Allometry In Males Research Articles

Static allometries do not reflect evolutionary allometry in exaggerated weaponry of male New Zealand sheetweb spiders (Cambridgea spp.).

Across the animal kingdom, exaggerated weaponry is frequently used by one sex to contest access for potential mates. Within species, if disproportionate investment in weaponry confers an advantage to larger individuals, this may result in positive static allometry. It is predicted that the same selective pressures may also lead to positive evolutionary allometry, where larger species bear disproportionately large weapons on average, compared with smaller species. Furthermore, in species with stronger sexual selection, the static allometries of those weapons are expected to steepen. All adult males across the New Zealand sheetweb spider genus Cambridgea bear exaggerated chelicerae, which are used to compete for control of females' webs. Here, we characterize the distribution of chelicera lengths within each sex of 12 Cambridgea species to show that chelicerae almost always exhibit positive static allometry in males while female chelicera lengths are consistently isometric. We use comparative phylogenetic methods to demonstrate that the slopes of static allometries steepen in males of larger species but that the ratio of average chelicera length to cephalothorax width is tightly conserved across taxa, leading to an isometric evolutionary allometry. While static allometries indeed steepen in larger species, possibly due to stronger sexual selection, this conservation of relative trait size suggests that chelicera length is subject to other stabilizing selective pressures. Changes to species body plans might be constrained, while still allowing for disproportionate investment in weapon traits in the upper range of intraspecific body sizes.

Monthly variations in growth pattern and condition factor of Periophthalmodon septemradiatus (Gobiiformes: Periophthalminae) living along the Bassac River in Viet Nam.

Periophthalmodon septemradiatus (Hamilton, 1822) is a unique mudskipper living in the mudbanks from estuarine to riverine regions in the Vietnamese Mekong Delta (VMD), but there is no data on its growth patterns and condition factors that are helpful for fish resource assessment and adaptation understanding. This study was conducted at five sites, from the lower (Soc Trang province) to middle (Can Tho city) and upper (An Giang province) reaches of Bassac River in VMD, to provide knowledge on growth patterns and condition factors to this mudskippers. Fish samples were caught using traps and hands for 24 months, from July 2017 to June 2019, at these five sites. The total length and weight of 3,417 individuals (1,340 females and 2,077 males) varied by sex, season and site (p < 0.001 for all cases). This species exhibited a positive allometric type as the slope (b = 3.06 ± 0.01) of the length-weight relationship (LWR) was >3 (p < 0.001) for both sexes. The growth pattern changed with sex as mudskipper showed isometry in females but positive allometry in males. Maturity also affected fish growth type since it shifted from negative allometry in immature groups to positive allometry in mature groups. The growth pattern of fish changed from isometry in the dry season to positive allometry in the wet season. The mudskipper showed isometry at the lower reaches in Soc Trang but positive allometry at the middle and upper reaches in Can Tho and An Giang. The condition factor (CF) did not change according to sex, size and season, but with month and site variables. The present environmental condition in these studied sites tended to be not good enough for this fish as CF (0.95 ± 0.01) was less than 1 (p < 0.001). The findings provided basic information on the growth and adaptation of P. septemradiatus being helpful in fish adaptation understanding and resource conservation in VMD.

Exaggerated sexually selected weapons maintained with disproportionately low metabolic costs in a single species with extreme size variation

Abstract Larger individuals typically have lower mass‐specific metabolic rates compared to small ones (hypometric scaling). This trend is most evident across species where body size differences can be extreme. Yet, within‐species studies are critical to decipher the morphological and physiological mechanisms responsible. However, in most species, the relatively small range in body and appendage size of the same life stage often precludes such intraspecific comparisons. Sexually selected weapons are among the most exaggerated traits in nature; these traits can account for a large portion of body mass and contribute to whole‐body energetic maintenance costs. Often larger individuals possess disproportionally larger weapons relative to their body size, yet little is known about how large individuals meet the predicted increased energetic demands of maintaining disproportionately large weapons. New Zealand giraffe weevils, Lasiorhynchus barbicornis (Coleoptera: Brentidae) exhibit an extreme 30‐fold range in male body mass as well as hypermetric scaling of sexually selected rostra used as weapons in male–male competition. We compare intra‐ and interspecific resting metabolic rates by compiling measurements across 26 arthropod species whose size ranges overlap with L. barbicornis. The scaling of metabolic rate across this pooled interspecific sample was not significantly different from the intraspecific scaling of metabolic rate we found in L. barbicornis (slope of log–log relationship with body mass = 0.67). Male and female L. barbicornis had a similar scaling of metabolic rate with body size, despite differing substantially in the scaling of rostra, legs and antennae. However, large structures that scaled with positive allometry in males (rostra and legs) were increasingly composed of cuticle in larger individuals. The largest males invest ~60% less metabolically active tissue into rostra (weapons) compared to the smallest males. Our findings reveal hypometric scaling relationships of inter‐ and intraspecific metabolic rates across a shared size range in a diverse group of arthropods. Our intraspecific study uncovers a tremendous difference in weapon architecture as individuals scale up in size; we find that large males bear disproportionally larger weapons at a lower relative metabolic cost. This cost‐saving mechanism may play a key role in shaping the hypermetric scaling of sexually selected traits. A free Plain Language Summary can be found within the Supporting Information of this article.

