Lysmata pederseni, a protandric simultaneously hermaphroditic shrimp that inhabits the tube sponge Callyspongia vaginalis, is monogamous in the central and southeastern Caribbean Sea. We tested the null hypothesis of monogamy in a northern Caribbean population. In the Florida Keys, shrimps did not inhabit host individuals in pairs with a frequency greater than expected by chance alone. Hermaphrodites inhabited sponges solitarily and often brooded embryos. Hermaphrodites do not store sperm and need to be inseminated shortly after molting to fertilize a new batch of eggs. Thus, males and/or other hermaphrodites are likely switching among host individuals in search of sexual partners. Field experiments demonstrated low shrimp host fidelity. Host residence time was *2 times shorter for males than for hermaphrodites. We inferred a polygynandrous mating system from L. pederseni from the Florida Keys, with male-role and young hermaphrodites often moving among sponges in search of older, more sedentary, female-role hermaphrodites. We expected shrimps to use water-borne chemical cues originating from conspecifics or sponges to locate sexual partners. Experiments demonstrated that shrimps were attracted to waterborne cues originating from sponges but not conspecifics. We have described the mating system of a reef-associated shrimp in a fast-pace shifting seascape increasingly dominated by sponges and vanishing stony corals. In the central and southeastern Caribbean Sea, with greater coral cover and lower sponge abundance than in the Florida Keys, the same species is monogamous. Whether or not similar shifts in the social organization of other coral reef-dwelling marine organisms are occurring due to contemporary changes in seascapes is a relevant topic that deserves further attention.

Theory predicts that monogamy is adaptive in resource-specialist symbiotic crustaceans inhabiting relatively small and morphologically simple hosts in tropical environments where predation risk away from hosts is high. We tested this prediction in Pontonia manningi, a hyper-symbiotic shrimp that dwells in the mantle cavity of the Atlantic winged oyster Pteria colymbus that, in turn, infects gorgonians from the genus Pseudopterogorgia in the Caribbean Sea. In agreement with theory, P. manningi were found dwelling as heterosexual pairs in oysters more frequently than expected by chance alone. Males and females also inhabited the same host individual independent of the female gravid condition or of the developmental stage of brooded embryos. While the observations above argue in favor of monogamy in P. manningi, there is evidence to suggest that males of the studied species are moderately promiscuous. That females found living solitary in oysters most often brooded embryos, and that males allocated more to weaponry (major claw size) than females at any given size suggest that males might be roaming among host individuals in search of and, fighting for, receptive females. All available information depicts a rather complex mating system in P. manningi: primarily monogamous but with moderately promiscuous males.

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Historically, Lysmata vittata has been reported with a near global non-polar distribution. Early studies reported a wide morphological variation in this species, which served as a basis for further synonymization of at least four species. Herein, we investigated the species diversity within L. vittata complex and tested whether L. rauli and L. durbanensis are valid species instead of junior synonyms of L. vittata. Our integrated morphological and molecular data strongly supports the validity of at least six taxonomic entities within the broader L. vittata complex, including L. rauli and L. durbanensis and three undescribed species. Multivariate analyses highlighted prominent morphological differences in accessory branch structure of dorsolateral antennular flagellum, number of carpal and meral segments of the second pereopod, and color pattern which segregated shrimps into distinct morpho-groups. Phylogenetic analyses supported morphological groupings and recovered five widely divergent lineages, which corresponded to the morphological groupings: L. vittata sensu stricto; L. rauli sensu stricto; L. sp. CHINA; L. sp. AUS1; and L. sp. AUS2. Therefore, we formally resurrect L. rauli to valid species status and posit it is native to the subtropical and tropical Indo West-Pacific. Although data were limited, we also formally resurrect L. durbanensis to valid species status from southern Africa. Our results imply L. vittata and L. rauli are exotic species in the western Atlantic, New Zealand, and the Mediterranean. This study provides a solid framework to continue untangling the historic L. vittata species complex, which is likely to include additional species to the ones included in the present study.

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Currently, 14 of the 50 species of Lysmata are known to possess a long accessory branch with more than two articles. Historically, Lysmata intermedia and Lysmata moorei were the only two ‘long-branch’ species inhabiting the southwestern Atlantic. Here we describe, based on morphological, molecular and colour pattern data, a new species of Lysmata possessing a long accessory branch from Pernambuco, northeastern Brazil. Our maximum-likelihood analysis recovered Lysmata elisa sp. n. as a sister species to Lysmata jundalini. Both species are closely related to Lysmata holthuisi and L. intermedia. The four aforementioned species comprise the L. intermedia species complex. The new species may be morphologically distinguished from the other closely related species by different sets of characters, which include details of the dorsolateral antennular flagellum, armature of ischium of the second pair of pereiopods, intraorbital process shape and relative proportions of pereiopods. Our results reinforce the importance of refining biodiversity data through the application of integrative taxonomic approaches to expand the knowledge of local and global biodiversity. The biodiversity of Lysmata deserves special attention, as they are intensively exploited in the aquarium trade.

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Animal populations can present different mating tactics according to the number of mates males or females obtain. The limiting factors for copulation in symbiont crustaceans are the size, structural complexity, and density of refuge, in addition to predation risk outside their shelters. In this study, we aimed to investigate, comparatively, if the occupation of the same refuge leads to the same reproductive tactic in two symbiont species of alpheid shrimps. For this purpose, we described the population distribution of Alpheus estuariensis, a burrowing shrimp, and Salmoneus carvachoi, a tenant species, in an intertidal mudflat in Pernambuco, northeastern Brazil. Alpheus estuariensis experiences sexual dimorphism relative to body and cheliped size, where males present larger body sizes and allocate more resources to cheliped growth than females. However, both males and females presented positive allometric growth in chelipeds. In S. carvachoi, male‐phase individuals reach smaller body sizes than hermaphrodites. Male‐phase shrimps experience positive allometry in cheliped size, while hermaphrodites do not. Both species were found inhabiting burrows as sexual pairs more frequently than expected by chance. Paired females of A. estuariensis were found carrying embryos more frequently than solitary ones. We did not find a statistical difference between the frequency of single and paired ovigerous hermaphrodites in S. carvachoi. Our results indicate that A. estuariensis is social monogamous, while S. carvachoi is promiscuous. We argue that different biological and ecological aspects of these species could explain the distinct tactics adopted by them. Alpheus estuariensis digs its own refuge and is a much larger species, bearing a more robust cheliped than S. carvachoi. Thus, we argue that the burrowing species (A. estuariensis) should monopolize and defend their refuge, and S. carvachoi only takes advantage of this resource.