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.

Does the female seahorse still prefer her mating partner after a period of separation?

For species showing sexual monogamy, once one male and one female form a mating pair bond, they will be faithful to each other in subsequent breeding events. However, if their pair bond is broken for some reason, do they continue to prefer their partner when they come together again for mating? In other words, can the broken pair bond of sexually monogamous species be repaired? This is an interesting question but not yet well answered. To address this question, in the present study we used the lined seahorse (Hippocampus erectus), a typical sexually monogamous species, to study the partner preference of a female individual who experienced a complete separation followed by a reunion with her partner. Our main findings are as follows: (i) The female seahorse no longer prefers her partner after a separation, whether it is a former partner or a recent partner. No preference for partner-males may indicate that the broken pair bond cannot be repaired. (ii) The female seahorse maintains sexual fidelity to her partner in the absence of separation. However, once the health of her partner decreases, the female will switch mate, and her courtship with the new partner can take place during the pregnancy of her original partner. The first finding may provide insight into whether monogamous species still have an opportunity to reselect a new partner in the future to correct their poor choice once they have mated with a low-quality partner. The answer is that they can still gain an opportunity as long as the pair bonds with their current partners are broken. The second finding may reveal the conditions and timing at which a female seahorse switches her mate. These findings help us better understand the mating system of the seahorse H. erectus.

Female lined seahorses (Hippocampus erectus) recognize their mates based on olfactory cues

Recognition of mates from others is crucial for monogamous species to maintain their long-term pair bonds. The seahorse is widely recognized as a monogamous species, and its mate recognition cue is still not well understood. In the present study, we used the lined seahorse (Hippocampus erectus) as an experimental animal and investigated the effect of blocking olfactory, visual or behavioral (i.e., greeting) cues on mate recognition. Our results show that as long as the female seahorse can smell her mate, she will remain faithful to her mate and persistently select her mate as her next mating partner, regardless of whether the visual and/or behavioral cues between her and her mate are blocked. This finding implies that olfaction is a critical cue for a female seahorse to recognize her mate.

Diversity of Seahorse Species (Hippocampus spp.) in the International Aquarium Trade

Seahorses (Hippocampus spp.) are threatened as a result of habitat degradation and overfishing. They have commercial value as traditional medicine, curio objects, and pets in the aquarium industry. There are 48 valid species, 27 of which are represented in the international aquarium trade. Most species in the aquarium industry are relatively large and were described early in the history of seahorse taxonomy. In 2002, seahorses became the first marine fishes for which the international trade became regulated by CITES (Convention for the International Trade in Endangered Species of Wild Fauna and Flora), with implementation in 2004. Since then, aquaculture has been developed to improve the sustainability of the seahorse trade. This review provides analyses of the roles of wild-caught and cultured individuals in the international aquarium trade of various Hippocampus species for the period 1997–2018. For all species, trade numbers declined after 2011. The proportion of cultured seahorses in the aquarium trade increased rapidly after their listing in CITES, although the industry is still struggling to produce large numbers of young in a cost-effective way, and its economic viability is technically challenging in terms of diet and disease. Whether seahorse aquaculture can benefit wild populations will largely depend on its capacity to provide an alternative livelihood for subsistence fishers in the source countries. For most species, CITES trade records of live animals in the aquarium industry started a few years earlier than those of dead bodies in the traditional medicine trade, despite the latter being 15 times higher in number. The use of DNA analysis in the species identification of seahorses has predominantly been applied to animals in the traditional medicine market, but not to the aquarium trade. Genetic tools have already been used in the description of new species and will also help to discover new species and in various other kinds of applications.

Home range use in the West Australian seahorse Hippocampus subelongatus is influenced by sex and partner’s home range but not by body size or paired status

Genetic monogamy is the rule for many species of seahorse, including the West Australian seahorse Hippocampus subelongatus. In this paper, we revisit mark-recapture and genetic data of H. subelongatus, allowing a detailed characterization of movement distances, home range sizes and home range overlaps for each individual of known sex, paired status (paired or unpaired) and body size. As predicted, we find that females have larger home ranges and move greater distances compared to males. We also confirm our prediction that the home ranges of pair-bonded individuals (members of a pair known to reproduce together) overlap more on average than home ranges of randomly chosen individuals of the opposite or same sex. Both sexes, regardless of paired status, had home ranges that overlapped with, on average, 6–10 opposite-sex individuals. The average overlap area among female home ranges was significantly larger than the overlap among male home ranges, probably reflecting females having larger home ranges combined with a female biased adult sex ratio. Despite a prediction that unpaired individuals would need to move around to find a mate, we find no evidence that unpaired members of either sex moved more than paired individuals of the same sex. We also find no effect of body size on home range size, distance moved or number of other individuals with which a home range overlapped. These patterns of movement and overlap in home ranges among individuals of both sexes suggest that low mate availability is not a likely explanation for the maintenance of monogamy in the West Australian seahorse.

