Age,growth and maturity of tub gurnard (Chelidonichthys lucerna Linnaeus 1758; Triglidae) in the inshore coastal waters of Northwest Wales,UK

The tub gurnard Chelidonichthys lucerna has been identified by ICES as a potential commercial species in the northeast Atlantic with recommendations made to monitor landings and discards and to derive information on population biology for stock assessment purposes, however, data are lacking for the species in the northeast Atlantic. Therefore, aims of this study were to provide data on the size/age‐structure and patterns of growth, maturity and mortality of C. lucerna in Northwest Wales, UK, and in doing so to provide data on the biological characteristics of the most northerly population studied to date for comparison with the existing data for southerly Mediterranean populations. Data on the age, growth and maturity of C. lucerna were collected by otter trawling (73 mm cod‐end stretched mesh size) in the coastal waters of Northwest Wales, UK in October (2000–2011, excluding 2006). Total length (TL) of fish sampled ranged between 10.5–41.0 cm (males) and 10.4–57.5 cm (females). The majority of the female fish were between 20–30 cm TL (60.2%) and the majority of the male fish between 20–30 cm TL (58.3%) respectively. TL/weight (W) relations for male and female fish were similar and the combined data was described by W = 0.0067 TL^3.10. Age of fish ranged between 1–7 years old for female fish and 1–5 years old for male fish respectively with the majority of female fish 3 years old (40%) and the majority of male fish 3 years old (37%). The age structures of female and male tub gurnards were not significantly different with the older age classes consisting predominantly of female fish. Both males and females exhibited similar asymptotic growth patterns and the combined von Bertalanffy growth function was TL_t = 51.6 (1 ? e ^{[?0.25(t + 0.41)]}). Instantaneous rates of total mortality were calculated as 1.04 year^?1 for males and 1.11 year^?1 for females. The size (L₅₀) and age at first maturity (A₅₀) were estimated to be 29.1 cm TL and 2.8 years for males, 27.7 cm TL and 2.7 years for females and 28.0 cm TL and 2.8 years for both sexes combined. The results of this study provide the first information on the biology and population dynamics of C. lucerna in the Irish Sea, the first data collected in the northeast Atlantic since 1985 and the most northerly population studied to date.     

Population biology of the red gurnard (Aspitrigla cuculus L.; Triglidae) in the inshore waters of Eastern Anglesey and Northwest Wales

ICES has identified red gurnard Aspitrigla cuculus (L.) as a potential commercial species and recommended that monitoring programmes should be conducted to derive information on biological parameters for stock assessment purposes. In this paper, data on the population biology of red gurnard in the coastal waters of Northwest Wales and Eastern Anglesey are presented. Total length (TL) of fish sampled ranged from 15.4 to 35.0 cm (males) and 10.5 to 43.1 cm (females), with the majority of females between 20 and 30 cm TL (70.0%) and males between 20 and 30 cm TL (71.0%). TL/weight (W) relations were similar between immature and mature individuals for both sexes and between both sexes (all maturity stages combined), producing a combined data equation W = 0.005 TL^3.19. Age of fish ranged from 1 to 7 years and 1 to 6 years, respectively, for females and males, with the majority of females age 3 (37%) and the majority of males age 2 (49%). The age structures of female and male red gurnards were significantly different, with the older age classes consisting predominantly of female fish. Both males and females exhibited similar asymptotic growth patterns; the combined von Bertalanffy growth function was . Instantaneous rates of total mortality were calculated as 1.13 year^?1 for males and 0.98 year^?1 for females. The size (L₅₀) and age at first maturity (A₅₀) were estimated to be 26.3 cm TL and 3.6 years for males, 28.1 cm TL and 3.5 years for females and 25.6 cm TL and 3.7 years for both sexes combined.     

Spawning seasonality and body sizes at sexual maturity in the bluespine unicornfish,Naso unicornis (Acanthuridae)