New biological data for Salaria fluviatilis (Asso, 1801) (Blenniidae) from North of Algeria

The family Blenniidae inhabiting freshwater systems has been scarcely studied, particularly in North Africa. The present study focused on the biology of Salaria fluviatilis (Asso, 1801) from Kabylie region (northern Algeria). A total of 198 specimens were sampled from May 2015 to May 2016. The total length of this fish varied between 4.9 and 12.7 cm and its total weight varied between 1.2 and 23.7 g. The age of this fish species varied between 1 and 3 years. The sex ratio of S. fluviatilis was in balance (SR = 0.98). The results of our investigation of length-weight relationship show highly significant correlation (p &gt; 0.001); the correlation coefficient r varied between 0.966 and 0.985. The specimens of this fish species exhibited positive allometry in males (b = 3.195) and negative allometry in females (b = 2.782). Growth parameters were higher in males than in females, except for the growth rate (K) where the values were close. The growth performance index (Φ) was similar in both sexes, which confirms the same growth potential for males and females. In Kabylie region, the breeding period of S. fluviatilis occurred between April and July. Our results of analysis of the hepatosomatic index (HSI) and Fulton’s K show that liver energy reserves are not devoted for reproduction; however, muscle reserves seem to support this physiological activity. In perspective, more biological studies must be carried out on this rare and poorly studied fish.

The nature of allometry in an exaggerated trait: The postocular flange in Platyneuromus Weele (Insecta: Megaloptera).

The origin and function of exaggerated traits exhibited by a great number of species with sexual dimorphism remain largely unexplored. The usual model considered as the evolutionary mechanism for the development of these structures is sexual selection. The nature of growth of the postocular flange (POF) in three species of the dobsonfly genus Platyneuromus (Megaloptera, Corydalidae, Corydalinae) is analyzed to explore sexual size dimorphism and allometric scaling. Results involve positive allometry of POF in males of two species, and negative allometry in males of one species, in general with a female-biased sexual dimorphism. We suggest an ancestral condition of dual incipient ornamentation in Platyneuromus, with a subsequent departure of size and shape of POF in males, triggered by sexual selection. Different sexual selection intensities may explain the parallel or divergent growth of POF within the scheme of dual ornamentation. Empirical behavioral data as well as a phylogenetic framework are necessary to clarify possible causes of phenotypic development, time of origin, and evolution of the POF.

Ontogenetic changes in morphometry of the spiny lobster, Panulirus homarus homarus (Linnaeus, 1758) from southern Indian coast

The morphometric relationships, carapace length vs total length (CL vs TL), carapace length vs weight (CL vs W), total length vs weight (TL vs W), carapace length vs carapace width (CL vs CW), carapace length vs length of third walking leg (CL vs III WL) and carapace length vs width of second abdomen (CL vs II ASW) were estimated for the Scalloped spiny lobster Panulirus homarus homarus fished from east and west coast of Tamilnadu. A total of 462 males and 350 females were measured for the study. Carapace length of lobsters ranged from 20.4 mm to 101.0 mm CL. The power function y=axb was applied in all the relationships. The first derivative dY/dCL-abCLb-1 where Y is TL was used to study the growth tendency of TL in relation to CL. A negative allometry, b<1 was found for males which was also reflected in their decreasing growth rate of TL in relation to CL. The CL vs W relationship showed a general negative allometry, b<3 in both sexes. Size at the onset of sexual maturity in male was determined based on allometric growth of third pereopod. Slopes of regression of log transformed data showed positive allometric growth of third pereopod for males (b=1.17) compared to negative allometry in females (b=0.97). Females had (b=1.04) positive allometry in second abdomen width compared to negative allometry in males (b=0.934). However, their size at maturity could not be precisely predicted as the b value of the two regression lines of immature (1.09) and mature females (1.01) did not indicate any significant deviation from 1. The size at maturity of males (63.0 mm) was determined where the two regression lines of mature and immature specimens crossed. The results obtained in the present study support the predictions of resource allocation theory in crustaceans and demonstrate marked shifts in resource allocation through ontogeny in both sexes.

