Maturation and spawning of four commercially important penaeid shrimps of Pakistan

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1 Indian Journal of Marine Sciences Vol. 31(2), June 2002, pp Maturation and spawning of four commercially important penaeid shrimps of Pakistan *Zarrien Ayub & Muzammil Ahmed *Centre of Excellence in Marine Biology, University of Karachi, Karachi-75270, Pakistan [ * zarrien@sat.net.pk ] Received 2 July 2001, revised 21 February 2002 The spawning seasons of four species of penaeid shrimp, Penaeus penicillatus (Alcock), P. merguiensis (de Man), Metapenaeus affinis (Milne Edwards) and Parapenaeopsis stylifera (Milne Edwards) from Pakistan s inshore waters were studied on the basis of ovarian colour and ovarian histology. During maturation shrimp ovaries pass through a series of colour changes. Females with green ovaries (green yellow, green white, light green and dark green) were considered as spawning females while those with other than green ovaries (translucent, white, cream and yellow) were considered as nonspawning. The maturation stages of the ovaries recognized histologically in these shrimps are: undeveloped, developing, nearly-ripe, fully-ripe, resorbing and resorbing/developing. Presence of nearly-ripe or fully-ripe ovaries in these species throughout the year suggests that these shrimps have the potential for spawning throughout the year. However, P. penicillatus, P. merguiensis and M. affinis showed two spawning peaks, the first one in winter-spring (February-May) and second one in July, September and/or October. In P. stylifera the spawning was more frequent from November to February. The present study indicated that the spawning seasons based on the colouration of the ovaries and on ovarian histology are more or less similar. [ Key words: Maturation, spawning, penaeid shrimps Pakistan ] One of the most important aspects of the reproductive biology of penaeid shrimps is the study of the histological basis of ovarian maturity and the spawning seasons, which is required for the management of the stock as well as for aquaculture. Such studies have not been made in Pakistan waters. Some studies 1-5 have been conducted in Pakistan to determine the spawning seasons of penaeid shrimps. However, none of these were based on histological observations of ovaries. In other parts of the world, the ovarian development of several penaeid species has been described histologically It is a well-known biological observation that the ovaries of shrimps pass through a series of colour changes during their maturation process. Based on these colour changes 3-5 maturation stages have been recognized In the present investigation, the spawning season of the commercially important penaeid shrimps [viz., Penaeus penicillatus (Alcock), Penaeus merguiensis (De Man), Metapenaeus affinis (Milne Edwards) and Parapenaeopsis stylifera (Milne Edwards) (Arthropoda/Crustacea/Decapoda/ Penacidae)] was studied with the help of ovarian colour and ovarian histology. Material and Methods The shrimps were collected once a month from September, 1992 to October, Each month the collection trips were made by the trawler Al- Ahmedi in the area, which extended from Phitti Creek to Cape Monze. This area is shallow (depths of m) and about 10 km from the shore (Fig.1). An otter trawl with the net (length 25 m, width 8 m and mesh size from wings to cod end 3, 2.5, 2 and 1 cm) was operated three times during each survey. The duration of each haul was about three hours. The shrimps caught in the three hauls were mixed and then sorted into Penaeus penicillatus and P. merguiensis (Jaira), Metapenaeus affinis (Kalri) and Parapenaeopsis stylifera and P. hardwickii (Kiddi) species on board. Later a sub-sample of each group was obtained from the trawler for species determination and ovarian studies. The ovarian maturity of females was determined by two methods. First, the colouration of the ovaries was noted and then the histological study was conducted. For this purpose, the ovaries present in the first abdominal segment were fixed in Davidson s Fluid for 48 hours as earlier workers 10,14 did not find any difference in the state of ovarian development between anterior, middle and posterior regions. After fixation, the ovaries were transferred to 70 % alcohol. Every month the ovaries of specimens of each species

