TESTING THREE MODELS ON THE ADAPTIVE SIGNIFICANCE OF PROTANDRIC SIMULTANEOUS HERMAPHRODITISM IN A MARINE SHRIMP

Does the Presence of Boring Sponges Alter Timing of Sex Change in Slipper Shell Snails, Crepidula fornicata?

AbstractThe Atlantic slipper limpet, Crepidula fornicata, is a sequential hermaphrodite whose size at sex change is plastic with respect to social and population cues. As an organism allocates energy between growth, reproduction, and maintenance, the increased cost of one process may affect another. Thus, C. fornicata affected by a stressor might have to alter the balance of energy allocation, potentially leading to changes in the timing of sex change. One such biotic stressor, the boring sponge Cliona celata, has been demonstrated to affect growth and condition of numerous molluscs. In this study, we explored whether the presence of the boring sponge affected the rate of sex change for C. fornicata, using both field surveys and an in situ manipulation. Population surveys suggested that while boring sponges may reduce tissue condition of slipper limpets, this relationship is highly variable and likely confounded by site variables. Using population metrics (size at sex, L₅₀), the presence of a boring sponge did not appear to affect sex change. Likewise, during the in situ experiment where we manipulated stack sex ratios, we did not find any differences in the timing of sex change between male-male and male-female stacks, even though C. fornicata without sponges grew significantly faster. Thus, results from field surveys and our in situ experiment suggest that sex change in this species appears robust to biotic stressors. This study suggests that C. fornicata will undergo sex change when conditions dictate, even if it is experiencing some level of biotic stress, a strategy that likely allows it to maximize lifetime reproductive output.

The first mitochondrial genome of the genus Exhippolysmata (Decapoda: Caridea: Lysmatidae), with gene rearrangements and phylogenetic associations in Caridea

The complete mitochondrial genome (mitogenome) of animals can provide useful information for evolutionary and phylogenetic analyses. The mitogenome of the genus Exhippolysmata (i.e., Exhippolysmata ensirostris) was sequenced and annotated for the first time, its phylogenetic relationship with selected members from the infraorder Caridea was investigated. The 16,350 bp mitogenome contains the entire set of 37 common genes. The mitogenome composition was highly A + T biased at 64.43% with positive AT skew (0.009) and negative GC skew (- 0.199). All tRNA genes in the E. ensirostris mitogenome had a typical cloverleaf secondary structure, except for trnS1 (AGN), which appeared to lack the dihydrouridine arm. The gene order in the E. ensirostris mitogenome was rearranged compared with those of ancestral decapod taxa, the gene order of trnL2-cox2 changed to cox2-trnL2. The tandem duplication-random loss model is the most likely mechanism for the observed gene rearrangement of E. ensirostris. The ML and BI phylogenetic analyses place all Caridea species into one group with strong bootstrap support. The family Lysmatidae is most closely related to Alpheidae and Palaemonidae. These results will help to better understand the gene rearrangements and evolutionary position of E. ensirostris and lay a foundation for further phylogenetic studies of Caridea.

Life history traits and reproductive performance of the caridean shrimp Lysmata boggessi, a heavily traded invertebrate in the marine aquarium industry

The most intense commercial harvest of marine aquarium species in North America occurs in the coastal waters surrounding Florida, yet very often little information exists on the life histories, population dynamics, or reproductive characteristics of these organisms. The peppermint shrimp Lysmata boggessi is one such species and is heavily targeted along the west coast of Florida. It is known primarily among aquarists for its ability to control pest anemones and in the scientific community for its unique sexual system, protandric simultaneous hermaphroditism. However, no study has addressed fishery interactions or long-term population dynamics for L. boggessi. We used monthly fisheries-dependent sampling, with a trained observer present, for a full year to assess seasonality in sex phase ratio (males to males + hermaphrodites), size at sex change, fecundity, embryo volume and reproductive output of an exploited L. boggessi population. L. boggessi exhibited distinct seasonality in size distribution, sex phase ratio, size at sex phase change and reproductive activity. The peak reproductive season was in spring, when the population was dominated by small but fecund hermaphrodites. Reproduction decreased during fall and winter and sex phase ratios favored male phase shrimp that exhibited delayed sex change. This population and individual level information is the first of its kind for L. boggessi and fills a much needed data gap for the informed management of this fishery.