We herein evaluate several reproductive metrics of Hawaiian Archipelagic populations of the bluespine unicornfish (Naso unicornis), an economically and ecologically important, broadly distributed tropical Pacific reef fish, based on multi-year, fishery-dependent and fishery-independent collections. Sex-specific spawning seasonality was characterized for fish collected mostly from Oahu (Main Hawaiian Islands, MHI) using a gonadosomatic index. Histological slides preparations were used to score gonad developmental phase and to classify individuals of either sex as immature or mature. Sex-specific median body lengths at maturity (L₅₀) were estimated by logistic fits of proportion mature versus length class. Spawning was highly seasonal in Hawaii, with a single brief (May–June) peak spawning period. Proportionate gonad-to-body weight values were relatively low, averaging only about 0.1 % and 0.6 % across all months of year and 0.16 % and 1.03 % during May–June for males and females, respectively. Median lengths at sexual maturity differed between the sexes. L₅₀ values for fish collected throughout all months of year were 30.1 ± 0.5 (standard error) cm Fork Length (FL) for males and 35.5 ± 0.7 cm FL for females. Spawning seasonality and L₅₀ estimates for bluespine unicornfish in Hawaii suggest that the species spawns several months earlier in the calendar year and matures at larger body lengths in Hawaii versus Guam, the Northern Mariana Islands, and Pohnpei in the Federated States of Micronesia. Estimated lengths at sexual maturity are compared to the minimum length (14 inches or 35.6 cm FL) mandated for this species in Hawaii: median size at maturity occurs at a length appreciably less than (males) or approximately equal to (females) minimum legal size. A likely disproportionately large contribution of old females to population replenishment is discussed relative to the minimum size limit.     

Rule of age and size at maturity: individual variation in the maturation history of resident white-spotted charr

Individual growth and maturation histories, age, and size at maturity of resident white-spotted charr Salvelinus leucomaenis were examined in a tag-recapture study in a natural river over 3 years. Slow-growing fish reached sexual maturity not only at an older age, but also at a smaller size than fast-growing fish, although females had a larger threshold size at maturity than males at each age. It is suggested that these patterns result from adaptive phenotypic plasticity that depends on individual growth conditions.     

Sexual size dimorphism,growth, and maturity of the fluvial eight-barbel loach in the Kako River,Japan

The maturation and growth pattern of the fluvial eight-barbel loach Lefua sp. (Japanese name: nagare-hotoke-dojo), an endangered species, was investigated using an individual identification-recapture method from 1995 to 1998 in an upper reach of a headwater tributary of the Kako River, Hyogo Prefecture, Japan. Based on observations of the gonads through the abdominal skin, the loach was estimated to breed mostly from May to July. All the males matured by age 1⁺, and all the females matured by age 2⁺. Gamete release in all individuals of both males and females was predicted from recaptured loaches during each breeding season. The standard length of mature females was significantly larger than that of males, showing sexual size dimorphism (SSD). The maximum sizes recorded were 75.4 mm SL for females and 61.2 mm SL for males. Both males and females of immature specimens grew mainly from May to November, including the breeding season, with no significant differences in growth rates between them. After sexual maturity, both males and females grew mainly from July to October (or November), after the breeding season, and the females exhibited higher growth rates than males. Therefore, SSD of the species seems to be attributable to the different growth rates after maturity. The longevity of the loach was estimated to exceed ten years based on individual growth patterns of various sizes during the survey period. It is likely that the loach has an iteroparous life history, breeding every year, and moderate growth rates after maturity.     

Life-history strategies of Acrossocheilus fasciatus (Barbinae, Cyprinidae) in the Huishui Stream of the Qingyi watershed, China

Life-history traits of Acrossocheilus fasciatus were examined using 384 specimens collected monthly during May 2009 and April 2010 in the Huishui Stream of the Qingyi watershed, China. Using scales for age determination, female and male fish comprised five and four age groups, respectively. The monthly changes in marginal increment ratio suggested that annuli on scales were formed during March through May. Total lengths back-calculated significantly increased with age for both sexes and varied significantly between the two sexes at each age. The fact that females had larger body size and grew faster than males indicated the sexual size dimorphism for this species. Both sexes got their 50% maturity at age 3, when females and males were 105.3 and 112.1?mm total length, respectively. Based on the monthly changes in the gonado-somatic index and egg-development process, fish spawned from April through August. Absolute fecundity ranged from 295 to 3,573 eggs per fish and increased significantly with age. But relative fecundity, ranging from 11.77 to 69.96?eggs/g, was not significantly different among age groups. Compared with the life-history traits of an upstream population in the Puxi Stream (a headwater stream within this study watershed), the downstream population of A. fasciatus in the Huishui Stream (a 4th-order stream) exhibits larger body size, faster somatic growth, later sexual maturity, and lower reproductive investment. These variations in life-history strategies between the two populations could perhaps be explained by the spatial heterogeneity in habitat environment along the upstream–downstream gradient in this watershed.     