2 120 Indian J. Mar. Sci., Vol. 31, No. 2, June 2002 Fig. 1 Map showing the area between Phitti creek and Cape Monze, where the trawler operated ( study area). were fixed for the study. The process of dehydration, clearing, infiltration and embedding was followed as given in Humason 15. The ovaries embedded in paraffin were sectioned at 7 µm and then stained with Delafield s haematoxylin and counter-stained with eosin. A repeat sampling was carried out from July, 1996 to May, 1997, for histological study of the ovary of these species to confirm the results obtained from the first year of study. The samples were collected from the same area. The colouration of the ovaries was also noted. Results and Discussion The shrimps were classified as non-spawning or spawning on the basis of the colour of their ovaries. Females with green ovaries (green-yellow, greenwhite, light-green and dark-green) were considered as spawning females or mature females while those with other than green ovaries (translucent, white, cream and yellow) were considered as non-spawning ones. Six arbitrary maturation stages of female ovary were recognized in these species. These were designated as undeveloped, developing, nearly-ripe, fully-ripe, resorbing and resorbing/developing (Fig. 2). Shrimp females with ovaries in fully-ripe stage (fully-ripe oocytes have cortical bodies at the periphery) were considered as spawners. The smallest female with fully-ripe ovary measured 123 mm in Penaeus penicillatus, 126 mm in Penaeus merguiensis, 113 mm in Metapenaeus affinis and 80 mm in Parapenaeopsis stylifera. In P. penicillatus and P. merguiensis the mm size-class showed the greatest number of females with fully-ripe ovaries. In M. affinis the mm size-class and in P. stylifera the mm size-class contained the largest number of females with fully-ripe ovaries. The size at which the females of P. penicillatus and P. merguiensis become sexually showed that the size at their first maturity is almost the same, being 123 mm and 126 mm, respectively. The size-class in which the greatest number of spawners occurred was also the same for these species. A parallelism of sorts, therefore, seems to occur in the two species which were at one time considered synonymous with each other and with P. indicus (in fact both were lumped under P. indicus, before seperation into three species). The sizes at first sexual maturity for females of some species have been reported from India and the sizes are smaller than those obtained presently. For P. stylifera it was 75 (ref. 16 ) and 70 mm (ref. 12 ), for M. affinis it was 94 (ref. 12 ), 120 (ref. 17 ) and 98 mm (ref. 18 ) and for P. indicus 134 (ref. 12 ) and 130 mm (ref. 19 ). Spawning seasons based on ovary colour Observations on spawning seasons based on the colouration of the ovaries are given in Fig. 3. The occurrence of the spawning females of the four species throughout the study period indicated that these species have the potential to spawn throughout the year. During the study period, September 1992 to October 1993, two peaks of spawning were observed in

3 Ayub & Ahmed : Maturation and spawning of shrimps 121 Fig. 2 Photomicrographs showing the developmental stages of the ovary in M. affinis: A) section of an undeveloped ovary containing oogonia and primary oocytes; B) developing ovary containing primary oocytes and yolkless oocytes; C) nearly-ripe ovary showing yolky oocytes; D) fully-ripe ovary showing oocytes with cortical rods; E) resorbing ovary showing dispersed follicle cells; F) resorbing/developing ovary showing follicle cells and primary oocytes. [CN = Chromatin nucleolar oocytes; PO = perinucleolar oocytes; YO 1 = yolkless oocytes 1; YO 2 = yolky oocytes 2; CR = oocytes with cortical rods; FC = follicle cells; N = nucleus; AO = atretic oocytes; 200X] [Scale = 50 µm]

4 122 Indian J. Mar. Sci., Vol. 31, No. 2, June 2002 P. penicillatus, P. merguiensis, and M. affinis. The first peak occurred from February to May, while the second peak occurred either in September or October. In the study period July 1996 to May 1997, the first peak of spawning in P. penicillatus, P. merguiensis and M. affinis was observed during February to May, as was the case in the earlier study. However, the second peak of spawning occurred in July and November in P. penicillatus, in September and October in P. merguiensis and in July and October in M. affinis. In P. stylifera the spawning females were observed throughout the study period in quite large numbers. Spawning seasons based on histological study of ovary The percentages of females with ovaries in fullyripe stage are given in Fig. 3. During , the first peak of spawning in P. penicillatus and P. merguiensis was observed from March to May and second peak in July and October, respectively. During , the two peaks were same as in the earlier study. However, the first peak included the month of February as well. In M. affinis and P. stylifera the females with ovaries in fully-ripe stage were few in numbers. During , spawners of M. affinis were found in September, November and April to May, while in P. stylifera they were found in April only. However, during , two speaks of spawning were observed in M. affinis, the first one from February to May and the second from July to October. In P. stylifera, the spawning was more in the winter period (November to February). In M. affinis and P. stylifera, the females with ovaries in fully-ripe stage were less in numbers as compared to P. penicillatus and P. merguiensis. According to Crocos 20 and Anderson et al. 21 the duration of cortical rod stage in the ovaries was 7 to 12 days in Fig. 3 Percentages of two categories of spawning females (based on ovarian colour and fully-ripe stage of ovarian histology) in P. penicillatus, P. merguiensis M. affinis and P. stylifera

