Zoological Studies 51(6): 768-782 (2012) Population Structure and Reproduction of the Mysid Shrimp Acanthomysis thailandica (Crustacea: Mysidae) in a Tropical Mangrove Estuary, Malaysia Tueanta Ramarn1,3, Ving-Ching Chong1,2,*, and Yukio Hanamura4 1Institute of Biological Sciences, University of Malaya, Kuala Lumpur 50603, Malaysia 2Institute of Ocean and Earth Science,University of Malaya, Kuala Lumpur 50603, Malaysia 3Department of Biology, Faculty of Science, Thaksin University, Phattalung Campus, Phattalung 93110, Thailand 4Japan International Research Center for Agricultural Sciences, Owashi 1-1, Tsukuba 350-8686, Japan (Accepted March 12, 2012) Tueanta Ramarn, Ving-Ching Chong, and Yukio Hanamura (2012) Population structure and reproduction of the mysid shrimp Acanthomysis thailandica (Crustacea: Mysidae) in a tropical mangrove estuary, Malaysia. Zoological Studies 51(6): 768-782. The mysid shrimp Acanthomysis thailandica Murano, 1988 in the Matang mangrove estuary showed large spatial and temporal variations in population density. Large numbers occurred on the mudflat where the mean salinity was 25 ppt, but rapidly declined towards the upper estuary (14 km upstream) where the salinity fell to 15 ppt. Mysids were present year-round and exhibited marked monthly variations in abundance, with modal peaks in May, Sept., and Dec. 2009 and Apr. 2010 in the mudflat area. All developmental stages were observed, except juveniles which were absent from the middle to upper estuary. Females predominated over males and matured at a smaller size. Juveniles and empty (= post-spawning) females comprised the largest groups (at about 17% for each stage) of the population on the mudflat suggesting that the mudflat serves as the main spawning and nursery ground. Brooding females occurred together with juveniles in all monthly samples, indicating that reproduction was year round. Brood size was independent of female body length, and ranged 5-25 embryos per brood. Mean lengths of eggs (0.26 ± 0.03 mm), and eyeless (0.58 ± 0.06 mm) and eyed larvae (0.73 ± 0.10 mm) were also independent of total length. http://zoolstud.sinica.edu.tw/Journals/51.6/768.pdf Key words: Reproductive biology, Spatial and temporal abundance, Developmental stages, Mudflat area, Crustacea. Mysid shrimps (Crustacea: Mysidae) are al. 2001), thus providing a trophic link between major organisms in many estuarine and coastal primary producers and secondary consumers. ecosystems (Azeiteiro and Marques 1999, Munilla Both field and laboratory studies on mysids and Vicente 2005). They occur in high numbers, are widespread throughout the world especially and their ecological importance, particularly their in temperate regions (Mauchline 1973, Jones role in food chains, is becoming increasingly et al. 1989, Pothoven et al. 2000, Baldó et al. apparent (Roast et al. 1998, Drake et al. 2002). 2001, Scharf and Koschel 2004, Mendonça et al. Mysids play important roles in the trophodynamics 2009, Rappé et al. 2011). In contrast, biological of coastal ecosystems (Fanelli et al. 2009), feeding and ecological data pertaining to tropical mysids on detritus, phytoplankton, and zooplankton are scanty. This is unfortunate because mysid (Kouassi et al. 2006). In turn, they serve as populations in some tropical habitats can be prey food for other marine organisms (Beyst et very significant (Hanamura et al. 2007 2009). * To whom correspondence and reprint requests should be addressed. Tel: 603-79674220. Fax: 603-79674178. Present address: National Research Institute of Fisheries Science, Fisheries Research Agency, Fuku-ura-2-12-4, Kanazawa-ku, Yokohama 236-8648, Japan. E-mail:[email protected] 768 Ramarn et al. – Reproduction of Acanthomysis thailandica 769 Moreover, life-history traits of mysids can differ mudflat. Four sampling stations were established over geographical areas, while temperature is from station S1, located on the coastal mudflat, known to affect their ecophysiology (Mauchline through stations S2 (4.4 km upstream), S3 1980, Wittmann 1984). Hence, comparative (9.6 km), and S4 (14.1 km) in the main channel studies between temperate and tropical mysids of Sangga Besar River (Fig. 1). Mean water pertaining to their biology and behavior are needed depths at these stations were 1.02 ± 0.39 (mean ± to further understand the ecological role of these S.E.), 1.34 ± 0.93, 2.1 ± 1.29, and 2.42 ± 1.16 m, small hyperbenthic crustaceans. respectively. Mysid shrimp were sampled monthly Acanthomysis thailandica Murano, 1988 is from Oct. 2008 to June 2010 (except in Nov. the most abundant mysid species in the Matang 2008), during low tides using a mud sledge (Pullen Mangrove Forest Reserve (MMFR), Malaysia, et al. 1968). The sledge net had a 0.53 × 0.16-m forming large aggregations especially on a coastal mouth area and a 2.35-m-long net of 500-µm mudflat where they comprised 70% of the total mesh size. It was pulled over the mud bottom by mysid population (this study). In spite of their high first paying out a fixed 30-m length of a tow line abundance in the Matang mangrove ecosystem, from a