The sex change of the caridean shrimp Hippolyte inermis Leach: temporal development of the gonopore morphology

Sex reversal is a process observed in several marine organisms, including some lineages of caridean shrimps. We investigated the gonopore shape and size, to study the sex reversal using Hippolyte inermis as a model. A method was developed which can be applied to identify the sex in juveniles of H. inermis, especially, useful when the standard method of sex assessment is not applicable. The position and the shape of gonopores was recorded under a light macroscope. The sex of mature individuals was then determined by observing the presence/absence of the appendix masculina. In addition, analysis of ontogenetic changes of gonopores were performed to compare their morphology with other species of shrimps whose gonopore morphology was previously known. Female gonopores are located at the far proximo-medial end of the third pair of pereiopod coxae and distally they bear cup-shaped structures, whilst male gonopores are located at the far proximo-medial end of the fifth pair of pereiopod coxae and they have a simpler structure. The shape and structure of gonopores in H. inermis resembled that of other caridean decapods. Intersex individuals were never observed, although this species was demonstrated to be protandric. This observation confirmed previous assumptions indicating that the process of sex reversal is very fast in H. inermis and that it takes place within a single moult. The identification of sex based on the position and shape of gonopores is feasible in this species, and it provides helpful insights for studying sex reversal in small decapods.

Integrative taxonomy of the ornamental 'peppermint' shrimp public market and population genetics of Lysmata boggessi, the most heavily traded species worldwide

The ornamental trade is a worldwide industry worth >15 billion USD with a problem of rampant product misidentification. Minimizing misidentification is critical in the face of overexploitation of species in the trade. We surveyed the peppermint shrimp ornamental marketplace in the southeastern USA, the most intense market for peppermint shrimps worldwide, to characterize the composition of species in the trade, reveal the extent of misidentification, and describe the population genetics of the true target species. Shrimps were bought from aquarium shops in FL, GA, SC, and NC. We demonstrated, contrary to popular belief (information from dealers), that the most heavily traded species in the market was Lysmata boggessi, an endemic species to the eastern Gulf of Mexico, and not Lysmata wurdemanni. Importantly, only when color pattern or genetic markers in conjunction with morphological traits were employed, was it was possible to unequivocally identify L. boggessi as the only species in the trade. The intensity of the market for peppermint shrimps in the USA has led to L. boggessi being the most traded species worldwide. Misidentification in the shrimp aquarium trade is accidental and involuntary, and is explained by remarkable similarity among congeneric species. Using sequences of the 16S-mt-DNA marker, we found no indication of population genetic structure in the endemic L. boggessi across  550 km of linear coast. Therefore, this species can be considered genetically homogeneous and a single fished stock. Still, we argue in favor of additional studies using more powerful markers (e.g., SNPs) capable of revealing genetic structure at a finer spatial-scale. Our results will help advance management and conservation policies in this lucrative yet understudied fishery. Future studies of other ornamental fisheries will benefit from using an integrative taxonomic approach, as we demonstrate here.

Molecular phylogeny of broken-back shrimps (genus Lysmata and allies): a test of the 'Tomlinson-Ghiselin' hypothesis explaining the evolution of hermaphroditism

