Females increase reproductive investment when mated to less sexually attractive males in a serially monogamous fish

Reproductive investment decision is an integral part of life-history theory. Differential allocation hypothesis predicts that females should increase investment when mated to high-quality males, conversely, reproductive compensation hypothesis predicts that females should increase investment when mated to low-quality males. Empirical research dominantly focuses on polygamous species and rarely on serially monogamous species. So, the question remains: which hypothesis does serially monogamous species fit? And if it fits reproductive compensation hypothesis, do females only compensate once or continuously for multiple times when mating to low-quality males? Here, we used a serially monogamous fish, the lined seahorse (Hippocampus erectus), to investigate the reproductive investment pattern of females in relation to male quality (measured by sexual attractiveness). We found that females allocated more resources into eggs when they mated to less-sexually-attractive males, indicating the investment pattern of lined seahorse falls in with the prediction of reproductive compensation hypothesis. This finding may imply that the sex role of seahorses is reversed, and female is the side imposed on a greater sexual selection pressure. On this basis, we compared the investment difference of females in two consecutive breeding events when mated to less-sexually-attractive males. We found that females allocated less resources into eggs in the second breeding than in the first one. Females reduced their reproductive compensation in the second breeding, which may be attributed to the improvement in the quality (e.g., paternal care ability) of their mates after the first breeding, thus eliminating the need for them to invest more in the second breeding.

Population dynamics of the long-snouted seahorse (Hippocampus guttulatus Cuvier, 1829) in the Mar Menor coastal lagoon

A 10-year monitoring program was developed to quantify the population dynamics of the long-snouted seahorse population in the Mar Menor coastal lagoon. Based on 985 underwater visual censuses, we estimated the long-snouted seahorse (Hippocampus guttulatus Cuvier, 1829) population size in the Mar Menor lagoon and its reduction in size in the last decades, as well as the effect of eutrophication crises in 2016 and 2019 on the species. The annual recruitment for the 2013-2020 period was estimated by comparing the relative abundance of early seahorse life stages in the ichthyoplankton. The density ranged from 0.0458 specimens/m3 at the beginning of the sampling period to 0.0004 at the end, showing a statistically significant difference between the three analyzed periods (Hgl=2  = 14.0, p = 0.001). The long-snouted seahorse population from the Mar Menor lagoon exemplifies the impact of fishing activities and human pressure, especially euxinic episodes and habitat destruction. As a result of this, the Mar Menor population has decreased from several million specimens to a few thousand, in only three decades. This species showed considerable resilience, the seahorse population began to recover once fishing activity stopped. In contrast, the long-snouted seahorse showed high vulnerability to habitat loss and an episodic flooding event. Adult seahorses showed preferences for highly complex habitats, especially Caulerpa prolifera-Cymodocea nodosa mixed meadows and habitats of high complexity and anthropogenic origin, such as harbors, jetties, or breakwaters. In contrast, juvenile seahorses preferred monotonous seabeds with low complexity, such as the sandy beds that are characteristic of the Mar Menor lagoon littoral.

Impacts of acute hypoxia on the short-snouted seahorse metabolism and behaviour

Seahorses are one of the most unique and enigmatic animals, recognized as flagship species for several conservation issues. Unfortunately, seahorses' populations have been declining and their unique lifestyle may constrain the ability of these animals to evolve in the future climate scenarios. They inhabit shallow coastal waters that display daily or seasonal environmental fluctuations. Yet, few studies have scrutinized the impacts of climate changes on these iconic species. Within this context, the objective of this work was to test the effects of an extreme hypoxia exposure (~27 % dissolved oxygen for approximately 7 h) on the metabolism, behaviour and food intake of the temperate seahorse Hippocampus hippocampus. Regarding metabolism, hypoxia exposure led to a significant reduction in metabolic rates and an increase in ventilation rates. Seahorses showed signs of movement lethargy under oxygen depletion. The results show that a small but extreme exposure to hypoxia is tolerable by seahorses despite inducing metabolic and behavioural changes, that may jeopardize the future development and survival of these iconic organisms.

