a piece of coral was placed inside or outside a damselfish territory on the abundance of many of the bioeroding taxa. We discuss multiple reasons for these indirect effects of the territories, including that deposit feeding bioeroders may benefit from the dense algal turf found inside the territories whereas suspension feeding bioeroders may benefit from substrate with less filamentous algae found outside territories. Considering our results in the context of the large areas of coral reefs that typically are defended as territories by damselfishes, these fish are likely to have a considerable impact on the boring community of a coral reef.     

Effects of a low oxygen environment on parental effort and filial cannibalism in the male sand goby, Pomatoschistus minutus

In fish, brood cycling parental males sometimes eat some orall of their eggs, a behavior termed filial cannibalism. Wetested predictions of filial cannibalism models related to thecost of parental care in the male sand goby, Pomatoschistusminutus, by increasing the parental effort (fanning expenditure)through reduced levels of dissolved oxygen to 39% in an experimentalgroup, whereas a control group had fully saturated water. Malesshowed both full-clutch cannibalism and partial-clutch cannibalismin both treatments. Giving the males one to three females tospawn with, we found that small clutches were completely eatenmore often than were larger ones, whereas partial-clutch cannibalismwas not affected by clutch size. Although treatment did notaffect filial cannibalism, it did affect a male's energy statesuch that males in the low oxygen treatment lost more body fat,indicating a greater fanning effort. This shows that males inthe low oxygen treatment allocated more energy to the presentbrood, potentially at the expense of future reproductive success.Our study strongly suggests that filial cannibalism in malesand gobies represents a strategic life-history decision asan investment in future reproductive success, and is not triggeredby a proximate need for food necessary for the male's own survival.Furthermore, males in the low oxygen treatment built nests withlarger entrances, and were less likely to rebuild their nestsafter destruction. Presumably, this makes fanning easier butthe nest more vulnerable to predators, suggesting a trade-offbetween fanning and nest defense.     

Sexually selected nest-building--Pomatoschistus minutus males build smaller nest-openings in the presence of sneaker males

Both natural selection and sexual selection may act on nest-building. We tested experimentally how different regimes of egg-predation and male-male competition influence nest-building before mating, using the marine fish sand goby, Pomatoschistus minutus. Males with sneaker males present built the smallest nest-openings, smaller than males held alone or with Pomatoschistus microps males (which may predate eggs and compete over nest-sites but not compete over fertilizations). Males with visual access to other nest-building males tended also to build smaller openings than males held alone or with P. microps. Males with egg-predators present built nests with openings not differing significantly from any other treatment. Our results indicate that the small nest-openings found in the sneaker male treatment are sexually selected through protection against sneaking or by female choice. Across treatments, time span before a male started to build his nest also explained variation in nest-opening width; males starting late built larger nest-openings.     

Costs of breeding and their effects on the direction of sexual selection

A recent life-history model has challenged the importance of the operational sex ratio and the potential reproductive rates of males and females as the factors most important for the control of sexual selection, arguing that the cost of breeding, interpreted as the probability of dying as a consequence of the current breeding attempt, is the single most important factor that best predicts a mating system. In one species of bushcricket, the mating system can be reversed by resource manipulation. Here, we examine the costs of breeding in this system. Consistent with the model, increased costs of breeding can explain female competition and increased male choosiness under resource limitation. However, this is due to differences in the time required for a breeding attempt, rather than differences in breeding mortality which did not differ between the sexes. In general, males lived longer than females and we discuss the possible reasons behind this pattern of sex-biased non-breeding mortality.     

Genetic mating patterns studied in pools with manipulated nest site availability in two populations of Pomatoschistus minutus

Pomatoschistus minutus show paternal care in a resource defence mating system. We investigated the effect of nest-site availability on parasitic spawning. Each experimental pool contained four potentially nest-building males, four females and nests-sites in shortage (2) or excess (6). Both treatments were conducted in two populations; one with natural nest-sites in excess, one with a nest-site shortage. Microsatellite-DNA revealed that all nest-holders had fertilized most of the eggs they tended. Yet, 35% of the nests contained eggs fertilized by another male and 14.4% of the males had performed parasitic spawning. There was no site or treatment effect. Several females spawned in two nests, which coincided with parasitic spawnings, suggesting a cost to the nest-holder in terms of lost mating success. Nest-holders with and without eggs and non-nesting males all spawned parasitically, generating a significantly lower opportunity for sexual selection compared to if there had been no parasitic spawning.     

The dynamics of operational sex ratios and competition for mates

In sexually reproducing animals, individuals of one sex may have to compete for access to mating partners of the opposite sex. The operational sex ratio (OSR) is central in predicting the intensity of mating competition and which sex is competing for which. Thanks to recent theoretical and empirical advances, particularly by exploring the concept of OSR, sexual selection studies today are becoming more fine-tuned and dynamic. The original role of parental investment in predicting sexual selection has recently been complemented by the use of sexual differences in potential reproductive rates (PRR).