Analysis Of Length-frequency Distributions Research Articles

Investigating key biological parameters of Nile tilapia (Oreochromis niloticus L.) in a large Asian reservoir to better develop sustainable fisheries

The Nile tilapia (Oreochromis niloticus) was introduced in Southeast Asia, such as in Thailand, in the mid-1960s for aquaculture purposes (Pullin et al., 1997; De Silva et al., 2004). The species was later promoted for aquaculture development in the early 1990s in Lao PDR (Garaway et al., 2000). In Lao People’s Democratic Republic (PDR), like in other countries, this exotic species is well established due to its self-reproduction (De Silva et al., 2004; CABI, 2018) facilitated by its particular life history traits and plasticity (Ishikawa et al., 2013). The species was then suspected to be introduced in the seventies in Laos and subsequently in the Nam Theun watershed where the NT2 Reservoir was impounded in 2008. Population parameters of this alien fish were investigated using the FiSAT II software with the most popular methods of bioparameters assessment to check their interchangeability within a same stock and the same year 2016. The length-at-age data analysis (using otoliths analysis, N = 258) gave slightly different results with the length frequency distribution analysis of fish landings (11 820 individuals). Furthermore, experimental fishing data provided irrelevant parameters due to insufficient representativeness of the sample size. The growth parameter K was estimated to be 0.23 year−1, with asymptotic length L∞ = 52.5 cm based on the length frequency distribution analysis with the fish landing data. According to these landing data, the total, natural and fishing mortality were Z = 1.41 year−1, M = 0.30 year−1 and F = 1.11 year−1. The exploitation rate E = 0.79 is over the Emax = 0.594 obtained by relative yield and biomass per recruit. This estimated stock of 165 tons (more than 700 000 tilapias) was characterized by high mortality (no population growth). These results showed that the population is overfished with too many juveniles caught (L50 = 210.4 mm; 50% mature stages at 295 mm). To maximize the yield per recruit, increase the biomass and sustain this fishery, enlarging the gillnet mesh size of the gill-net is recommended. This example highlights the variability of the parameters calculated from different methods and thus weaken worldwide and even inter-site comparisons. Despite this issue, the Growth Performances Indices ([see formula in PDF]) gathered into the literature can serve as baseline and confirmed the wide phenotypic plasticity of the species due to environmental factors. Analyses revealed difference between fast growing domesticated fish rose for aquaculture with [see formula in PDF] higher to the tilapia growing in natural and challenging environments.

A holistic approach to the age validation of Mullus barbatus L., 1758 in the Southern Adriatic Sea (Central Mediterranean)

The growth of Mullus barbatus has been widely studied using different methods, but no previous study has focused on age validation. The uncertainty in estimating the age of the red mullet by otolith reading is linked to the number of false-growth increments laid down before the annulus. The capture of red mullets in the early life stage allowed us to estimate their size at the metamorphosis from the pelagic to the demersal phase. The comparison between the metamorphosis size and the back-calculated length of the first growth increment clarified the position of the false growth increment on the otolith. Moreover, the analyses of the otolith marginal increments in adult and juvenile specimens allowed us to define the deposition patterns of their annuli. The modal components of the length–frequency distribution analysis (LFDA) were identified in the winter survey (ELEFAN and Bhattacharya methods), and they did not show significant differences from the length back-calculation of the annuli. Moreover, no significant differences were found between the growth curves calculated by otolith reading (back-calculation and direct otolith reading) and the LFDA. The agreement between the length–frequency results and the otolith age estimation either corroborated or indirectly validated the growth pattern estimated in the otoliths of the red mullet, mainly when the direct validation methods (e.g. mark-recapture, captivity, radiochemical) were difficult to implement, like the case of this species. The comparison of the results of the present work to previous Mediterranean studies showed agreement with the slow growth pattern.

Reproduction and Population Structure ofAcanthocardia tuberculata(Linnaeus, 1758) (Bivalvia: Cardiidae) in Southern Spain: Implications for Stock Management

ABSTRACT The cardid bivalve Acanthocardia tuberculata is harvested in southern Spain and on the Atlantic coast of Morocco for the Spanish canning industry. The reproductive cycle and population structure of this species were studied in southern Spain from June 1999 to May 2000, using histology and condition indices, as well as modal size classes in length-frequency distribution analysis. The samples were collected monthly during autumn and winter months and fortnightly during spring and summer months. A total of 4,047 specimens were examined, from which 3,539 specimens were processed for biomass analyses and 508 for the study of the gametogenic cycle. Shell lengths of individuals ranged from 6 to 76 mm (48.15 ± 15.15 mm, mean ± SD), and the sex ratio was 1:1. The reproductive cycle of A. tuberculata showed an annual pattern, with an extended sexual activity from January to July, during which successive spawning events were observed, and with a clear-cut resting period from September to November. Small-siz...