Nocturnal surveys of lined seahorses reveal increased densities and seasonal recruitment patterns

Although the nighttime ecology of organisms remains understudied, nocturnal surveys play an integral part in assessing fish assemblages and the selective forces shaping them. Eleuthera (Bahamas) contains an unusual population of lined seahorses (Hippocampus erectus) in an anchialine lake, possessing morphological characteristics distinct from those found in the ocean. Population surveys for seahorses and their potential predators were conducted at midnight and midday during wet and dry seasons, with belt transects perpendicular to the shoreline that increased in depth away from shore. Nocturnal surveys uncovered seahorse densities 259% higher than daytime transects on average. Sex ratios were consistently male-biased, and the frequency of animals from different reproductive categories varied significantly by time of day, with gravid males observed around the clock but females and nongravid males observed more often at night. Spatial and seasonal recruitment was detected for the first time in this species, with an increase in juveniles detected in the shallow ends of transects during dry season surveys. Juvenile recruitment is poorly understood across syngnathid fishes, so the detection of early recruits at night has broad implications for this fish family. Seahorses from all reproductive categories were perched significantly higher in the water column during the night regardless of their depth or season. Predator densities followed a similar pattern with higher densities observed at night, indicating that elevated nocturnal perch height may be a response to predator presence. However, the selective agents driving these nocturnal behaviors have yet to be identified. Considering H. erectus is listed on the IUCN Red List as "Vulnerable," the increase in nocturnal population size and the detection of juveniles has crucial implications for understanding their ecology, recruitment, and conservation.

Age-specific survivorship and fecundity shape genetic diversity in marine fishes

Genetic diversity varies among species due to a range of eco-evolutionary processes that are not fully understood. The neutral theory predicts that the amount of variation in the genome sequence between different individuals of the same species should increase with its effective population size (N e ). In real populations, multiple factors that modulate the variance in reproductive success among individuals causeN e to differ from the total number of individuals ( N ). Among these, age-specific mortality and fecundity rates are known to have a direct impact on theN e / N ratio. However, the extent to which vital rates account for differences in genetic diversity among species remains unknown. Here, we addressed this question by comparing genome-wide genetic diversity across 16 marine fish species with similar geographic distributions but contrasted lifespan and age-specific survivorship and fecundity curves. We sequenced the whole genome of 300 individuals to high coverage and assessed their genome-wide heterozygosity with a reference-free approach. Genetic diversity varied from 0.2% to 1.4% among species, and showed a negative correlation with adult lifespan, with a large negative effect (s l o p e = - 0.089 per additional year of lifespan) that was further increased when brooding species providing intense parental care were removed from the dataset (s l o p e = - 0.129 per additional year of lifespan). Using published vital rates for each species, we showed that theN e / N ratio resulting simply from life tables parameters can predict the observed differences in genetic diversity among species. Using simulations, we further found that the extent of reduction inN e / N with increasing adult lifespan is particularly strong under Type III survivorship curves (high juvenile and low adult mortality) and increasing fecundity with age, a typical characteristic of marine fishes. Our study highlights the importance of vital rates as key determinants of species genetic diversity levels in nature.

Mutual mate choice and its benefits for both sexes

In mating interactions, it is common in nature for both sexes to choose simultaneously. However, this mutual mate choice and its consequences for progeny has received relatively little study; an approach where both male and female condition is manipulated is thus desirable. We compared both sexes' preferences in Tenebrio molitor beetles when individual condition varied (healthy vs infected with a fungus), and observed the direct benefits of those preferences. We predicted that: (a) females and males in good condition would prefer high quality mates; (b) preferences would be weaker when the choosing individual is in poor condition (and thus less selective given, for example, time and energetic constrains); and, (c) high quality mates would lay a larger number of total eggs and/or viable eggs than low quality mates. We found that both males and females in good condition were not more likely to choose mates that were also in good condition. However, poor-condition animals were more likely to prefer similar quality animals, while high-condition animals did not necessarily prefer mates of similar condition. Choosing sick males or females had a negative impact on egg number and viability. Our results suggest a non-adaptive mate choice in this species. Possibly, a deteriorated condition may drive individuals to invest more in attracting mates, because their chances of surviving the infection are very low. However, we do not discount the possibility that the fungus is manipulating individuals to increase its transmission during mating.