Temporal variation in body size and weapon allometry in the New Zealand giraffe weevil

1. Body size and exaggerated traits can show high phenotypic plasticity in response to environmental variation. Trait size can vary among generations but also fluctuate within a breeding season in response to resource availability.2. This study documents patterns of temporal variation in body and weapon size, and in weapon allometry over 3 years for a wild population of New Zealand giraffe weevils [Lasiorhynchus barbicornis (Fabricius)], the males of which display an extremely elongated rostrum used as a weapon during contests for females.3. It was predicted that body size and rostrum allometry would decrease during a breeding season, but in spite of significant annual and seasonal variation there was little evidence to support these predictions. Weapon allometry in males was more variable between years and over the breeding season than females, suggesting that male rostrum size may be more susceptible to environmental change than female rostrum size.

Biological traits of Cyzicus grubei (Crustacea, Spinicaudata, Cyzicidae) in south-western Iberian Peninsula

2 In this study, characteristics of the biology of the spinicaudatan Cyzicus grubei were determined from a population in a temporary pond in the south-western Iberian Peninsula from January 2011 to July 2011. The results indicated the existence of a single cohort for the duration of the flooding period. Non-ovigerous females and males were present in the pond throughout the study period. However, ovigerous females were present from mid-April to early July. Males had valves significantly larger and higher than females. The relationship between the valve length and the valve height showed positive allometry in males and negative allometry in females and the whole population (mature and immature individuals). The smallest mature male was 8.13 mm in valve length, whereas the smallest ovigerous female was 8.21 mm in valve length. Females outnumbered males in winter and early spring, and males were more abundant in late spring and early summer. The mean number of eggs on ovigerous females was 574.74±245.29, ranging from 241 to 1068 eggs/female. The number of eggs was linearly correlated with the valve length of the female.

Sexual Size and Shape Dimorphism in an Agamid Lizard Sitana Ponticeriana (Sauria: Agamidae)

The fan-throated lizard, Sitana ponticeriana, is a medium-sized ground living agamid distributed throughout India, preferably in dry and more or less open country habitats. A total of each of 80 males and females were collected from Konark-Balukhand Wildlife Sanctuary area and subjected to various morphometric measurements to investigate the sexual size and shape dimorphism of the species. Sexually immature individuals were much less dimorphic than the adults. All body parts of males were larger than those of females. Both the sexes also differ in allometry of all head characters with body size, with males showing a disproportionate increase of head size and dimensions and head shape changed with increasing head size. Similarly, the foot (front and hind) of both sexes differs in length. The hind foot was double in length than that of the front. Males had larger hind foot (1.68cm) than females (1.56cm). A strong positive relatioship (r =0.8) was observed between SVL and HFL in females. Analysis of variance at p<0.05 level of significance was done foe several variables. The head was twice longer than width. The HL and HW showed positive allometry in males and females.

Testing the performance of a fragment of the COI gene to identify western Palaearctic stag beetle species(Coleoptera, Lucanidae).

The taxonomy of stag beetles (Coleoptera: Lucanidae) remains challenging, mainly due to the sexual dimorphism and the strong allometry in males. Such conjecture confounds taxonomic based conservation efforts that are urgently needed due to numerous threats to stag beetle biodiversity. Molecular tools could help solve the problem of identification of the different recognized taxa in the “Lucanus cervus complex” and in some related Palaearctic species. We investigated the potential use of a 670 bp region at the 3’ end of the mitochondrial cytochrome c oxidase subunit I gene (COI) for barcoding purposes (different from the standard COI barcoding region). Well resolved species and subspecies were L. tetraodon, L. cervusakbesianus, L. c. laticornis, as well as the two eastern Asian outgroup taxa L. formosanus and L. hermani. Conversely, certain taxa could not be distinguished from each other based on K2P-distances and tree topologies: L. c. fabiani / L. (P.) barbarossa, L. c. judaicus / an unknown Lucanus species, L. c. cervus / L. c. turcicus / L. c. pentaphyllus / L. (P.) macrophyllus / L. ibericus. The relative roles of phenotypic plasticity, recurrent hybridisation and incomplete lineage sorting underlying taxonomic and phylogenetic discordances are discussed.