The 'Tomlinson-Ghiselin' hypothesis (TGh) predicts that outcrossing simultaneous hermaphroditism (SH) is advantageous when population density is low because the probability of finding sexual partners is negligible. In shrimps from the family Lysmatidae, Bauer's historical contingency hypothesis (HCh) suggests that SH evolved in an ancestral tropical species that adopted a symbiotic lifestyle with, e.g., sea anemones and became a specialized fish-cleaner. Restricted mobility of shrimps due to their association with a host, and hence, reduced probability of encountering mating partners, would have favored SH. The HCh is a special case of the TGh. Herein, I examined within a phylogenetic framework whether the TGh/HCh explains the origin of SH in shrimps. A phylogeny of caridean broken-back shrimps in the families Lysmatidae, Barbouriidae, Merguiidae was first developed using nuclear and mitochondrial makers. Complete evidence phylogenetic analyses using maximum likelihood (ML) and Bayesian inference (BI) demonstrated that Lysmatidae+Barbouriidae are monophyletic. In turn, Merguiidae is sister to the Lysmatidae+Barbouriidae. ML and BI ancestral character-state reconstruction in the resulting phylogenetic trees indicated that the ancestral Lysmatidae was either gregarious or lived in small groups and was not symbiotic. Four different evolutionary transitions from a free-living to a symbiotic lifestyle occurred in shrimps. Therefore, the evolution of SH in shrimps cannot be explained by the TGh/HCh; reduced probability of encountering mating partners in an ancestral species due to its association with a sessile host did not favor SH in the Lysmatidae. It is proposed that two conditions acting together in the past; low male mating opportunities and brooding constraints, might have favored SH in the ancestral Lysmatidae+Barbouridae. Additional studies on the life history and phylogenetics of broken-back shrimps are needed to understand the evolution of SH in the ecologically diverse Caridea.

Simultaneously hermaphroditic shrimp use lipophilic cuticular hydrocarbons as contact sex pheromones

Successful mating is essentially a consequence of making the right choices at the correct time. Animals use specific strategies to gain information about a potential mate, which is then applied to decision-making processes. Amongst the many informative signals, odor cues such as sex pheromones play important ecological roles in coordinating mating behavior, enabling mate and kin recognition, qualifying mate choice, and preventing gene exchange among individuals from different populations and species. Despite overwhelming behavioral evidence, the chemical identity of most cues used in aquatic organisms remains unknown and their impact and omnipresence have not been fully recognized. In many crustaceans, including lobsters and shrimps, reproduction happens through a cascade of events ranging from initial attraction to formation of a mating pair eventually leading to mating. We examined the hypothesis that contact pheromones on the female body surface of the hermaphroditic shrimp Lysmata boggessi are of lipophilic nature, and resemble insect cuticular hydrocarbon contact cues. Via chemical analyses and behavioural assays, we show that newly molted euhermaphrodite-phase shrimp contain a bouquet of odor compounds. Of these, (Z)-9-octadecenamide is the key odor with hexadecanamide and methyl linoleate enhancing the bioactivity of the pheromone blend. Our results show that in aquatic systems lipophilic, cuticular hydrocarbon contact sex pheromones exist; this raises questions on how hydrocarbon contact signals evolved and how widespread these are in the marine environment.

The symbiotic lifestyle and its evolutionary consequences: social monogamy and sex allocation in the hermaphroditic shrimp Lysmata pederseni

Sex allocation theory predicts female-biased sex allocation for simultaneous hermaphrodites with a monogamous mating system. Mating systems theory predicts that monogamy is advantageous in environments where refuges are discrete, scarce, relatively small, and when predation risk is high outside of these refuges. These predictions were tested with the Caribbean shrimp Lysmata pederseni, a simultaneous hermaphrodite which has an early male phase and lives inside tubes of the sponge Callyspongia vaginalis. This host sponge is a scarce resource that, together with the high predation risk typical of tropical environments, should favor monogamy in the shrimp. Field observations demonstrated that shrimps were frequently encountered as pairs within these tube sponges. Pairs were equally likely to comprise two hermaphrodites or one hermaphrodite and one male. Several of these pairs were observed for long periods of time in the field. Experiments demonstrated that hermaphrodites tolerated other hermaphrodites but not males in their host sponge. These results suggest that pairs of hermaphroditic L. pederseni are socially monogamous; they share the same host individual and might reproduce exclusively with their host partners for long periods of time. Nevertheless, males appeared less likely to establish long-term associations with hermaphrodites as indicated by the rate of their disappearance from their hosts (greater than that of hermaphrodites). Sex allocation was female biased in monogamous hermaphrodites. On average, hermaphrodites invested 34 times more to female than to male reproductive structures. Monogamy and female-biased sex allocation seem to be evolutionary consequences of adopting a symbiotic lifestyle in simultaneous hermaphrodites.