Seasonal and spatial variation in the reproductive biology of the dwarf seahorse Hippocampus zosterae

Factors associated with the reproductive ecology of the dwarf seahorse Hippocampus zosterae were investigated. Fish from a Tampa Bay (FL, USA) seagrass ecosystem were collected, photographed and returned to the wild, with photos analysed to determine patterns of body size, density, sex ratio and reproductive state across site and season to understand the population dynamics of H. zosterae over time. Animal density did not vary significantly with site and season, indicating there is little evidence of seasonal migration in this species. Densities reported in this study were higher than the mean density for all seahorse species Hippocampus spp. There was no sexual dimorphism in body length and both sexes reached sexual maturity at the same size. The ratio of gravid to non-gravid males was found to shift by season but not by site, with breeding detected year-round in this population compared with populations further north in their range. Peak breeding (70% gravid males) was observed in the late summer-autumn (August-October) in the site furthest from shore. The largest fish for both sexes were recorded during the summer and autumn months in the mid-shore, deepest site. Sex ratio shifted by site with even sex ratios near the shore but significantly female-biased sex ratios detected at sites near open water. Lastly, an increase in marking dates with decreased time intervals between collections did not yield a higher recapture rate, compared with sampling in 2010. However, the Tampa Bay population of dwarf seahorses demonstrated stable densities across 3 years with year-round breeding, indicating that it is a robust population worthy of long-term monitoring for conservation efforts.

When more is not merrier: Using wild population dynamics to understand the effect of density on ex situ seahorse mating behaviors

Seahorses are considered one of the most iconic examples of a monogamous species in the animal kingdom. This study investigates the relationship between stocking density and mating and competitive behavior from the context of the field biology of the dwarf seahorse, Hippocampus zosterae (Jordan & Gilbert). Animals were housed in 38 liter tanks at a range of densities and sex ratios (from 2-8 animals per tank), and their reproductive and other social behaviors were monitored from tank introduction through copulation. At low tank densities and even sex ratios but comparatively high field densities, frequency of both mating and competitive behaviors was low in trials. A higher level of males in tanks across all densities increased competition, activity levels, and aggression leading to egg transfer errors and brood expulsion, resulting in lower reproductive success. Across seahorse species, mean and maximum wild densities were consistently lower than those used in ex situ breeding, with adult sex ratios that were significantly female biased. However, significant variation exists in wild seahorse densities across species, with higher densities detected in focal/mark recapture studies and on artificial habitat structures than reported with belt transect sampling techniques. Interchange of knowledge gained in both aquarium and wild contexts will allow us to better understand the biology of this genus, and improve reproduction in captivity. Interpreting ex situ reproductive behaviors of seahorses within various densities reported from natural populations will help us predict the impact of conservation efforts and increase the likelihood of long-term persistence of populations for this threatened genus.

Monthly population density and structure patterns of an endangered seahorse Hippocampus capensis: a comparison between natural and artificial habitats

This study investigated aspects of the population ecology of the endangered Knysna seahorse Hippocampus capensis within different habitat types. High densities of H. capensis were found within artificial Reno mattress habitat, within the Knysna Estuary, South Africa. Monthly surveys at three sites were conducted from October 2015 to August 2016 to compare population densities of H. capensis in this artificial habitat with natural eel grass Zostera capensis habitat. Hippocampus capensis densities varied significantly across all sites and highest population densities were consistently observed within the Reno mattress habitat. Hippocampus capensis were also found to be significantly larger within the Reno mattress habitat and pooled data showed that males were significantly larger than females. The overall sex ratio for all three sites was female biased, although this varied across seasons at two sites. The findings suggest that artificial Reno mattresses provide novel habitat for this endangered species and consideration should be given to the usefulness of these structures in future conservation actions.

Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy

Why do some animals mate with one partner rather than many? Here, I investigate factors related to (i) spatial constraints (habitat limitation, mate availability), (ii) time constraints (breeding synchrony, length of breeding season), (iii) need for parental care, and (iv) genetic compatibility, to see what support can be found in different taxa regarding the importance of these factors in explaining the occurrence of monogamy, whether shown by one sex (monogyny or monandry) or by both sexes (mutual monogamy). Focusing on reproductive rather than social monogamy whenever possible, I review the empirical literature for birds, mammals and fishes, with occasional examples from other taxa. Each of these factors can explain mating patterns in some taxa, but not in all. In general, there is mixed support for how well the factors listed above predict monogamy. The factor that shows greatest support across taxa is habitat limitation. By contrast, while a need for parental care might explain monogamy in freshwater fishes and birds, there is clear evidence that this is not the case in marine fishes and mammals. Hence, reproductive monogamy does not appear to have a single overriding explanation, but is more taxon specific. Genetic compatibility is a promising avenue for future work likely to improve our understanding of monogamy and other mating patterns. I also discuss eight important consequences of reproductive monogamy: (i) parentage, (ii) parental care, (iii) eusociality and altruism, (iv) infanticide, (v) effective population size, (vi) mate choice before mating, (vii) sexual selection, and (viii) sexual conflict. Of these, eusociality and infanticide have been subject to debate, briefly summarised herein. A common expectation is that monogamy leads to little sexual conflict and no or little sexual selection. However, as reviewed here, sexual selection can be substantial under mutual monogamy, and both sexes can be subject to such selection. Under long-term mutual monogamy, mate quality is obviously more important than mate numbers, which in turn affects the need for pre-mating mate choice. Overall, I conclude that, despite much research on genetic mating patterns, reproductive monogamy is still surprisingly poorly understood and further experimental and comparative work is needed. This review identifies several areas in need of more data and also proposes new hypotheses to test.

A synthesis of European seahorse taxonomy, population structure, and habitat use as a basis for assessment, monitoring and conservation

Accurate taxonomy, population demography, and habitat descriptors inform species threat assessments and the design of effective conservation measures. Here we combine published studies with new genetic, morphological and habitat data that were collected from seahorse populations located along the European and North African coastlines to help inform management decisions for European seahorses. This study confirms the presence of only two native seahorse species (Hippocampus guttulatus and H. hippocampus) across Europe, with sporadic occurrence of non-native seahorse species in European waters. For the two native species, our findings demonstrate that highly variable morphological characteristics, such as size and presence or number of cirri, are unreliable for distinguishing species. Both species exhibit sex dimorphism with females being significantly larger. Across its range, H. guttulatus were larger and found at higher densities in cooler waters, and individuals in the Black Sea were significantly smaller than in other populations. H. hippocampus were significantly larger in Senegal. Hippocampus guttulatus tends to have higher density populations than H. hippocampus when they occur sympatrically. Although these species are often associated with seagrass beds, data show both species inhabit a wide variety of shallow habitats and use a mixture of holdfasts. We suggest an international mosaic of protected areas focused on multiple habitat types as the first step to successful assessment, monitoring and conservation management of these Data Deficient species.

Life history and ecology of the elusive European short-snouted seahorse Hippocampus hippocampus

To improve the understanding of the life history and ecology of one of Europe's most elusive fishes, the short-snouted seahorse Hippocampus hippocampus, data from wild populations in a shallow coastal lagoon in southern Portugal were analysed. The data were collected from 17 tagged seahorses on a focal-study grid as well as from >350 seahorses encountered during underwater visual surveys and a fishery-independent study using beach seines. These populations of settled juveniles and adults had a mean population density of 0·009 m^-2 . During the study period (2000-2004), reproduction peaked in July and August. Juveniles recruited to the lagoon at c. 66 mm standard length (L_S ) and 0·5 years of age and established small home ranges (0·8 to 18·2 m² ). First reproduction was estimated at 100 mm and 1 year of age. Based on a fitted von Bertalanffy model, H. hippocampus grew quickly (growth coefficient K = 0·93) to a maximum theoretical size L_∞  = 150 mm and have a maximum lifespan of c. 3·2 years. Courtship behaviours were consistent with the maintenance of pair bonds and males brooded multiple batches of young per year. Estimated annual reproductive output averaged 871 young (±632). Together these analyses provide the first life-history parameters for this species and indicate that H. hippocampus bears characteristics of opportunist and intermediate strategists. Such populations are predicted to exhibit large fluctuations in abundance, making them vulnerable to extended periods of poor recruitment.