Yield characteristics and morphometric relationships of giant freshwater prawn, Macrobrachium rosenbergii (de Man, 1879)

Meat yield of 233 individuals of giant freshwater prawn, Macrobrachium rosenbergii (de Man, 1879), and their morphometric relationships were examined. Morphometric parameters of different length and weight measurements were recorded for both the sexes separately and subjected to analysis. The meat yield of prawns was size and sex dependent with females showing significantly greater (P < 0.05) tail meat yield than males. The tail meat as a body weight percentage ranges from 26.12 to 40.75 and 32.97 to 42.11 in males and females, respectively. Much of the differences in yield are related to the size and weight of the head and claws. The head weight of males was observed to be higher compared to females in all the size classes. Regression slopes of total length versus body weight and total length versus head weight were significantly higher than critical isometric value indicating high positive allometric growth in both the sexes, with males showing significantly higher (P < 0.05) slope than females. The relationship parameters of total length versus tail weight and total length versus tail meat showed a positive allometric growth in females, whereas it is isometry and negative allometry in males. The morphometric relationship of tail length versus all weight measurements showed positive allometry and negative allometry in females and males, respectively.

Sexual selection on forelimb muscles of western grey kangaroos (Skippy was clearly a female)

Studies of sexual selection have tended to concentrate on obvious morphological dimorphisms such as crests, horns, antlers, and other physical displays or weapons; however, traits that show no obvious sexual dimorphism may nevertheless still be under sexual selection. Sexual selection theory generally predicts positive allometry for sexually selected traits. When fighting, male kangaroos use their forelimbs to clasp and hold their opponent and, standing on their tail, bring up their hind legs to kick their opponent. This action requires substantial strength and balance. We examined allometry of forelimb musculature in male and female western grey kangaroos (Macropus fuliginosus) to determine whether selection through male-male competition is associated with sex differences in muscle development. Forelimbs of males are more exaggerated than in females, with relatively greater muscle mass in males than the equivalent muscles in females. Furthermore, while muscles generally showed isometric growth in female forelimbs, every muscle demonstrated positive allometry in males. The significant positive allometry in male forelimb musculature, particularly those muscles most likely involved in male-male combat (a group of muscles involved in grasping: shoulder adduction, elbow flexion; and pulling: arm retraction, elbow flexion), clearly suggests that this musculature is subject to sexual selection. In addition to contributing to locomotion, the forelimbs of male kangaroos can also act as a signal, a weapon, and help in clasping, features that would contribute towards their importance as a sexually selected trait. Males would therefore benefit from well-developed musculature of the arms and upper body during competition for mates.

Static allometry and sexual size dimorphism in Centruroides margaritatus (Scorpiones: Buthidae)

Animal body traits are scaled relative to overall body size depending on the evolutionary context. Most naturally selected traits are scaled approximately isometrically (constitute a constant proportion of the body size at different body sizes), whereas those under sexual selection tend to present positive static allometry (be proportionally larger in larger individuals). However, there are body traits that might be influenced by both natural and sexual selection. We studied the courtship behavior of the scorpion Centruroides margaritatus (Gervais 1841) and analyzed the static allometry of several body traits. We hypothesized that those traits that were actively used in courtship and seemed to be sexually dimorphic could be under sexual selection. The main sexually dimorphic traits were body size (female larger) and metasoma length (male longer). Although metasoma length of males had a steeper allometric slope (larger males had longer metasoma) than that of females, the slopes did not differ significantly. All body traits measured showed isometry with body size, except that the pecten presented negative allometry in males. Thus the length of the metasoma of males, thought to be influenced by sexual rather than natural selection, did not present positive allometry as expected. Males used the metasoma actively while courting females.

