Reproductive biology and relative growth in the spider crab Maja crispata ( Crustacea : Brachyura : Majidae ) *

A wealth of literature deals with a variety of reproductive aspects of spider crabs, covering topics on reproductive behaviour, reproductive effort, seasonality, growth and sexual maturity (Paul and Paul, 1996; Jones and Hartnoll, 1997, among others). Knowledge of reproductive potential, seasonality, growth and sexual maturity in spider crabs has been of significant importance for some species being SCI. MAR., 67 (1): 75-80 SCIENTIA MARINA 2003

The present study was performed to provide baseline data on population parameters that have not yet been investigated in the spider crab Maja crispata, such as sex ratio, achievement of sexual maturity, copulation, reproductive season, reproductive effort and brood sequence as well as relative growth.Such baseline information is crucial for future studies of this species' dynamics in the Mediterranean Sea, in environments that are threatened by invasive algae (Ceccherelli et al., 2000).

MATERIALS AND METHODS
Sampling sites were located in the area of Castello Aragonese on the island of Ischia-Italy, in the Tyrrhenean Sea (40º42'N and 13º55'W) (Fig. 1).Four sampling periods (February, May/June, August/September and November/December 1982) were used in a mosaic habitat complex consisting of rocky sublittoral, Posidonia meadows, and dead Posidonia-rhizomes at a depth of between 0 and 5 m, well protected from the open sea (Fig. 1).
Crabs were captured using SCUBA equipment from sampling sites chosen randomly by tossing a 1m 2 quadrate haphazardly.Capture methodology is described in detail in Carmona-Suárez (2002).For relative growth analysis, additional specimens were obtained from local fishermen.Captured crabs were taken to the laboratory of the Marine Biological Station in Ischia.Some of them were transported alive to the Marine Biological Department (University of Vienna-Austria) for further observations.The rest were stored at -10ºC.After thawing, they were sexed, measured and their gonadal stage was determined.In females, seminal receptacles were visually examined, in order to establish the grade of "fullness".
They were classified either "full" or "empty".In ovigerous females, numbers of eggs were quantified.
The different maturity stages were determined by a power regression analysis, using the methodology described by Hartnoll (1978).Morphological (chelae in males and pleon in females) and physiological (gonadal development) changes were taken into account, transforming data to logarithm base 10.Carapace length (CL) in both sexes was measured from the basal region between the frontal teeth to the posterior-median edge.In males, chelae width (CW) was measured at the articulation between the propodus and the dactylus.In females, width of the abdomen (AW) was measured at the 5th abdominal segment.Measurements were taken with a 0.1-mm precision caliper.Gonadal development was determined by changes in colour and consistency of the gonads.
Eggs from ovigerous females were carefully detached from the pleopods.A sample from each clutch was observed under the microscope for measurements using a graduated reticule.The number of eggs per female was estimated using both a gravimetric (Fielding and Haley, 1976) and a volumetric method (Díaz et al., 1983).
Brood sequence was observed in crabs maintained in closed-water-circulating aquaria with filtering systems and sufficient aeration in a temperature-controlled room in the Marine Biological Department in Vienna (Austria).A day/night light period was used.Copulation observations were carried out in situ, in aquaria using recently captured animals (Ischia), and in aquaria with animals kept for longer periods (Vienna).In both cases, each crab pair was held in a separate aquarium.
Sex ratios were compared with a G-test; carapace length in females and egg counting methods were compared with a t-student test.Regression analyses were conducted using standard least squares (Sokal and Rohlf, 1995), and differences between regression coefficients were analysed with an F test.All calculations were done with the computer program Statistica (Statsoft, 1992).