Population biology of grey gurnard (Eutrigla gurnardus (L.); Triglidae) in the coastal waters of Northwest Wales

The grey gurnard Eutrigla gurnardus (L.) has been identified by ICES as a potential commercial species in the NE Atlantic with recommendations made to derive information on population biology for stock assessment purposes. However, data on the population biology of this species is limited. In this study, data on the age, growth and maturity of grey gurnard were collected by otter trawling in the coastal waters of northwest Wales and Eastern Anglesey. Total length (TL) of fish sampled ranged between 2.1–33.0 cm (male) and 1.9–36.9 cm (female) with the majority of female (70.8%) fish between 11 and 20 cm TL and male fish (70.5%) between 11 and 18 cm TL. The percentage of fish >20 cm TL was larger for females (30.4%) compared to males (17.6%). Total weight (TW) for female and male grey gurnard in the stratified subsample ranged from 1.9 to 499.9 g for females and 2.1–390.0 g for males, with the majority of female (66.3%) and male (76.1%) fish between 10 and 60 g. TL/TW relations for male and female fish and both sexes combined were: TW = 0.006TL^3.07, TW = 0.007TL^3.03 and TW = 0.007TL^3.05 respectively. Age structure (based on otolith reading) ranged between 0.5 and 7.5 years old for females and 0.5 to 5.5 years old for male with the majority of female (41.7%) and male (46.0%) fish aged as 1.5 years old. The age structure of female and male grey gurnards was significantly different with the majority of older fish (>2.5 years) being female. The von Bertalanffy growth functions were calculated as L_t = 32.4[1 ? e^{?0.24(t + 1.41)}] for males, L_t = 45.9[1 ? e^{?0.16(t + 1.37)}] for females and L_t = 44.0[1 ? e^{?0.18(t + 1.20)}] for both sexes combined. Instantaneous rates of total mortality were similar for males and females and the combined Z value 1.00 year^?1 with the natural mortality rate estimated as 0.33 year^?1. The size at 50% maturity (L₅₀) was estimated to be 25.3 cm TL for males, females and for both sexes combined. Age at 50% maturity (A₅₀) was 3.2 years for both males and females. The results of this study provide the first information on the population biology of E. gurnardus in the Irish Sea, the first detailed study in the NE Atlantic since 1985 and helps to address the data gap identified by ICES in knowledge of the population biology of this species.     

Sex and reproduction of the alfonsino Beryx splendens (Pisces, Berycidae) from the Macaronesian archipelagos

This paper provides comparative information on the reproductive biology of the alfonsino, Beryx splendens Lowe, 1834, species with commercial interest in the Azores, Madeira and the Canary Islands. A total of 846 individuals from Azores (14.0–42.0 cm fork length), 621 from Madeira (17.2–50.0 cm fork length) and 643 from the Canaries (18.2–38.9 cm fork length) were used for the study. The alfonsino is gonochoric with no evidence of sexual dimorphism. Females are more abundant than males; this dominance probably reflects certain differences in the spatial distribution and/or the catchability of males and females in the Macaronesian archipelagos. The spawning season was distinct for the three Macaronesian areas, with an observed North–South variation in the reproductive period: September–March in the Azores, March–June in Madeira and July–September in the Canary Islands. The size at sexual maturity estimated for Madeira and the Canary Islands is similar (32 and 30 cm fork length, respectively), while for the Azores it is reached at smaller length (23 cm fork length). The differences observed in the size at sexual maturity can be explained by the different exploitation levels in each archipelago. Life‐history parameters of the alfonsino suggest that this species has a specialistic life‐history strategy and fisheries based on this species are more susceptible to growth overfishing and population depletion.     

Length–weight relationship and condition factor of Lepidocephalichthys goalparensis Pillai and Yazdani, 1976 in Assam,India

The present study describes the length–weight relationship (LWR), condition factor (K) and size at first maturity of a cobitid loach, Lepidocephalichthys goalparensis Pillai and Yazdani, 1976 of the family Cobitidae in Assam, India. A total of 716 specimens (males = 324, females = 392) ranging from 3.06 to 7.01 cm total length (TL) and 0.17–2.27 g body weight (BW) were analyzed. The overall value of the allometric coefficient b for the LWR indicated negative allometric growth (<3.00) for both males (2.774) and females (2.993). The size at first maturity (TL₅₀) for female L. goalparensis was estimated to be 5.6 cm TL. The condition factor values ranged from 0.56 to 1.35 in mature females. This is the first time that the biological parameters have been studied in this species.     