Sexual and mating system in a caridean shrimp symbiotic with the winged pearl oyster in the Coral Triangle

AbstractFor symbiotic crustaceans, theory predicts that monogamy is adaptive when species inhabit scarce, relatively small and morphologically simple hosts in tropical environments where predation risk away from hosts is high. We tested this prediction in the shrimp Paranchistus pycnodontae, which inhabits the mantle cavity of the winged pearl oyster Pteria penguin in the Coral Triangle. In various symbiotic crustaceans, males are smaller than females, and this sexual dimorphism has been used as evidence of sex change (protandry) in these organisms. Preliminary observations in Pa. pycnodontae suggested that males were smaller than females. Thus, we first investigated the sexual system of Pa. pycnodontae to determine if the species was protandric. Morphological identification and size frequency distributions indicated that the population comprised small males, small immature females and larger mature females, which was confirmed by dissections. No transitional individuals were found. Thus, Pa. pycnodontae is a gonochoric species with reverse sexual dimorphism. Pa. pycnodontae inhabit as heterosexual pairs in the mantle cavity of hosts more frequently than is expected by chance alone. Pairing was size assortative; both carapace length and propodus length of the major cheliped were positively correlated between males and females forming pairs. Males occur with females in the same host, independent of the female gravid condition or of the stage of development of the brooded eggs. Lastly, the major cheliped did not exhibit positive allometry in males. All the available information suggests that Pa. pycnodontae has adopted a socially monogamous mating system with males and females forming exclusive pairs from other adults. Symbiotic shrimps can be used as a model system to understand behavioural diversity within socially monogamous marine invertebrates.

Weaponry, color, and contest success in the jumping spider Lyssomanes viridis

Weaponry and color badges are commonly theorized to function as visual signals of aggressiveness or fighting ability. However, few studies have supported a signaling function of weaponry, and the role of color in invertebrate competitive interactions remains virtually unexplored. Jumping spiders (Salticidae) make excellent invertebrate models for studying weaponry and color because males of many species are colorful and possess exaggerated chelicerae, which are used as weapons in escalated contests. To determine whether color or weaponry might function as visual signals in male-male competitions, we investigated relationships between contest success, cheliceral length, and red coloration in Lyssomanes viridis. Males having longer chelicerae than their opponents were significantly more likely to win (p=0.0008). Males who won, despite being smaller than their opponents, had significantly less red chelicerae than their opponents (p=0.01). Male and female cheliceral length, as well as foreleg length, correlated tightly with body size. Cheliceral and foreleg length showed significantly stronger positive allometry in males than in females. We conclude that male chelicerae and forelegs are under strong positive selection for their use in physical fights and/or as visual signals of fighting ability.

Refuge size, group living and symbiosis: testing the “resource economic monopolization” hypothesis with the shrimp Betaeus lilianae and description of its partnership with the crab Platyxanthus crenulatus

Theory predicts that refuge size is most relevant in driving the population distribution of marine organisms. Relatively small refuges are expected to harbor single or pairs of conspecifics because defense against intruders is energetically inexpensive. Relatively large shelters should harbor aggregations because guarding behaviors turn energetically expensive. Here, we used the intertidal shrimp Betaeus lilianae to test the hypothesis that species inhabiting large refuges live in aggregations and not solitarily or in pairs. Also, we provided information on the lifestyle of this species, specifically regarding a newly discovered partnership with the crab Platyxanthus crenulatus. In agreement with theoretical expectations, Betaeus lilianae was found living in aggregations in rock pools, characterized by their large size. Shrimp aggregations featured female-biased sex ratios more frequently than expected by chance alone and had no particular complex social structure. There was no effect of pool size and shrimp group size on sex ratio and no significant relationship between the difference in body size of the largest and second largest male and shrimp group size was observed. Relative growth analyses showed that the major claw had positive allometry in males and females but relative claw growth was greater in males. The information above permitted rejecting several alternative hypotheses on the mating system of B. lilianae: it is neither socially monogamous nor features a promiscuous pure-search mating system. Additional studies are needed to reveal the mating behavior of the studied species. Field observations and laboratory experiments demonstrated that B. lilianae associate preferentially with the crab P. crenulatus.

