(Herklots) off Lago`S Coast, Nigeria

European Journal of Scientific Research ISSN 1450-216X Vol.30 No.3 (2009), pp.402-408 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm

The Biology of the Smooth Swim Crab, Portunus Validus (Herklots) off Lago`S Coast, Nigeria Lawal Are Aderonke Omolara Corresponding Author Department of Marine Sciences University of Lagos, Lagos -Nigeria E-mail: [email protected] Tel: +2348033020969 Bilewu Barakat Department of Marine Sciences, University of Lagos Lagos -Nigeria Abstract The size composition, growth pattern, food and feeding habits, fecundity and sex ratio of the smooth swim crab, Portunus validus (Herklots) off the Lagos Coast, Nigeria were investigated. The carapace length of the 618 specimens examined ranged between 4.5cm and 11.4cm (carapace width 9.2cm to 19.5cm) and weighed 82.3g to 694.0g. The crabs exhibited negative allometric growth in both sexes. There was high correlation between carapace width and body weight of the crabs with correlation coefficient (r) ranging between 0.889 and 0.911. The condition factor ranged between 3.78 and 5.46 with a mean of 4.85. The condition factor decreased with increase in crab size. The crabs fed mainly on fishes, molluscs, crustaceans, plant parts and algae. There were distinct differences in the feeding habits in relation to size. The small crabs fed mainly on shrimp appendages while larger crabs fed mostly on fishes and molluscs. The sex ratio was 1: 2.2 which was significantly different from the expected 1: 1 ratio. The fecundity ranged between 806,000 and 6,240,000. The average fecundity was 2,200,310. The egg diameter ranged from 0.28mm to 0.45mm. The mean egg diameter was 0.34mm

Introduction The smooth swim crab, P. validus occurs off tropical West Africa from Mauritania to Angola and inhabits shallow waters between 0m – 50m (Monod, 1956). It is found from bottoms consisting of shells, muddy sand, fine sand mud or on shells and mud (Buchanan, 1958; Crosnier, 1964). It is an economically important crustacean for food, commerce and biological research (Kailola et al, 1993). It is caught in beach seines, trawl nets and fish pots. Very little information is available on its biology of this species off the Nigeria Coast. The size composition, growth pattern, food habits and reproductive biology were investigated in this study as a contribution to the database on this species in Nigerian marine waters.

The Biology of the Smooth Swim Crab, Portunus Validus (Herklots) off Lago`S Coast, Nigeria

403

Materials and Methods Collection of Specimens Specimens of P. validus were caught by fishing trawlers operating off Lagos coast between Badagry and Lekki areas as by-catch. Trawl nets with 76mm cod end mesh size were used at depths of 10 -30m. Samples of P. validus were obtained from the trawlers during their landings at Ijora and Apapa fisheries terminals. A total of 618 specimens were collected between October 2006 and April 2007. The specimens were preserved in ice-chest with ice – blocks and transferred to a deep freezer in the laboratory pending further analysis. Laboratory Procedure In the laboratory, the male and female crabs were separated using the method described by Kwei (1978). The carapace length (Cl) of the crab was measured to the nearest 0.1cm from the edge of the frontal region to the tip of the carapace backwall. The carapace width (Cw) was taken to the tips of the two lateral spines of the carapace. The weight of the crab was determined using a Satorious top loading balance to the nearest 0.1g. Data for the growth pattern were compiled from the length / weight relationship and condition factor. The carapace length / weight relationship was expressed by: Log W = Log a+ b Log L. (Bagenal, 1978) Where W = weight of crab in g, L = carapace length in cm, a = regression constant and b = regression coefficient The condition factor, K of the crab was determined using the formula: 100W K= (Bannister, 1976) Lb Where W= weight of crab in g, L= carapace length in cm, b = regression coefficient. For the food analysis, the cardiac stomach was dissected out and contents examined under a binocular microscope. The food analysis was by the numerical and occurrence methods (Kwei, 1978; Hyslop, 1980). Reproductive Biology The sexes of the crabs were determined using the method described by Kwei (1978). Eggs were removed from the ventral surface of the carapace of fecund females for fecundity estimates. Fecundity was estimated using the gravimetric method. The eggs diameter was measured using a calibrated eyepiece micrometer.