Growth differences and dimorphic expression of growth hormone (GH) in female and male Cynoglossus semilaevis after male sexual maturation

Half-smooth tongue sole, Cynoglossus semilaevis, is an ideal model to investigate the regulatory mechanisms of sexual growth dimorphism in fish species. The aim of the study was to investigate the effect of differential age of sexual maturity for females and males on growth and GH mRNA expression in C. semilaevis. The body weight differences between the sexes were not significant in C. semilaevis at age 5 months when females and males were all immature. Significant differences in body weight between the sexes were found after early sexual maturation of males at the age of 9 months. The body weight of 21-month-old females (621.4 ± 86.4g), still not immature, was even 3.28 times higher than that of the males (189.7 ± 14.4g). The cDNAs encoding GH in C. semilaevis was cloned. The GH gene is 2924bp long and consists of six exons and five introns. The results of qRT-PCR showed that GH mRNA levels of the immature females were not significantly different from that of immature males at age 5 months. However, GH mRNA levels of the immature females were significantly higher compared with those of the mature males at age 9 months (P<0.05). At age 11 months, GH mRNA levels of females were even 6.4-fold higher than that of males. In conclusion, for the first time we show that early sexual maturity of males is the main cause of sexual growth dimorphism in C. semilaevis and exert significant effect on GH mRNA expression.     

Age,growth, maturity and extinction risk of an exploited and endangered skate,Atlantoraja castelnaui,from off Uruguay and northern Argentina

The spotback skate Atlantoraja castelnaui (Arhynchobatidae) is a large and threatened skate species subjected to fishing pressure, endemic to the Southwest Atlantic that occurs from Rio de Janeiro, Brazil, to San Jorge Gulf, Argentina. The age, growth, age at maturity and the maximum intrinsic rate of population increase r_max of A. castelnaui were studied using 152 specimens collected from off Uruguay and north Argentina (35°–42° S), between June 2013 and February 2020. Vertebrae from 143 individuals were used for ageing (females: n = 83, size range 404–1300 mm total length, TL; males: n = 60, size range 400–1270 mm TL). Maximum ages determined for females and males were 30 and 28 years, respectively. To fit growth models, non-linear and Bayesian estimation approaches were considered. For the first approach, a set of four candidate growth (size-at-age) models were fitted: three-parameter von Bertalanffy, two-parameter von Bertalanffy with fixed L₀, Gompertz and Logistic. In the second approach, von Bertalanffy, Gompertz and Logistic were fitted. For non-linear estimation, model selection indicated that the entire set of candidate growth models were supported by the data. The von Bertalanffy was selected as the best model for Bayesian estimation. There were no differences in growth between sexes. For the sexes combined, the von Bertalanffy growth model by Bayesian method was considered the most adequate to describe the growth of A. castelnaui (growth mean parameters ± S.D. : L_∞ = 1210.29 ± 40.68 mm; k = 0.12 ± 0.01 years⁻¹; L₀ = 179.20 ± 11.62 mm). The age at maturity was estimated at 16.21 and 14.04 years for females and males, respectively. The maximum intrinsic rate of population increase r_max was estimated as 0.252 years⁻¹. Life-history traits and r_max provided in the present study suggest that this species has a relatively low productivity and may be vulnerable to an intense fishing pressure.     

Sexual size dimorphism, growth and maturity of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the Inabe River, Mie prefecture, central Japan

The population structure of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the upper reaches of the Inabe River, Mie Prefecture, central Japan, was investigated by a mark-and-recapture method from July 1989 to January 1991. Breeding of the species occurred from mid February to early May, peaking from mid February to late March. The mean size of mature males observed in March 1990 was significantly larger than that of females, showing apparent sexual size dimorphism. Data analysis of the growth of 1658 marked individuals revealed that the species matured at 2 years of age in both sexes. Whereas 1 year old males reached ca. 50–70 mm SL, females were less than 50 mm SL at the same age, size dimorphism already being apparent. Immature males exhibited higher growth rates than females during their first and second years, some of the former outstripping mature males of the preceding year class in total length. After attaining sexual maturity, both males and females grew mainly from July to December, with no significant differences in mean growth rate between them. Sexual size dimorphism of the species seems to be attributable to different growth rates between the sexes during their immature stage.     