Population dynamics and reproduction ofHolothuria arenicola(Holothuroidea: Echinodermata) in coastal waters of Pakistan, North Arabian Sea

In Pakistan, although the sea cucumber fishery has not yet been developed,Holothuria arenicolais abundant on its coast. Nineteen months of sampling was carried out to study the population structure, allometric relationships and reproductive status ofH. arenicolastocks in Manora and Buleji rocky shores. All measured biometric characters showed significant temporal variations and width/length, weight/length, gutted weight/length and gutted weight/weight relationships followed negative allometry, indicating a change in body shape as the animal grows. Length-frequency distribution analysis was bimodal in both summer and autumn consisting of small-sized and medium-sized individuals while in winter and spring three modes were found at Manora. At Buleji, the population was unimodal in all seasons consisting of small-sized individuals. Sex inH. arenicolacannot be determined externally but can be identified on examination of gonad colour which is orange-like in females and creamy yellow in males. The sex was further confirmed on histological examination and maturation was divided into five stages, i.e. undetermined, early developing, late developing, mature and partially spawned. The sex ratio inH. arenicolawas approximately 1:1 and increase in gonad index was observed during spring and early summer, followed by a decrease in GI in autumn and winter, which showed the spawning followed by resting phase. The GI showed a significant negative correlation with salinity and non-significant correlation with temperature. There is a need to undertake more biological/ecological studies onH. arenicolain order to take effective measures for its management.

Growth and mortality parameters of Orthopristis ruber (Perciformes: Haemulidae) from Los Frailes Archipelaeo. Venezuela

Orthopristis ruber is a species in high demand in Eastern Venezuela, but production has been decreasing in recent years. For this reason, our objective was to estimate the growth and mortality parameters of this resource. Monthly samples were collected from June 2011 to May 2012, obtaining 2980 specimens in El Tirano and Puerto Abajo. Data on total length (cm), total weight (g), and sex were recorded for each specimen. No sexual dimorphism was shown with respect to length (t(S)=1.113, p>0.05), so one length-weight ratio was established for both sexes (Wt=0.0612*Lt(2.54)); and they both exhibited minor allometric growth. Growth was estimated by analysis of length frequency distributions using FiSAT software. The estimated growth parameters (L.=39.03cm, W.=679.60g, k=0.48/year and t(O) = -0.32 year) showed moderately rapid growth. Length frequency data were adjusted to the von Bertalaniffy model, and indicated an exponential tendency of accelerated growth during the first years of life, followed by slow growth until the fish reached its maximum length. The coefficient of variation of the growth index (theta) demonstrated no differences in growth pattern. The natural mortality rate (M=0.97/year), from fishing (F=1.57/year), and total mortality (Z=2.54/year), were high, as well as the exploitation rate (E=0.62/year). We concluded that O. ruber has been fully exploited by artisanal fishers, and suggest a continuous study on population dynamics, to recommend optimum management techniques for the fishery.

Experimental Grow-Out of the Omani AbaloneHaliotis mariaeWood, 1828, in Land-Based Tanks in Mirbat, Oman