Tests of sex allocation theory in simultaneously hermaphroditic animals

Sex allocation is a crucial life-history parameter in all sexual organisms. Over the last decades a body of evolutionary theory, sex allocation theory, was developed, which has yielded capital insight into the evolution of optimal sex allocation patterns and adaptive evolution in general. Most empirical work, however, has focused on species with separate sexes. Here I review sex allocation theory for simultaneous hermaphrodites and summarize over 50 empirical studies, which have aimed at evaluating this theory in a diversity of simultaneous hermaphrodites spanning nine animal phyla. These studies have yielded considerable qualitative support for several predictions of sex allocation theory, such as a female-biased sex allocation when the number of mates is limited, and a shift toward a more male-biased sex allocation with increasing numbers of mates. In contrast, many fundamental assumptions, such as the trade-off between male and female allocation, and numerous predictions, such as brooding limiting the returns from female allocation, are still poorly supported. Measuring sex allocation in simultaneously hermaphroditic animals remains experimentally demanding, which renders evaluation of more quantitative predictions a challenging task. I identify the main questions that need to be addressed and point to promising avenues for future research.

Sex allocation in a simultaneously hermaphroditic marine shrimp

Two fundamental questions dealing with simultaneous hermaphrodites are how resources are optimally allocated to the male and female function and what conditions determine shifts in optimal sex allocation with age or size. In this study, I explored multiple factors that theoretically affect fitness gain curves (that depict the relationship between sex-specific investment and fitness gains) to predict and test the overall and size-dependent sex allocation in a simultaneously hermaphroditic brooding shrimp with an early male phase. In Lysmata wurdemanni, sperm competition is absent as hermaphrodites reproducing in the female role invariably mated only once with a single other shrimp. Shrimps acting as females preferred small over large shrimps as male mating partners, male mating ability was greater for small compared to large hermaphrodites, and adolescent males were predominant in the population during the breeding season. In addition, brooding constraints were not severe and varied linearly with body size whereas the ability to acquire resources increased markedly with body size. Using sex allocation theory as a framework, the findings above permitted to infer the shape of the male and female fitness gain curves for the hermaphrodites. The absence of sperm competition and the almost unconstrained brooding capacity imply that both curves saturate, however the male curve levels off much more quickly than the female curve with increasing level of investment. In turn, the predominance of adolescent males in the population implies that the absolute gain of the female curve is greater than that of the male curve. Last, the size-dependent female preference and male mating ability of hermaphrodites determines that the absolute gain of the male curve is greater for small than for large hermaphrodites. Taking into consideration the inferred shape of the fitness gain curves, two predictions with respect to the optimal sex allocation were formulated. First, overall sex allocation should be female biased; it permits hermaphrodites to profit from the female function that provides a greater fitness return than the male function. Second, sex allocation should be size-dependent with smaller hermaphrodites allocating more than proportionally resources to male reproduction than larger ones. This size-dependent sex allocation permits hermaphrodites to profit from male mating opportunities that are the greatest at small body sizes. Size-dependent sex allocation is also expected because the male fitness gain curve decelerates more quickly than the female gain curve and experiments indicated that resources are greater for large than small hermaphrodites. These two predictions were tested when determining the sex allocation of hermaphrodites by dissecting their gonad and quantifying ovaries versus testes mass. Supporting the predictions above, hermaphrodites allocated, on average, 118 times more to the female than to the male gonad and the proportion of resources devoted to male function was higher in small than in large hermaphrodites. A trade-off between male and female allocation is assumed by theory but no negative correlation between male and female reproductive investment was observed. In L. wurdemanni, the relationship between sex-specific investment and fitness changes during ontogeny in a way that is consistent with an adjustment of sex allocation to improve size-specific reproductive success.