Results Size composition and growth pattern A total of 618 specimens of P. validus were examined. The carapace length ranged between 4.5cm and 11.4cm (carapace width 9.2cm to 19.5cm) and weight 82.3g to 694.0g and consisted of small, medium and large sized crabs (Figure 1). All size groups were caught during each month of the study.

404

Lawal Are Aderonke Omolara and Bilewu Barakat Figure 1: Carapace length frequency distribution of P. validus off Lagos Coast. 45 40 35

% Frequency

30 25 20 15 10 5 0 5

6

7

8

9

10

11

Carapace Length (cm)

Length / weight Relationship: The length – weight relationship values for P. validus from off Lagos Coast for the male, female and combined sexes are given below: For male: Log Y = - 0.5380 + 2.5023 Log X (n = 192, r = 0.9110) For female: Log Y = - 1.1475 + 3.0494 Log X (n =426 , r = 0.8910) Combined sexes: Log Y = - 0.9331 + 2.8566 Log X (n =618 , r = 0.8888) The male crabs exhibited negative allometric growth while the females showed isometric growth. The length–weight relationships are shown in Figures 2,3 and 4. Condition Factor (K) The condition factor of P. validus which is indicative of the state or overall well being of the crab is given in Table 1. The k-values ranged between 3.78 and 5.46 (mean 4.85). The condition factor was slightly higher in the males than the females except in the larger crabs. In both sexes, the k-values decreased with size. Table 1:

Condition factor (K) by sex and size of P. validus off Lagos Coast.

Carapace Length (cm) Small crab (4.5 - 6.4) Medium crab (6.5 - 8.4) Large crab (8.5 - 11.4)

N

Cl

Male W

K

N

Female Cl W

76

6.0

119.0

5.46

92

6.0

108

7.7

220.0

4.89

308

8

10.5

477.7

3.78

26

Combined Sex Cl W

K

N

110.6

5.02

168

6.0

114.4

5.22

7.3

185.9

4.72

416

7.4

194.8

4.77

10.3

443.6

4.11

34

10.3

451.6

4.12

KEY N= Number of Crabs,Cl= Carapace length (cm), W= Crab weight (g), K= Condition factor.

K


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Food Habits 50 (8.1%) of the 618 specimens of P. validus examined had empty stomachs. The crabs were opportunistic omnivores, feeding mainly on fishes, bivalves and gastropods, crustaceans, annelids and occasionally on algae and plant materials (Table 2). Fish parts were the most important food item constituting 42.4% by number and 27.3% by occurrence. Molluscs accounted for 33.6% by number and 28.4% by occurrence methods. Annelids occurred least accounting for 0.2% by number and 1.0% by occurrence The crabs were divided into three size groups in order to facilitate the comparisson of their food habits. The small size group (4.5cm – 6.4cm) fed mostly on shrimp appendages (47.4% and 6.6% by numerical and occurrence methods respectively). The middle sized (6.5cm – 8.4cm) fed mostly on fish parts while the large crabs (8.5cm – 11.4cm) fed mostly on fish parts and molluscs. Figuer 2: Log carapace length / Log weight relationship of male P. validus off Lagos Coast. 2.9

Log Y = - 0.5380 + 2.5023 Log X (n = 192, r = 0.9110)

2.7

Log Total Weight

2.5

2.3

2.1

1.9

1.7

1.5 0.9

1.0

1.1

1.2

1.3

1.4

Log Carapace width

Figure 3: Log carapace length / Log weight relationship of female P. validus off Lagos Coast. 3.0

Log Y = - 1.1475 + 3.0494 Log X (n =426 , r = 0.8910)