Hatched larvae, juveniles, and adults of the Omani abalone Haliotis mariae were cultivated in land-based tanks with filtered running seawater during 3 experiments: (1) from January 2001 to December 2005, (2) from January 2006 to November 2009, and (3) from January 2009 to December 2011. Larvae and early juveniles were fed daily with diatoms, whereas late juveniles and adults were fed with artificial food and the green algae Ulva fasciata. Shell length and total wet weight from 40–80 randomly selected specimens in various experiments were measured monthly, and sex and gonad maturity stage were defined visually. The water parameters in tanks reflected the ambient condition of the Arabian Sea. The growth pattern of the abalone was very similar in the 3 experimental trials. Larvae and early juveniles have very fast growth rates, reaching an average 53–56 mm and 19–25 g after 1 y. Better results were obtained in the second and third experiments, when at the end of 2 y of rearing, abalone mean length was 67–72 mm and the weight was 46–54 g. At the age 3 y, abalone attained a mean length of 83–85 mm and a weight of 89–93 g. After 6 y, the abalone from the first experiment (2001 to 2005) showed, on average, 88.5 mm and 110.9 g. The length increments were the highest during the first year of the culture, whereas weight increments reached maximum values during the third (second and third trials) and fourth year (first trial) of rearing. After 4 y, abalone growth increments in weight decreased significantly. Males generally but not always have larger increments in length and weight when compared with females. During cultivation, a decrease in abalone weight was sometimes observed that, in most cases, coincided with decrease in water temperature during the southwest monsoon period (June to September). Some abalone sexually matured in tanks after 1 y, and mature males and females occurred almost year-round, with peaks in April to May and November to December. The length at first maturity was found to be 65.2 mm for males and 67.1 mm for females. Growth parameters for a von Bertalanffy function were calculated using data from different experiments. The growth rate coefficient in the first experiment was lower (K = 0.65), but in third experiment it appeared higher (K = 0.80) than in natural conditions from data other authors calculated based on length-frequency distribution analysis, which indicates faster growth rates of abalone in tanks when compared with those in the wild. Results of the trials showed that it is feasible to produce Omani abalone in land-based farms from eggs to commercial size during a 3 4-y cycle.

Growth discontinuity in males of the deep‐water giant red shrimpAristaeomorpha foliaceain the Mediterranean Sea

AbstractThe deep‐water giant red shrimp,Aristaeomorpha foliacea(Risso, 1827) (Decapoda, Aristeidae), represents a highly valuable resource for bottom trawling in the Mediterranean Sea. Recent assessments have described both a worsening of the status of the traditionally exploited stocks and low levels of annual‐based instantaneous natural mortality (M 0.4–0.7 year−1) in the unexploited stocks. The mortality (M) figures, however, are in contrast with the longevity (Tmax) of 3–4‐year males and 4–5‐year females, estimated from classical length frequency distribution analysis. Reduced growth (after the onset of sexual maturity) and pile‐up of older individuals in the larger size classes have been considered to be reasonable explanations for the contrast between M and Tmax. We propose that a discontinuity of the growth models could address this contrast. Because a clear discontinuity in sexual maturity is evident only in males, length frequency distribution data for different sets of males collected from the South of Sicily and the Maltese Islands deep bottoms (400–800 m) were fitted with both the classic (c) and double‐phased (d) von Bertalanffy growth formula (VBGF). According to the dVBGF, adult males sharply reduce their growth rate after reaching an age between 1.2 and 1.5 year, which corresponds to the estimates of age at sexual maturity (tmbetween 1 and 1.5 years). The reduction in growth determines a higher Tmax(7.3–9.5 year) and lower M (0.4–0.6 year−1) than previously derived on the basis of cVBGF estimates (Tmax3–4 years and M 1.2 year−1). The dVBGF results suggest that sexual dimorphism and length–sex segregation in giant red shrimps reflect an alternative life history strategy that is based mainly on growth reduction in adult males and it might be adopted for implementing more conservative assessments.

Growth and mortality of Cichla spp. (Perciformes, Cichlidae) introduced in Volta Grande Reservoir (Grande River) and in a small artificial lake in Southeastern Brazil

The growth and mortality parameters were estimated through the analysis of length frequency distribution for species of Cichla spp. introduced into a lake in Leme (SP), and in Volta Grande reservoir (SP-MG). In Leme, Cichla kelberi presented larger frequency in the inferior classes of lengths, larger instantaneous rate of natural mortality, and smaller number of cohorts than C. kelberi and C. piquiti in Volta Grande. The values of growth performance obtained for the species were similar, corroborating the validity of the estimated growth and mortality parameters. The increase of the growth rate in small and less diverse environments occurs due to predation. The genus Cichla adapts well in locations in which it is introduced, however this adaptation shows itself to be strongly adjusted to each particular location, determining great plasticity and establishment capacity.