Parental care improves immunity in the seahorse (Hippocampus erectus)

In the present study, the sexual dimorphism in immune response in the seahorse Hippocampus erectus in which males compete for mates and invest heavily in parental care was assessed. Variability in immunocompetence in virginal seahorses with differing levels of sexual maturity (i.e., immaturity, early maturity and maturity) and with different mating statuses (i.e., virginal, experienced mating failure and experienced mating success) were analyzed by evaluating immune parameters in the plasma. Additionally, ultrastructural characteristics of the inner epithelium of the brood pouch were compared between males that had experienced mating failure and those that had succeeded. Generally, immunity in sexually mature virgin males was greater than in females, and mating competition significantly reduced males' immunity. However, parental care gave males stronger immune and metabolic abilities and resulted in their immunity significantly rebounding after a successful mating. The present study quantitatively clarifies, for the first time, how parental care and mating competition jointly affect immunity. Moreover, previous findings that females display more efficient immune defenses than males in conventional species (i.e., males are as competitor and females as care giver) and that males' immunity is higher than females' in the pipefish (i.e., females are as competitor and males as care giver) in combination with the present results indicate that parental care is a key factor for sexual dimorphism in immunity. The care-giving sex has strong immunity regardless of the sex in charge of mating competition or not.

A conceptual review of mate choice: stochastic demography, within-sex phenotypic plasticity, and individual flexibility

Mate choice hypotheses usually focus on trait variation of chosen individuals. Recently, mate choice studies have increasingly attended to the environmental circumstances affecting variation in choosers' behavior and choosers' traits. We reviewed the literature on phenotypic plasticity in mate choice with the goal of exploring whether phenotypic plasticity can be interpreted as individual flexibility in the context of the switch point theorem, SPT (Gowaty and Hubbell 2009). We found >3000 studies; 198 were empirical studies of within‐sex phenotypic plasticity, and sixteen showed no evidence of mate choice plasticity. Most studies reported changes from choosy to indiscriminate behavior of subjects. Investigators attributed changes to one or more causes including operational sex ratio, adult sex ratio, potential reproductive rate, predation risk, disease risk, chooser's mating experience, chooser's age, chooser's condition, or chooser's resources. The studies together indicate that “choosiness” of potential mates is environmentally and socially labile, that is, induced – not fixed – in “the choosy sex” with results consistent with choosers' intrinsic characteristics or their ecological circumstances mattering more to mate choice than the traits of potential mates. We show that plasticity‐associated variables factor into the simpler SPT variables. We propose that it is time to complete the move from questions about within‐sex plasticity in the choosy sex to between‐ and within‐individual flexibility in reproductive decision‐making of both sexes simultaneously. Currently, unanswered empirical questions are about the force of alternative constraints and opportunities as inducers of individual flexibility in reproductive decision‐making, and the ecological, social, and developmental sources of similarities and differences between individuals. To make progress, we need studies (1) of simultaneous and symmetric attention to individual mate preferences and subsequent behavior in both sexes, (2) controlled for within‐individual variation in choice behavior as demography changes, and which (3) report effects on fitness from movement of individual's switch points.

An assessment of West African seahorses in fisheries catch and trade

This study provides the first assessment of a heavily traded West African seahorse species, Hippocampus algiricus, and the first information on short-snouted seahorse Hippocampus hippocampus biology in Africa. A total of 219 seahorses were sampled from fisher catch in Senegal and The Gambia, with estimated height at reproductive activity for H. algiricus (161 mm) larger than mean ± S.D. catch height (150 ± 31 mm). Catch composition, height at reproductive activity and potential biases in fishery retention are discussed with regard to the current Convention on International Trade of Endangered Species (CITES) guidelines.