Log Total Weight

2.8

2.6

2.4

2.2

2.0

1.8 0.9

1.0

1.1

1.2

Log Carapace Width

1.3

1.4

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Lawal Are Aderonke Omolara and Bilewu Barakat

Figure 4: Log carapace length / Log weight relationship of combined sexes in P. validus off Lagos Coast 2.9

Log Y = - 0.9331 + 2.8566 Log X (n =618 , r = 0.8888)

LogTotal weight

2.7 2.5

2.3 2.1

1.9 1.7 0.9

1.0

1.1

1.2

1.3

1.4

Log Carapace Width

Table 2:

Summary of the stomach contents of P. validus off Lagos Coast (Oct. 2006 - April 2007).

Food Items PISCES Fish scales Fish bones Fish eyes MOLLUSCA Bivalve shells Gastropod shells CRUSTACEA Crab appendages Shrimp appendages ANNELIDA Polychaetes (parts) ALGAE PLANT MATERIALS SAND GRAINS UNIDENTIFIED MASS

Numerical Method Number %

Occurrence Method Number %

2560 5670 1220

11.5 25.4 5.5

79 147 18

8.0 17.3 2.0

4350 3150

19.5 14.1

139 113

16.2 12.0

1722 3441

7.7 15.4

128 109

14.3 12.2

51 191 -

0.2 0.9 -

9 27 24 17 83

1.0 3.0 2.7 1.9 9.3

Reproductive Biology 194 male and 424 female crabs were obtained giving a sex ratio of 1: 2.2 (male / female) which was significantly different from the expected 1: 1 sex ratio. Immature, developing, ripening, ripe and spent maturity stages were observed. P. validus was highly fecund (Plate 2). The fecundity estimates ranged from 806,000 to 6,240,000. The average fecundity was 2,200,310. The egg diameter ranged between 0.28mm and 0.45 mm (mean 0.34 mm)


407

Plate 2a: Dorsal view of berried female Portunus validus

Plate 2b: Ventral view of berried female Portunus validus

Discussion The carapace length of P. validus in this study ranged from 4.5 cm to 11.4 cm. The length frequency distribution showed a unimodal distribution of which most crabs (80.3%) were in the medium size group and were of the same year class. Similar results were obtained for Callinectes latimanus by Kwei (1978) and for C. amnicola by Lawal-Are and Kusemiju (2000). The length-weight relationship reflected the common general increase in weight with increasing carapace length. P. validus off Lagos Coast exhibited a positive growth as the exponent ‘b’ was greater than 3. The combined sex showed positive allometric growth having a high correlation value of 0.889. The condition factor in P. validus ranged from 3.78 to 5.46 the mean k value was 4.85. The condition factor decreased with increasing carapace length and weight. Similar results were documented for C. latimanus by Kwei (1978) and for C. amnicola by Lawal-Are and Kusemiju (2000). The number and variety of preys found in the stomach contents of P. validus showed the opportunistic nature of the crab. P. validus had a preference for animal food with predating tendencies as the stomach content showed the presence of fish parts, molluscs shells, and crustacean appendages. The stomach content showed the presence of plant parts and algae. Guillory (1996) reported that crabs were generally opportunistic predators. Juvenile and adult blue crabs have been characterised as opportunistic benthic omnivores, detritivores cannibals and scavengers, with food availability of prey

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Lawal Are Aderonke Omolara and Bilewu Barakat

(Guillory et al, 1996). Sanchez and Raz-Guzman, (1997) concluded that it was difficult to place blue crabs in one trophic level. The presence of annelids showed a probability of parasitic association or infection of the stomach The sex ratio obtained for P. validus was 1: 2.2 (male/female) which showed a significantly more abundant female population. Similar results were reported for Portunus pelagicus by White (1999) and Sumner and Malseed (2004). P. validus was highly fecund. The fecundity estmates ranged between 806,000 and 6,240,000 while average fecundity was 2,200,310. Brachyuran crabs have been reported to be highly fecund with females producing from 2 – 8 million eggs per spawn (Kwei, 1978; Lipske, 1995; Wakefield, 2002)

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