Latitudinal variation in sexual dimorphism in life‐history traits of a freshwater fish

Sexual dimorphism is common across the animal kingdom, but the contribution of environmental factors shaping differences between the sexes remains controversial. In ectotherms, life‐history traits are known to correlate with latitude, but sex‐specific responses are not well understood. We analyzed life‐history trait variation between the sexes of European perch (Perca fluviatilis L.), a common freshwater fish displaying larger female size, by employing a wide latitudinal gradient. We expected to find sex‐dependent latitudinal variation in life‐history variables: length at age, length increment, and size at maturity, with females showing consistently higher values than males at all latitudes. We further anticipated that this gender difference would progressively decrease with the increasingly harsh environmental conditions toward higher latitude. We hypothesized that growth and length increment would decrease and size/age at maturity would increase at higher latitudes. Our results confirmed female‐biased sexual size dimorphism at all latitudes and the magnitude of sexual dimorphism diminished with increase in latitude. Growth of both sexes decreased with increase in latitude, and the female latitudinal clines were steeper than those of males. Hence, we challenge two predominant ecological rules (Rensch's and Bergmann's rules) that describe common large‐scale patterns of body size variation. Our data demonstrate that these two rules are not universally applicable in ectotherms or female‐biased species. Our study highlights the importance of sex‐specific differences in life‐history traits along a latitudinal gradient, with evident implications for a wide range of studies from individual to ecosystems level.     

Growth of dark chub, Zacco temmincki (Cyprinidae), with a discussion of sexual size differences

I examined the annual and seasonal growth of dark chub, Zacco temmincki, in a Japanese river. Investigation of opercular rings showed that the fish reached a maximum age of 8 years. There was no significant sexual size difference at younger ages (3–5 years), but males were larger than females at older ages (6–7 years). Annual increments of length and weight for males that were recaptured were also larger than those for females. The fish spawn from June to August. Females grew for a short period from April to May, but males on average grew for a longer period from April to August. There was no sexual difference in growth rate except during the spawning period. Annual growth rate was negatively correlated with fish length in each sex. The sexual size differences at older ages of the fish might be due to the polygynous mating system in which most mature males could not obtain females and invested for somatic growth in the spawning period, and a short growing season that was overlapped considerably with the spawning period.     

Age and Growth of Blue Antimora <Emphasis Type="Italic">Antimora rostrata</Emphasis> (Moridae) in Southwestern Greenland Waters

The results of determination of age and study on growth of blue antimora Antimora rostrata from the waters of Southwestern Greenland are presented. The results are based on the analysis of 200 fish otoliths. In the catches, we found specimens of antimora with total lengths of 18?70 cm, body weights of 23?2731 g, at the age of 7?38 years. Minimal age of males (not considering juvenile individuals) was 10 years at body length of 27?33 cm; maximal age was 18 years at 42 cm. Minimal age of females was 9 years at length of 21–27 cm; maximal age was 38 years at 70 cm. The rate of linear growth in blue antimora from Southwestern Greenland waters is comparable to that in the fish from New Zealand and Ross Sea waters but considerably lower than indicated earlier for fish from the waters of Iceland, Greenland, and the Mid-Atlantic Ridge. The age of reaching sexual maturity in males and females is preliminary determined as 15 and 19?20 years, respectively.     

Size at the onset of sexual maturity in the anomuran crab, Aegla uruguayana (Aeglidae)

The size at maturity was studied in the crab Aegla uruguayana from the Areco River (31°14′ S, 59°28′ W), Argentina. Size at sexual maturity was determined according to three criteria: morphometric (change in the relative growth of reproductive characters), histological (first maturation of gonads) and functional (capability to mate and carry eggs). Regarding females, morphometric maturity occurred at a carapace length (CL) of 11.50 mm, considering abdomen width as a reproductive character. Gonad maturity of females could be observed at a minimum size ranging from 15 to 17 mm CL. The smallest ovigerous female observed in the field was 15.60 mm CL, although a relevant population incidence of ovigerous females (86.6%) has just been observed at values higher than 17 mm CL. As for males, the relative growth of the left chela length changed at a value of 15.40 mm CL, while morphological changes in sexual tube occurred between CL of 14 and 16 mm. Testicular maturation occurred at a CL ranging from 17 to 19 mm. The smallest size of males having spermatozoids in their vasa deferentia was 18.70 mm CL. The results obtained indicated that, in both sexes, functional maturity occurred after morphometric maturity and at a size similar to that of gonad maturity. Comparing sexes, females acquired sexual maturity (morphometric, gonad and functional maturity) at sizes statistically smaller than those of males.     