Coevolution of female fidelity and male help in populations with alternative reproductive tactics

In socially monogamous species, pair-bonded males often continue to provide care to all offspring in their nests despite some degree of paternity loss due to female extra-pair copulation. Previous theoretical models suggested that females can use their within-pair offspring as 'hostages' to blackmail their social mates, so that they continue to provide care to the brood at low levels of cuckoldry. These models, however, rely on the assumption of sufficiently accurate male detection of cuckoldry and the reduction of parental effort in case of suspicion. Therefore, they cannot explain the abundant cases where cuckolded males continue to provide extensive care to the brood. Here we use an analytical population genetics model and an individual-based simulation model to explore the coevolution of female fidelity and male help in populations with two genetically determined alternative reproductive tactics (ARTs): sneakers that achieve paternity solely via extra-pair copulations and bourgeois that form a mating pair and spend some efforts in brood care. We show that when the efficiency of mate guarding is intermediate, the bourgeois males can evolve to 'specialize' in providing care by spending more than 90% of time in helping their females while guarding them as much as possible, despite frequent cuckoldry by the sneakers. We also show that when sneakers have tactic-specific adaptations and thus are more competitive than the bourgeois in gaining extra-pair fertilizations, the frequency of sneakers and the degrees of female fidelity and male help can fluctuate in evolutionary cycles. Our theoretical predictions highlight the need for further empirical tests in species with ARTs.

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.

Phylogenetic analysis and genetic structure of the seahorse, Hippocampus fuscus from the Arabian and Red Sea based on mitochondrial DNA sequences

The unique body morphology and specialized life history traits make seahorses excellent flagship species for many issues in marine conservation and biological evolution. In this study, the complete mitochondrial genome of Hippocampus fuscus was determined. Phylogenetic analyses showed that H. fuscus had a close genetic relationship to Hippocampus reidi, which give us a new insight into the speciation and dispersal among seahorse genus. A total of 843 base pairs of cytochrome b (Cytb) gene and a 646 base pairs of cytochrome oxidase I (COI) gene were obtained from 36 H. fuscus and 26 H. reidi from three populations. Population genetic analysis revealed a relatively high genetic diversity across the populations of H. fuscus compared with H. reidi. Neighbour-joining (NJ) tree of COI and Cytb gene sequences showed that H. fuscus haplotypes formed one cluster. The result of median-joining network of haplotypes based on COI and Cytb indicated a lack of structure in populations of H. fuscus. This study addressed the information about the evolution history and genetic structure of an Arabian seahorse H. fuscus, which provides insight into the genetic conservation and sustainable management for this species.

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.

Host Genotype and Microbiota Contribute Asymmetrically to Transcriptional Variation in the Threespine Stickleback Gut

Recent studies of interactions between hosts and their resident microbes have revealed important ecological and evolutionary consequences that emerge from these complex interspecies relationships, including diseases that occur when the interactions go awry. Given the preponderance of these interactions, we hypothesized that effects of the microbiota on gene expression in the developing gut—an important aspect of host biology—would be pervasive, and that these effects would be both comparable in magnitude to and contingent on effects of the host genetic background. To evaluate the effects of the microbiota, host genotype, and their interaction on gene expression in the gut of a genetically diverse, gnotobiotic host model, the threespine stickleback (Gasterosteus aculeatus), we compared RNA-seq data among 84 larval fish. Surprisingly, we found that stickleback population and family differences explained substantially more gene expression variation than the presence of microbes. Expression levels of 72 genes, however, were affected by our microbiota treatment. These genes, including many associated with innate immunity, comprise a tractable subset of host genetic factors for precise, systems-level study of host–microbe interactions in the future. Importantly, our data also suggest subtle signatures of a statistical interaction between host genotype and the microbiota on expression patterns of genetic pathways associated with innate immunity, coagulation and complement cascades, focal adhesion, cancer, and peroxisomes. These genotype-by-environment interactions may prove to be important leads to the understanding of host genetic mechanisms commonly at the root of sometimes complex molecular relationships between hosts and their resident microbes.

Population Genetic Structure of the Dwarf Seahorse (Hippocampus zosterae) in Florida

The dwarf seahorse (Hippocampus zosterae) is widely distributed throughout near-shore habitats of the Gulf of Mexico and is of commercial significance in Florida, where it is harvested for the aquarium and curio trades. Despite its regional importance, the genetic structure of dwarf seahorse populations remains largely unknown. As an aid to ongoing conservation efforts, we employed three commonly applied mtDNA markers (ND4, DLoop and CO1) to investigate the genetic structuring of H. zosterae in Florida using samples collected throughout its range in the state. A total of 1450 bp provided sufficient resolution to delineate four populations of dwarf seahorses, as indicated by significant fixation indices. Despite an overall significant population structure, we observed evidence of interbreeding between individuals from geographically distant sites, supporting the hypothesis that rafting serves to maintain a degree of population connectivity. All individuals collected from Pensacola belong to a single distinct subpopulation, which is highly differentiated from the rest of Floridian dwarf seahorses sampled. Our findings highlight the utility of mtDNA markers in evaluating barriers to gene flow and identifying genetically distinct populations, which are vital to the development of comprehensive conservation strategies for exploited taxa.