First data on reproduction and growth of <i>Buenia affinis</i> Iljin, 1930 (Gobiidae) in the northern Adriatic Sea

De Buen’s goby, Buenia affinis , is a very small Mediterranean goby (38.8 mm TL) inhabiting sandy infralittoral bottoms and known only from a few published records. Some aspects of reproduction and growth of B. affinis were investigated from a population collected in the Kvarner area (north-eastern Adriatic Sea). Sampling was carried out by SCUBA diving between May 2003 and April 2004. The sex ratio did not differ significantly. The estimated length at sexual maturity was 26.1 and 27.4 mm for females and males, respectively. The spawning season extended from February to July. The absolute fecundity (F) ranged from 457 to 714 eggs per individual (mean ± SE, 556 ± 25). Based on the length frequency distribution analysis throughout the year and seasonal growth bands on sagittal otoliths, B. affinis was assumed to be an annual species.

Population dynamics, allometric relationships and reproductive status of Microcosmus sabatieri (Tunicata: Ascidiacea) in the Aegean Sea

The Mediterranean ascidian Microcosmus sabatieri is among the conspicuous benthic invertebrates, forming dense populations on moderately inclined cliffs. It is an edible species, commercially harvested at the southern part of the Aegean Sea. Considering that the biology of this species has not been adequately studied, a three-year survey was carried out focusing on population structure, allometric relationships and reproductive status of M. sabatieri stocks in Astypalea Island (Dodecanese, south Aegean). Population density showed insignificant temporal variation and lower values at the steeper station, while the pattern of dispersion was random. All measured biometric characters showed high plasticity, especially those related to siphons. All the examined morphometric relationships follow negative allometry. Ascidian length was a good predictor of biomass, and a strong relation was observed between mantle and tunic characters. Gonad somatic index showed a single spawning season per year, in winter. Length–frequency distribution analysis revealed the presence of one mode of medium sized individuals, implying that M. sabatieri stocks are under moderate fishing pressure in the study area.

Stage-dependent spatial synchrony revealed for fish populations in the Garonne River (SW France)

This study aimed to establish the presence of spatial synchrony for three riverine fish populations (stone loach Barbatula barbatula L., European minnow Phoxinus phoxinus L. and gudgeon Gobio gobio L.) in two spatially-close sites of the Garonne River (SW France). Sampling was carried out by electrofishing from 1994 to 1999. The fish were split into two age classes (0+ and >0+) using length-frequency distribution analysis. No autocorrelation was present in our time series. There were significant correlations of densities between sites for stone loach (>0+) and gudgeon (0+), but not for European minnow. A significant positive relationship was observed between maximal flow values and 0+ gudgeon density. No significant associations between flow variables and density were observed for stone loach. These results emphasize the role of external factors in effecting population dynamics, and suggest that their influences differ according to life stage and life-history characteristics.

Age structure of Pacific saury Cololabis saira based on observations of the hyaline zones in the otolith and length frequency distributions

The ages (n=11 178) and length frequency distributions (n=173 300) of Pacific saury Cololabis saira caught from September to November in 1989–2000 were examined. Most specimens were determined as either age 0 or 1 based on the number of annual rings in the otolith. The knob lengths (length from the tip of the lower jaw to the posterior end of the muscular knob on the caudal peduncle) at which 50% of the fish were age 1 ranged 277.8–304.7 mm. Analysis of length frequency distribution was also useful to divide the knob lengths of the two groups. The modal lengths of age 1 fish ranged 303.9–325.9 mm.

DETERMINATION OF THE RATE OF GROWTH OF PACIFASTACUS LENIUSCULUS IN LAKE POBŁĘDZIE, USING POLYMODAL LENGTH FREQUENCY DISTRIBUTION ANALYSIS

This study estimated the rate of growth of Pacifastacus leniusculus using the sizefrequency data analysis in Lake Pobledzie NE Poland. A sample of 1,342 individuals (645 males and 697 females) was collected with baited traps in late September 2002. The age composition of the crayfish population was determined by the Macdonald-Pitcher mixture analysis (computer program MIX. 3.11a) for both sexes separately. Six age groups of males and five of females were identified. The rate of growth was described with a polynomial growth model. The results showed relatively fast increases of both length and weight of Pacifastacus leniusculus in Lake Pobledzie and no difference in growth rates between males and females. It is concluded from this study that polymodal lengthfrequency distribution analysis can be successfully applied to data on crayfish catches even if juveniles were not completely covered by the sampling method applied. The sampling size needs, however, to be large enough. Further, advantages of applying this analysis are the easy data collected and processing making the method suitable for large data sets already existing, while a disadvantage is the problem of separating the early age classes. Comparison of growth rates among signal crayfish populations suggested a relative fast growth of Pacifastacus leniusculus in Lake Pobledzie. This population had a faster apparent growth rate than those reported for other crayfish populations from Poland.