moving boat, stopping the boat, and then mysid shrimps are poorly studied, unlike penaeid pulling in the net by hand onto the deck. The boat shrimps which are the most researched crustacean was 8-m long with a low freeboard. The collected of interest in mangroves (Ronnback et al. 2002, sample was completely emptied into a pail before Faunce and Serafy 2006, Chong 2007). This is large mangrove leaves and debris were removed, because of their small size, non-obvious economic and the entire contents were then washed into a value, and difficulty in identification. However, in 1-L sample bottle containing a borax-buffered 4% the Matang mangroves, mysid shrimps are con- seawater-formalin solution (Grabe et al. 2004). sumed by juvenile clupeid, sciaenid, mullid and Two more replicate samples following the gobiid fishes that use the mangrove as a feeding same sampling procedure were also taken. In ground (Chew et al. 2007). addition, environmental data such as salinity, Published works on Malaysian mysid shrimps temperature, pH, and dissolved oxygen (DO) are markedly lacking. Hanamura et al. (2007) were measured just above the mud bottom with reported on the abundance and distribution a Hydrolab 4e sonde (Hach Hydromet, Loveland, of mangrove hyperbenthic crustaceans which Colorado, USA). All monthly samplings were included mysid shrimps. Hanamura et al. (2008b) carried out during the daytime when hyperbenthic further described the reproductive biology of the animals are known to be concentrated near the mysid species, Mesopodopsis orientalis (Tattersal, bottom (Verslycke and Janssen 2002). 1908) (now M. tenuipes Hanamura, Koizumi & Sawamoto 2008) (Hanamura et al. 2008a) in a Laboratory analysis Malaysian mangrove estuary. With the exception of some descriptions by Hanamura et al. (2007 In the laboratory, A. thailandica and other 2008a b), there is no single study on A. thailandica mysid shrimps were sorted out and counted in Malaysian waters. Therefore, given the dense under a stereomicroscope. The total length (TL, populations of mysid shrimps in a mangrove mm) was measured from the anterior margin of habitat that is well known for its feeding and the rostrum to the posterior end of the telson, nursery ground function (Sasekumar et al. 1994, excluding the setae, with an ocular micrometer Ahmad Adnan et al. 2002, Chong 2007) and which mounted on a stereomicroscope. Individuals were supports the largest fisheries in the country, the sexed and classified into one of 7 developmental aim of the present study was to elucidate the stages based on the appearance of their sexual population structure and reproductive habits of this characteristics, namely, (1) juvenile (absence of neglected shrimp species. sexual characteristics); (2) immature or sub-adult male (penes on the 8th thoracic limb rudimentary, 4th pleopod extended, and process masculinus MATERIALS AND METHODS present, but with no or few setae); (3) mature male (penes and secondary sexual characteristics Sample collection completely developed), (4) immature or subadult female (rudimentary empty marsupium present); Sampling locations for mysid shrimp were (5) mature female (marsupium fully developed, located in the MMFR and an adjacent coastal with no eggs or larvae); (6) brooding or ovigerous 770 Zoological Studies 51(6): 768-782 (2012) female (marsupium with eggs or larvae); and molted larva with pigmented and stalked eyes. (7) empty female (developed marsupium void of Larval size was measured as the diameter of the contents) (Mauchline 1980, Delgado et al. 1997). egg stage, whereas the length from the frontal to All examinations and measurements were made the terminal tip of the body was measured for both from more than 200 specimens, if available. eyeless and eyed larval stages. The brood size and brood stage of females with unruptured marsupium were recorded each Data analysis month. The brood stage was classified according to Mauchline (1980), which is based on the Analysis of variance (ANOVA) and post- development of the marsupium contents (egg hoc Tukey’s honestly significant difference (HSD) or embryo) as follows: stage 1 (egg), round egg test were conducted to compare mysid shrimp containing developing embryo; stage 2 (eyeless abundances, water parameters, proportion of larva), hatched larva with observable antennae and brooding females carrying eggs, eyeless and eyed thoracic appendages; and stage 3 (eyed larva), larvae, body length of brooding females, and brood 25' 100°30' 35' 40' 4°55' 102° River 5° Selinsing Besar Straits of Malacca Sangga River Sepetang River S1 S2 S4 Kecil River S3 Kuala 50' Sangga Sepetang 2 m 45' 5 m Mudflat 10 m Mangroves 4°40' Fig. 1. Map showing the sampling area and stations in the Matang mangrove estuary, Peninsular Malaysia. Ramarn et al. – Reproduction of Acanthomysis thailandica 771 size (numbers of eggs, eyeless larvae, and eyed Monthly densities of A. thailandica in the larvae) among sampling stations. All datasets Matang mangrove showed marked temporal were first tested for normality and homoscedascity, fluctuations at all sampling stations.