Stomach Contents of Sergestid Shrimp Acetes japonicus from the Estuary of Tanjung Dawai Peninsular Malaysia

MATERIALS AND METHODS

Study area: Samples were collected monthly from the local fisherman of Tanjung Dawai (N 5° 40' 48" and E 100° 22' 5''), coastal waters (Fig. 1) between June 2008 and May 2009. However, it was not possible to collect any shrimp samples in November 2008 due to flood problem in the study area. The main fishing gear used by fishermen to catch acetates was the Set Bag Net (SBN). The net was normally set up during high tide and harvesting was done during the low tide. The SBN was fixed mostly between 2 and 3 m depth along the shallow waters in the upper estuary. The mesh size of the SBN was around 0.5 cm at cod end. Fresh samples for study were taken from fisherman’s net immediately after hauling and preserved in 5% formalin for laboratory analysis.

Stomach examination: Specimens were identified using a Nikon dissecting microscope (Nikon-122764, Japan). A. japonicus was identified using the reference of (Omori, 1975). A total of 330 stomachs (30 stomachs per month) of A. japonicus were analyzed but only 164 stomachs were either full or half full or quarter full and the remaining 166 were empty stomach. Each stomach was removed from the cephalothorax region using a pin under a dissecting microscope (Omori, 1975). The stomach contents were then incised and laid on a counter glass slide. A drop of distilled water was added to the sample. Food items and frequency of occurrence were counted. Food item identification was made using references by Crane (1973), Lokman (1990), Lewis (1979) and Bougis (1976).

Fig. 1:	Geographical location of the study site in the Tanjung Dawai (•), Kedah, Peninsular Malaysia

Stomach contents analysis: Percentage frequency of occurrence and percentage numerical abundance methods (Chrisfi et al., 2007) were adopted for the analysis of stomach content.

where, ni is the number of ith food item and m the number of food items.

RESULTS

Diet composition: In total, 23 food items belonging to eight major groups were identified from the stomach sacs of A. japonicus. Percentage of diet composition in the stomach of A. japonicus is presented in Fig. 2. Plant matter consisted 27.81% of the total composition and was the highest proportion observed. The second highest value was the crustacean appendages (18.06%). This was followed by sand and mud (17.43), debris (14.4%), unidentified items (8.12%), phytoplankton (6.13%), zooplankton (6.05%) and algae (4.46%). Among the phytoplankton diet recorded, dinoflagellate (3.61%) was the most dominant. This was followed by diatom (1.89%), Navicula (1.7%), centric diatom (1%), Euglena (0.77%), Coscinodiscus (0.41%) and Ceratium (0.31%). As for the zooplankton (Table 1), copepod (2.08%) made up the highest percentage and this was followed by the amphipods (1.99%), cladocerans (1.71%), ostracods (1.43%) and mysids (0.92%) (Table 1).

Seasonal variation of prey items: Monthly percentage frequency of occurrence (F_pi) of prey items are presented in Table 2. Phytoplankton was found in every month of the stomach samples. Slightly higher frequency of occurrence of phytoplankton was recorded in September (5.1%), January (6.9%), March (5.5%), April (6.6%) and May (8.4%).

Fig. 2:	Percentage of major groups of diet composition of A. japonicus from the estuary of Tanjung Dawai, Kedah

Table 1:	Combined mean percentage of diet composition of A. japonicus in the estuarine waters of Kedah

Among the phytoplankton composition, dinoflagellate, diatom and Naviculla were consistently dominant throughout the year. Zooplankton was observed around the year except in the month of July. The occurrence of zooplankton were particularly higher during March (11.1%) and April (16.2%). Copepod and ostracod showed diverse distribution throughout the year compared to other zooplankton. Algae were found from April-July and September-December. Apart from these months, no algae diets were found. Plant matter was found in most months of the year and its occurrence frequency ranged from 5.4 to 32.8%. Appendages of crustacean and debris were also recorded throughout the study period. The highest occurrence of plant matters was observed during June to August. High occurrences of crustacean appendages were found between October 2008 to February 2009. The highest percentage frequency of debris was seen in February and this is similar to the highest percentage frequency showed by the crustacean appendages (Fig. 3).