Biology of the Deep-Water Shrimp Heterocarpus Ensifer (Caridea: Pandalidae) Off the Canary, Madeira and the Azores Islands (Northeastern Atlantic)

An analysis of size and reproduction of Heterocarpus ensifer (Caridea: Pandalidae) off the Macaronesian archipelagos (Canary, Madeira, and the Azores islands, northeastern Atlantic) is done. The size range of samples was similar among the archipelagos studied, around |$9-39{\text{ mm}}$| carapace length (CL). The carapace length/wet weight relationship showed a negative allometry in males and varied in females. One-way analysis of regression slopes |$(b)$| among archipelagos pointed out significant differences in all groups with the exception of males. Growth parameters indicated that males are smaller in size, but grow faster than females. Growth performance |$(\Phi{\text{’}})$| estimates varied between 2.69 and 2.81, pointing out a very similar growth pattern among regions. Ovigerous females were present in all months sampled with a mass spawning period in spring and summer, which are the seasons when the water is warmer. Size at maturity was estimated in |$22.4{\text{ mm CL}}$| for the Canary Islands and |$25.7{\text{ mm CL}}$| for Madeira. In general, the life history of this deep-water species is similar in the three archipelagos.

Sexually dimorphic legs in a neotropical harvestman (Arachnida, Opiliones): Ornament or weapon?

The evolution of sexually dimorphic traits has been the focus of much theoretical work, but empirical approaches to this topic have not been equally prolific. Males of the neotropical family Gonyleptidae usually present a strong fourth pair of legs armed with spines, but their functional significance is unknown. We investigated the putative functions of the leg armature in the harvestman Neosadocus maximus. Being a non-visual species, the spines on male legs can only be perceived by females through physical contact. Thus, we could expect females to touch the armature on the legs of their mates if they were to evaluate it. However, we found no support for this hypothesis. We did show that (1) leg armature is used as a weapon in contests between males and (2) spines and associated sensilla are sexually dimorphic structures involved in "nipping behavior", during which a winner emerged in most fights. Finally, we demonstrate that five body structures directly involved in male-male fights show positive allometry in males, presenting slopes higher than 1, whereas the same structures show either no or negative allometry in the case of females. In conclusion, leg armature in male harvestmen is clearly used as a device in intrasexual contests.

Sexual differences in size and shape of the Mosor rock lizard [Dinarolacerta mosorensis (Kolombatovic, 1886)] (squamata: lacertidae): A case study of the Lovcen mountain population (Montenegro)

Sexual differences in size and shape of the Mosor rock lizard, Dinarolacerta mosorensis (Kolombatovic, 1886), from Lovcen Mountain (Montenegro) were examined on the basis of the intersex variation pattern of nine morphometric, eight pholidotic, and four qualitative traits. Sexual dimorphism was apparent for all morphometric characters except snout-vent length, while scalation and dorsal pattern exhibited small differences between sexes. The value of the sexual size difference (SSD) index based on snout-vent length was 1.028. The sex-specific allometric slopes for head dimensions and interlimb distance significantly diverged. Head dimensions, especially head height, showed strong positive allometry in males, while interlimb distance was the only character which showed positive allometry in females. Generally, males had significantly greater body size than females. This was true of all body measurements except interlimb distance. The influence of sexual and natural selection on the examined traits is discussed.

Social monogamy in the shrimp Pontonia margarita, a symbiont of Pinctada mazatlanica, off the Pacific coast of Panama

A previous study predicted the evolution of monogamy in symbiotic crustaceans inhabiting scarce, relatively small hosts in tropical environments where predation risk away from hosts is high. This prediction was tested in the shrimp Pontonia margarita, which inhabits the pearl oyster Pinctada mazatlanica. A total of 68 oysters were collected from the intertidal and shallow subtidal at two islands (Islas Secas [N 27° 55′, W 82° 03′] and Isla de La Coiba [N 27° 50′, W 97° 03′]) off the eastern tropical Pacific coast on 15 and 17 March 2007, respectively. The population structure, distribution, male-female association pattern, and relative growth of the major claw and pleura of the second abdominal segment of each shrimp retrieved were examined. Shrimps were found as heterosexual pairs in the mantle cavity of hosts more frequently than would be expected by chance alone. Males occurred with females in the same host, independent of the reproductive condition of the female or the stage of development of brooded embryos. This observation, and strong correlations between the host and shrimp body size in both males and females suggest a long-term association between males and females in each host. Sexual dimorphism in body size was minor, with males being just slightly smaller than females. In agreement with predictions for monogamous species, the major claw of males did not display positive allometry, which has been generally reported for polygamous shrimps. In turn, the pleura of the second abdomen presented negative allometry in males but positive allometry in females. All available information suggests that Pontonia margarita has a socially monogamous mating system with males and females forming exclusive pairs in their hosts.