Length‐weight relationships,condition factor,gonadosomatic index‐based size at first sexual maturity,spawning season and fecundity of Aspidoparia morar (Cyprinidae) in the Jamuna River (Brahmaputra River distributary), northern Bangladesh

The present study describes the length‐weight relationships (LWRs), length‐length relationships (LLRs), Fulton's condition factor (K_F), size at first sexual maturity, spawning season, sex ratio and fecundity of the Morari Aspidoparia morar (Hamilton, 1822) (Cyprinidae). Sampling was done using traditional fishing gear jhaki jal (cast net) from July 2010 to June 2011. Total length (TL), fork length (FL) and standard length (SL) were measured with digital slide calipers. Individual body weight (BW) and gonad weight (GW) were determined to an accuracy of 0.01 g for all specimens. The gonadosomatic index (GSI) was calculated and size at first maturity for males and females estimated using GSI and TL as indicators. Female ≥ size at first maturity was used to determine fecundity. A total of 1200 specimens (males = 552, females = 648) ranging from 4.06–12.84 cm TL and 0.53–16.75 g BW were analyzed. The overall coefficient b for the LWR indicated positive allometric growth (>3.00) in males and isometric growth in females (~ 3.00). ancova (analysis of covariance) revealed significant differences between males and females (P < 0.001). All LLRs were highly correlated (r² > 0.973, P < 0.001). Sizes at first sexual maturity for males and females were 6.0 and 7.0 cm TL, respectively. K_F changed little throughout the year and GSI peaked in November to April, indicating the spawning season (GSImax = 15.0 in females, 2.0 in males). Mature females were dominant during the entire spawning season except in April. Mean total fecundity was 6700 ± 3500, ranging from 1860 to 19680. In addition, relative fecundity ranged from 190 to 1200 (mean 560 ± 235) in the Jamuna River. To ensure sustainable management of this species, the protection of mature individuals during the peak spawning season is highly recommended.     

Size distribution,length–weight relationship and size at the onset of sexual maturity of the orange mud crab,Scylla olivacea,in Malaysian waters

The size distribution, length–weight relationship and size at the onset of sexual maturity of the orange mud crab (Scylla olivacea) from four geographically distinct locations (Taiping, Setiu, Kota Marudu and Lundu) representing Malaysian waters were analysed and estimated. Scylla olivacea was found in the size range of 47–134?mm carapace width. Males were significantly smaller in size but heavier than females. Geographical variation in carapace width and body weight were significant, but no interaction was found between sexes and locations. As shown by the length–weight relationships of S. olivacea, the males exhibited positive growth allometry whereas the females exhibited negative growth allometry. Males mature physiologically prior to attaining morphometric sexual maturity. Females, however, achieve physiological and morphometric sexual maturity in synchrony. No significant variation was found in the estimates of size at the onset of sexual maturity of males and females among different locations. We recommend the use of the third right walking leg merus length and carapace width to estimate the size at the onset of sexual maturity (morphometric maturity) for S. olivacea. Data obtained in this study serve as important baseline data for future mud crab resource management in Malaysia and were used to recommend minimum landing sizes for S. olivacea in each respective location based on the largest size at the onset of sexual maturity estimates were suggested.     

Relative growth and onset of morphological sexual maturity of the freshwater crab Sylviocarcinus pictus in a river of the Caatinga of northeastern Brazil

The aim of this study was to investigate the relative growth and onset of morphological sexual maturity of the freshwater crab Sylviocarcinus pictus. Specimens were collected every month from October 2013 to September 2014, at night, on a stretch of the river Guaribas, Piauí, Brazil. Crabs were sexed and their carapace width (CW, independent variable), carapace length (CL), cheliped propodus length (PL) and height (PH), gonopod length (GL), and abdomen width (AW) (dependent variables) were measured. These measurements were related to characterise relative growth and possible sex differences. On average males were larger than females (p = 0.0001). Size at the onset of morphological sexual maturity was defined by relating CL vs. PL for males (30.82 mm) and CL vs. AW for females (28.63 mm). These are considered secondary sexual characters and reflect initial size at morphological sexual maturity with greater precision. The differential cheliped growth of males may be related to courting and disputes with other males, while the allometric growth of the abdomen of females indicates increased probability of reproductive success.