RESULTS
One hundred and twenty-six square metres were searched in February, 256 m 2 in May/June, 453 m 2 in August/September and 195 m 2 in November/ December.A total of 104 males and 86 females were captured.There were no significant differences in the sex ratio from the Mendelian proportion (1:1) in each of the sampling periods and considering all the periods together (Table 1).More significant than the plain sex ratios is the relationship between individuals with mature and immature gonads, due to its direct connection with reproduction.A comparative table was generated for the determination of the gonadal stages, taking into account colour and consistence of the gonads (Table 2).Animals with mature gonads were present in all the sampling periods.Their abundance was higher, than that of individuals with immature gonads.The highest abundance of animals from both sexes with mature gonads was found in the warmer season (August/September), when females reached a pro-portion of 20:1 (mature: immature), and males reached a proportion of 4:1 (Fig. 2).
Relative growth analyses in males were conducted in three separate groups: crabs with morphological juvenile characteristics and immature gonads (JI); crabs with juvenile morphological characteristics and mature gonads (JM); and crabs with morphological adult characteristics and mature gonads (AM) (Fig. 3A).JI crab size ranged between 1.69 and 5.03 cm carapace length; JM crabs ranged between 3.59 and 6.11 cm, and AM crabs between 4.98 and 7.16 cm.Transition from JI to JM males takes place in a size range between 3.63 and 5.01 cm carapace length, with a significant change in the slope from 1.3 to 2 (F= 974.15; p < 0.001; df= 78) (Fig. 3A).On the other hand, the moult of puberty (transition from JM to AM) happens between 5 and 6.5 cm carapace length, with a significant change in the slope from 2 to 1.6 (F= 1289.53;p < 0.001; df= 79) (Fig. 3A).Regression equations are shown in Table 3. Morphological changes were evident in adult males.In these, the chela grows in length as well in width, developing an additional tooth on the internal side of the dactylus.
Regression equations are shown in Table 3. Females displayed mature gonads only after the puberty molt and throughout most of the year; only in November/December were some postpuberty females found with immature gonads.Also, almost all ovigerous females had mature gonads.
The breeding season appears to extend from May to September.From the whole adult female population in each sampling period, 36% of ovigerous females were found in May/June (n= 13), and 81% in August/September (n= 20).No ovigerous females were captured in February and in November/December.Sixteen ovigerous females were used to determine their number of eggs.Both methods showed that number of eggs per female varied between 5600 and 19000 (volumetric method), and between 4800 and 19800 eggs (gravimetric method); however, as the differences were not significant (t= 0.04; df= 30; p > 0.05), the results were pooled giving a mean estimate of 11473 eggs per female (SD= 4787.8;n= 32)   A total of 14 isolated copulatory crab-pairs were observed in Ischia and Vienna.Five adult males and 9 males with juvenile morphological characteristics were observed copulating with hard-shelled adult females.Six of these females were ovigerous.In Ischia, where natural light conditions were present, copulation took place mostly at night in May, June, August, September and November, in a temperature range between 16 and 27°C (Table 4).
Adult females with full seminal receptacles were captured during all the sampling periods, but the lowest percentages from the total adult females in each period occurred in November/December (February 71.4%, n= 7; May/June 69.2%, n= 13; August/ September 95.0%, n= 20; November/December 25.0%, n= 20).
Breeding sequences were observed in two mature females, each held in an aquarium together with an adult male.For these crabs the breeding sequence was 5 and 7. Intervals between broods varied from 21 to 32 days (Mean= 23.6 SD= 3.16;n= 18), with temperatures ranging from 19 to 24ºC (Mean= 22; SD= 1.66; n= 11) (Table 5).

DISCUSSION
From the reproductive point of view, the puberty moult in males (transition from JM to AM) may not be as important as the ability to reach gonadal maturity.This would explain the high incidence of males with mature gonads still exhibiting juvenile characters (57%), and the fact that these were observed copulating with mature females.Although females with mature gonads were present throughout the year, ovigerous females appeared only during the warmer period (May to September), coinciding with those indicated by Lo Bianco (1908) in the Gulf of Naples, and by Pesta (1918) in the Adriatic Sea.The absence of ovigerous females in February and November/December may be due to the low temperatures that dominate in the winter months.
The three differentiated male groups discovered in this study can be defined using the terminology given by Sampedro et al. (1999): males with juvenile morphological characteristics and immature gonads= immature juveniles; males with juvenile morphological characteristics and mature gonads= adolescent juveniles; and males with adult morphological characteristics and mature gonads= adults.
As determined in this study, reproduction begins in May, but it peaks between August and September when females have the highest percentage of mature gonads, the highest percentage of ovigerous individuals, and full seminal receptacles.As stated before, M. crispata can produce at least 5 to 7 broods in the laboratory, and may be even more productive in its natural environment.In a study conducted by Hines (1982), spider crabs bred 3 to 10 times a year, which is many times more than members of other families that generally breed only once or twice per year.
In spite of the low numbers of individuals captured, the reproductive cycle of M. crispata reflects a high degree of adaptation to the strong seasonal variations in the environments that it inhabits.Furthermore, females have the capability to copulate several times with different males, even if they are ovigerous.This assures egg fertilisation (where several clutches of eggs are produced during the year), high larval production, an increase in genetic flow, and thus an increase in the survival chances of the population.

FIG. 3
FIG. 3. -Relative growth in Maja crispata.A: (a) males with juvenile morphological characteristics and immature gonads, (b) males with juvenile morphological characteristics and mature gonads, (c) males with adult morphological characteristics and mature gonads.B: juvenile (a) and adult (b) females.
ACKNOWLEDGMENTS I should like to express my gratitude to the following persons: the staff of the Marine Biological Station of the Stazione Zoologica di Napoli in Ischia, Italy; the Department of Marine Biology of REPRODUCTIVE BIOLOGY IN MAJA CRISPATA 79

TABLE 1 .
-Comparison of sex ratios of Maja crispata.

TABLE 3 .
. Egg size ranged between 290 and 400 µm in diameter (n= 244; Mean= 318; SD= 24.2).-Relative growth in Maja crispata.Regression lines of carapace length (CL) against chela width (CW) of males with juvenile morphological characteristics and immature gonads (JI), males with juvenile morphological characteristics and mature gonads (JM), and males with adult morphological characteristics and mature gonads (AM).Regressions lines of carapace length (CL) against abdomen width (AW) of juvenile (JF) and adult (AF) females.Measurements in cm were log-transformed.

TABLE 5 .
-Brood sequence in two ovigerous females of Maja crispata maintained in aquaria in Vienna (Austria).Bs= Brood sequence; a= Intervals in days between each brood; b= Incubation time in days; °c= Mean temperature and standard deviation (n in parenthesis).* = Eggs eaten by the female; ** = Female died.