Percentage numerical abundance (C_i) of prey items is presented in Table 3. High percentage of phytoplankton was recorded in September (11.4%) and April to May (90%). Zooplankton was highest in March 2009 (16.6%). The lower number of zooplankton percentage was found between August and December 2008. Algae abundance was highest in the month of July (12.32%). Plant matter was found highest in June (30.2%) and lowest in September (5.9%). Crustacean appendages ranged from 10.5 to 19.6%. No definite pattern on food composition occurrence was observed. The percentage numerical abundance also did not fully agree with the frequency of occurrence amongst the diets. In term of numerical abundances, there were three major food components found in the stomach in each month (Fig. 4). These were phytoplankton, plant matters and crustacean appendages.

Table 2:	Percentage frequency of occurrence (Fpi) of food items of A. japonicus in the coastal waters of Kedah

Fig. 3:	Percentage frequency of occurrence (Fpi) of three groups of diets in the different sampling months

The highest occurrences of phytoplankton were observed in September and in January-May. The peak occurrences of plant matters were found in June-October and January.

Table 3:	Percentage of numerical abundance (Ci) of food items of A. japonicus in the coastal waters of Kedah

Fig. 4:	Percentage numerical abundance (Ci) of three groups of diets in the different sampling months

The peak occurrences of appendages of crustacean in the stomach were observed during the month of August- October and January-May (Fig. 4).

DISCUSSION

In the present study, diet compositions of A. japonicus were grouped into eight major categories viz phytoplankton, zooplankton, algae, appendages of crustacean, plant matter, debris, unidentified items and sand and mud. Amongst all diets, plant matter showed the highest percent (27.81%). This is similar in result with Amin et al. (2007) who found the plant matter was also highest in the stomach of A. indicus from the coastal waters of Malacca. Phytoplankton was the highest diet composition (45.51%) in A. intermedius in the coastal waters of south-western Taiwan (Chiou et al., 2005) and the dinoflagellate (27.03%) was also the most important diet discovered. However, phytoplankton is not the major food item in the present study. This could be attributed to the non specific food requirement characteristic of this shrimp and the diversity in food source at different geographical location. It is reported that A. paraguayensis grazing the phytoplankton group such as Ankistrodemus sp., Oedogonium sp., Bulbochaeta sp. and Navicula sp. (Collins and Williner, 2003). This result in different main food items may be due to the different resource availability in those places. It is reported by Zafar (2000) that the stomachs of A. indicus in the coastal waters of Bangladesh contained diatoms (30.08%), copepods (8.34%), sagitta (7%), foraminifera (1.66%), crustaceans appendages (1.25%), detritus (27.5%), sand and mud (22.58%) and molluscan parts (1.58%). Copepods were also recorded in the stomachs of A. japonicus but no diatoms were found in the stomachs of A. japonicus collected in the Ariake Sea (Ikematsu, 1957).

Deshmukh (2002) reported that detritus as the most frequent food item (39.70%) in the stomachs of A. indicus in the coastal waters of India. The foraminiferans, eggs and larvae were rare, whereas food items commonly observed in the foreguts of shrimp such as polychaetes, molluscs, filamentous algae and sand. It is concluded that the shrimp A. indicus is omnivorous. Although detritus is not the major found items in the stomach of A. japonicas but it is omnivores in the coastal waters of Tanjung Dawai. Xiao and Greenwood (1993) had reported that copepods were the primary item consumed by Acetes shrimp. The variety of food ingested by A. japonicus indicates that they are omnivore, similar as other species of Acetes (Xiao and Greenwood, 1993; Xu, 1957). Based on the results obtained, it can be said that A. japonicus is an active feeders and can be classified as omnivore since it consumed food of both plants and animal origins.

CONCLUSIONS

According to combined mean percent of diet composition, plant matter was observed as the highest percentage 27.81% in the stomach of A. japonicus. This was followed by crustacean appendages 18.06%, sand and mud 17.43%, debris 14.4%, unidentified items 8.12%, phytoplankton 6.13%, zooplankton 6.05% and algae 4.46%. There was no definite pattern of food consumption between months and different peak of occurrence was observed. The various percentage composition of food items further reiterated that this shrimp species is a bottom feeder omnivore.

ACKNOWLEDGMENTS

This study is part of a research project funded by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia (grant No. 05-01-04-SF0613). The authors like to acknowledge MOSTI for providing financial funding to carry out present research work and UPM for the providing of the research facilities and technical assistance.