Reproductive biology of Patagonotothen ramsayi (Regan, 1913) (Pisces: Nototheniidae) around the Falkland Islands

The reproductive biology of one of the most abundant notothenioids, Patagonotothen ramsayi, was investigated between February 2003 and November 2004 on the Falkland and Patagonian Shelves (Southwest Atlantic). Male and female P. ramsayi were mature at 27.6 and 24.8 cm L T, respectively. P. ramsayi is a total spawner, with a total fecundity ranging between 24,300 and 76,700 eggs. Spawning occurs on the shelf breaks between June and August with the peak in gonado-somatic indices in June. Analysis of length frequency distributions over the year and sex ratios of mature fish during the spawning season may indicate the presence of a nesting and nest guarding behaviour in male fish, similar to other rockcods. Features of its reproductive strategy, which enabled P. ramsayi to dominate the medium-sized demersal fishes on the Patagonian shelf are discussed and compared with those of other nototheniids and cottid sculpins from the Northern Hemisphere.

Life history and population dynamics of the Arctic sympagic amphipods Onisimus nanseni Sars and O. glacialis Sars (Gammaridea: Lysianassidae)

The two sympagic amphipod species Onisimus nanseni Sars and O. glacialis Sars (Lysianassidae) are important constituents of the macrofaunal community in Arctic sea ice but little, as yet, is known about their autecology. In this paper we provide a description of the life cycles and population dynamics of these two species that were sampled beneath the ice pack surrounding Svalbard/Norway in September 2002 and May 2003. The distribution of Onisimus spp. was patchy and their numbers were low in sea ice throughout the year (∼0.8 ind. m−2 in spring, ∼1.0 ind. m−2 in autumn). In May the juveniles accounted for 37% and 22% of the population of O. nanseni and O. glacialis, respectively. The population structure of O. glacialis was relatively constant in spring and autumn, but of O. nanseni a shift towards a more mature structure was observed in September. Ovigerous females were virtually absent in both species during both sampling periods. Length–frequency distribution analysis on spring- and autumn-data showed three cohorts for O. nanseni and four cohorts for O. glacialis and suggests a temporal shift in the reproductive cycles of these two species. Both species appear to be univoltine (one generation per year) and probably iteroparous (several broods per female lifespan). The offspring of O. nanseni is presumably released in late spring and shows secondary sexual characters the first year allowing for reproduction in year 0+. In contrast, O. glacialis leaves the brood pouch in early spring and sexually matures in year 1+. The growth constants were comparable for the two species. The estimated life span was 3.5 years for O. glacialis and 2.5 years for O. nanseni.

Life‐history traits of the common pandora Pagellus erythrinus L., interpreted using information from aquaculture experiments

The common pandora Pagellus eythrinus is considered a promising species for Mediterranean aquaculture, due to its high market price and good reproduction in captive conditions. The aim of this paper is to clarify aspects of reproduction and growth in the common pandora, using data from both aquaculture studies and experimental fishing. Spawning time and behaviour, frequency of egg deposition, larval performance, growth of juveniles and adults and morphometric characteristics were studied in fish originating from a broodstock acclimatised to captivity since 1997. At the same time observations on length structure and other morphometric measures, age and patterns of sexual maturity were gathered from ten trawl‐surveys between 1996 and 2002. The surveys were carried out in late spring and early autumn in the Central‐Southern Tyrrhenian Sea, area where the breeders had been captured. Wild fish growth patterns were estimated using length frequency distribution analysis and age‐based methods. The study showed that several micro‐cohorts enter in the fishery over an extended period, and growth is faster than previously reported.