5 Ayub & Ahmed : Maturation and spawning of shrimps 123 the species of Penaeus. The occurrence of few females in the fully-ripe stage of M. affinis and P. stylifera showed that in these two species the duration of ripe ovary stage was shorter, and even more so in P. stylifera. Therefore, spawning must have occurred soon after the formation of cortical bodies in both the species with the result that a very small number of their females with fully-ripe ovaries were found. The spawning seasons as reported in the present study for P. penicillatus, P. merguiensis and M. affinis are in agreement with the earlier studies 1-5 done in Pakistan waters. In P. stylifera, however, Hasan 4 had mentioned an intensive summer spawning while in the present study, fully-ripe females were found more in number from November to February. The present study indicated that the spawning seasons based on the colouration of the ovaries and on ovarian histology are more or less similar. Therefore, the assessment of spawning season on the basis of ovarian colour would be easier and reliable method whereas the ovarian histology is much more time consuming process. According to Penn 22 spawning in penaeid shrimps is often seasonal showing one peak or sometimes two peaks in a year. According to Garcia 23 the double peaked pattern of spawning among penaeid shrimps is more common. These peaks usually occur during the transition period, which separates the cold and the warm seasons, that is, one in spring and the other in autumn. When two peaks of spawning occur, the spring spawning is usually more regular in timing and amplitude. The autumn spawning varies from year to year and place to place as well as in time of occurrence. Similar double spawning peaks have been reported for several species from different areas 12, 14, 24. It is a well-established fact that most of the penaeid shrimp species migrate to offshore waters when they attain sexual maturity 23. This migration is intended for spawning purposes. However, in the present study a large number of ripe females of P. penicillatus and P. merguiensis were caught from the shallow waters during the peak season of spawning, i.e., in winterspring suggesting that no migration to deeper waters is required for spawning for these two species. Zupanovic 1 reported the migration of P. penicillatus and P. merguiensis to deeper waters from January onwards in Pakistan for spawning purpose. In this study the presence of a large number of ready-to-spawn females in the shallow waters indicates that the local shrimps may be spawning close to shore. Recently Ahmed 25 had suggested that the high salinity, which prevails in the creeks and coastal waters of Pakistan, seems to have changed the spawning behaviour of the local penaeid shrimps. The mature females which used to migrate offshore for spawning (as reported by Zupanovic 1 ) now seems to spawn in inshore waters. Hassan & Ahmed 4 had also reported the abundance of early larvae of penaeid shrimps in the creeks and channels. They suggested that this indicate high spawning activity on the adjacent shelf. Amjad 26 collected mature females of P. merguiensis from the shallow-bay area of Sonmiani, Damb village. However, Ahmed & Ayub 27, while studying the population of penaeid shrimps from Miani Hor Lagoon, Balochistan, did not find any females with mature ovary. In this study a total of 470 specimens of shrimps were collected during the period at irregular intervals with the salinity range Crocos & Kerr 14 had also reported the presence of large numbers of P. merguiensis spawning females in the shallower waters (< 15 m depth) from Gulf of Carpentaria, Australia, but were unable to explain the reason for this. It would, however, stand to reason that if high salinities were present in the inshore waters and bays/lagoons, penaeid shrimps would not have the need to go far from the shore to spawn - as has been suggested by Ahmed 25 for the Pakistani marine environment. 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