Conservation genetics of threatened Hippocampus guttulatus in vulnerable habitats in NW Spain: temporal and spatial stability of wild populations with flexible polygamous mating system in captivity

This study was focused on conservation genetics of threatened Hippocampus guttulatus on the Atlantic coast of NW Iberian Peninsula. Information about spatial structure and temporal stability of wild populations was obtained based on microsatellite markers, and used for monitoring a captive breeding program firstly initiated in this zone at the facilities of the Institute of Marine Research (Vigo, Spain). No significant major genetic structure was observed regarding the biogeographical barrier of Cape Finisterre. However, two management units under continuous gene flow are proposed based on the allelic differentiation between South-Atlantic and Cantabrian subpopulations, with small to moderate contemporary effective size based on single-sample methods. Temporal stability was observed in South-Atlantic population samples of H. guttulatus for the six-year period studied, suggesting large enough effective population size to buffer the effects of genetic drift within the time frame of three generations. Genetic analysis of wild breeders and offspring in captivity since 2009 allowed us to monitor the breeding program founded in 2006 in NW Spain for this species. Similar genetic diversity in the renewed and founder broodstock, regarding the wild population of origin, supports suitable renewal and rearing processes to maintain genetic variation in captivity. Genetic parentage proved single-brood monogamy in the wild and in captivity, but flexible short- and long-term mating system under captive conditions, from strict monogamy to polygamy within and/or among breeding seasons. Family analysis showed high reproductive success in captivity under genetic management assisted by molecular relatedness estimates to avoid inbreeding. This study provides genetic information about H. guttulatus in the wild and captivity within an uncovered geographical range for this data deficient species, to be taken into account for management and conservation purposes.

The dynamics of reproductive rate, offspring survivorship and growth in the lined seahorse, Hippocampus erectus Perry, 1810

Seahorses are the vertebrate group with the embryonic development occurring within a special pouch in males. To understand the reproductive efficiency of the lined seahorse, Hippocampus erectus Perry, 1810 under controlled breeding experiments, we investigated the dynamics of reproductive rate, offspring survivorship and growth over births by the same male seahorses. The mean brood size of the 1-year old pairs in the 1^st birth was 85.4±56.9 per brood, which was significantly smaller than that in the 6^th birth (465.9±136.4 per brood) (P<0.001). The offspring survivorship and growth rate increased with the births. The fecundity was positively correlated with the length of brood pouches of males and trunk of females. The fecundity of 1-year old male and 2-year old female pairs was significantly higher than that from 1-year old couples (P<0.001). The brood size (552.7±150.4) of the males who mated with females that were isolated for the gamete-preparation, was larger than those (467.8±141.2) from the long-term pairs (P<0.05). Moreover, the offspring from the isolated females had higher survival and growth rates. Our results showed that the potential reproductive rate of seahorses H. erectus increased with the brood pouch development.

Novel sex-related characteristics of the longsnout seahorse Hippocampus reidi Ginsburg, 1933

This paper presents novel sex-related characteristics in Hippocampus reidi, expressed as a prominent and pigmented keel (males only), and by a row of dorsolateral spots mostly (97.4%) found in males. While the keel was found even in the smallest males sampled, the dorsolateral spots appear to be associated with body size. Ex situ observations revealed that during courtship males highlighted both characters by turning pale their background body colour. Understanding such colour and behavioural patterns is relevant to increase our knowledge about sexual selection in a morphologically and biologically unique group of fishes.

Body size preferences in the pot-bellied seahorse Hippocampus abdominalis: choosy males and indiscriminate females

Male seahorses (genus Hippocampus) provide all post-fertilization parental care, yet despite high levels of paternal investment, these species have long been thought to have conventional sex roles, with female mate choice and male–male competition. Recent studies of the pot-bellied seahorse (Hippocampus abdominalis) have shown that sex-role reversal occurs in high-density female-biased populations, indicating that male mating preferences may lead to sexual selection on females in this species. Egg size, egg number, and offspring size all correlate positively with female body size in Hippocampus, and by choosing large mating partners, male seahorses may increase their reproductive success. While male brood size is also positively correlated with body size, small H. abdominalis males can carry exceptionally large broods, suggesting that the fecundity benefits of female preference for large partners may be limited. We investigated the importance of body size in reproductive decisions of H. abdominalis, presenting focal individuals of both sexes with potential mating partners of different sizes. Mating preferences were quantified in terms of time spent courting each potential partner. Male seahorses were highly active throughout the mate-choice trials and showed a clear behavioral preference for large partners, while females showed significantly lower levels of activity and equivocal mating preferences. The strong male preferences for large females demonstrated here suggest that sexual selection may act strongly on female body size in wild populations of H. abdominalis, consistent with predictions on the importance of female body size for reproductive output in this species.