AGE AND GROWTH OF CHAMELEA GALLINA FROM THE ALGARVE COAST (SOUTHERN PORTUGAL): INFLUENCE OF SEAWATER TEMPERATURE AND GAMETOGENIC CYCLE ON GROWTH RATE

The age and growth of Chamelea gallina from the Algarve coast (southern Portugal) were estimated using three methods: (1) counting of external shell rings; (2) examination of shell microstructure (acetate peels); and (3) length–frequency distribution analysis. The Von Bertalanffy growth curves obtained by these methods were, respectively: Lt = 37.55[1 − e−0.71(t + 0.01)]; Lt = 38.95[1 − e−0.47(t + 0.24)]; Lt = 42.15[1 − e−0.32(t + 0.76)]. Based on these results, and taking consideration of the biology of C. gallina along the Portuguese coast, counting of external rings is appropriate for a quick estimate of age, while the use of acetate peels is recommended for the estimation of Von Bertalanffy growth parameters. The analysis of acetate peels showed that periods of major growth occurred between December and July. Slower growth periods were registered at the beginning of autumn, especially in October, when all individuals of the population showed narrow growth increments associated with a cleft in the shell ventral margin. This slowing of the growth rate appears to be related to a decrease in seawater temperature. The gametogenic cycle may also influence growth rate, which decreased after spawning (September to November) and increased with the onset of gametogenesis (November).

How fast hake grows? A study on the Mediterranean hake (<i>Merluccius merluccius</i>, L.) comparing whole otoliths readings and length frequency distributions data

The hake (M. merluccius) is a target species of the Mediterranean trawl fleets. In this study, its growth was compared by two means: whole otolith readings and length frequency distribution analysis. Two growth hypotheses, fast and slow, were assumed using the results of otolith readings: (1) two rings were formed every year and (2) a single ring was formed every year. The monthly length distributions covered the period from 1992 to 1998, and the otoliths were obtained from monthly samplings between 1992 and 1995. The variability observed in the results for the number of rings counted was shown especially in the range of lengths for each group of individuals that had the same number of rings. A high correlation was found between the rings counted in the otoliths and the length of the individual in cm. The sex factor did not influence the relationship, unless the sexually undifferentiated individuals were included. The best fit with the growth data according to the length frequencies occurred in the two-ring hypothesis (Hypothesis 1), meaning faster growth rates than had been accepted till now, especially in females. We suggest the consideration of the hypothesis of faster growth in future studies.

Growth of Norway lobster, Nephrops norvegicus (Linnaeus 1758), in the Skagerrak, estimated from tagging experiments and length frequency data

Parameter values for the von Bertalanffy growth model are estimated for Nephrops norvegicus in the Skagerrak on the Swedish west coast using analysis of length frequency distribution from commercial catches and tag-recapture data. The asymptotic lengths (L) for males and females are estimated from size distributions using a modified Powell-Wetherall plot. The tagging experiment was conducted with Floy streamer tag and analysed in a ‘‘forced’’ Gulland-Holt plot to estimate the growth coefficient K for males. The estimates of L (72.9) and K (0.138) for males differ from those currently used in the ICES assessment for this area (76 and 0.16, respectively), and the results are discussed in relation to sensitivity to reference points from analytical stock assessments and possible implications for management decisions.

Biological data on Triops cancriformis mauritanicus (Ghigi,1921) and Cyzicus grubei (Simon, 1886) - crustacea, branchiopoda - in SW Portugal temporary ponds

The lack of information about Notostraca and Spinicaudata in general, and particularly about these taxa in Portugal, can be due to the fact that they are typical of temporary ponds. As most of the species only occur in this kind of habitats, which are highly vulnerable to human action, the study of these systems is of great importance in order to define strategies for their conservation. In April 1996, Triops cancriformis mauritanicus - Branchiopoda, Notostraca - was found in six lagoons from the Vila do Bispo area and in two of those was also found Cyzicus grubei (Simon) - Branchiopoda, Spinicaudata - a species considered endemic to the Iberian Peninsula. In spite of being previously cited as part of the Portuguese fauna (in 1944 and 1951) they were only recently reported in Southwest Portugal (Machado et al., 1999) and there are no other recent records in Portugal. The dimensions of T. c. mauritanicus (maximum standard length = 68.3 mm) are well within those referenced for the Iberian Peninsula, but the maximum valve length of C. grubei (15.1 mm) is about 25% higher than that reported for this species. The morphometric relation studied for T. c. mauritanicus (standard length / carapace length) show sexual dimorphism, while that studied for C. grubei (valve height / valve length) show no difference between sexes. For both species, modal analysis of length frequency distributions suggests the presence of only one cohort. Concerning the sex ratio and although there seems to be a 1:1 ratio for both T. c. mauritanicus and C. grubei, we have found that this ratio is variable among samples.