Feeding Ecology of the Sesarmid Crab Neosarmatium trispinosum Davie (Crustacea: Brachyura) from the Mangroves of the Ryukyu Islands, Japan MS ISLAM, T UEHARA, S SHOKITA AND MA RAHMAN Faculty of Science, University of the Ryukyus, Okinawa, Japan

General aims Specific aims Investigate the feeding habits of Neosarmatium trispinosum in the mangrove ecosystems of the Ryukyu Islands, Japan. 1. Stomach contents analysis 2. Consumption of mangrove leaves 3. Leaf preference based on colors 4. C/N ratio in leaves, sediments and faces 5. Nutritional analysis of mangrove leaves Aims and Background

Methodology Nutritional analysis Mangrove leaves Crab tissues C/N ratio analysis Leaves and sediments Crab faeces Feeding experiment Leaf consumption Leaf preference Stomach analysis Stomach fullness Stomach contents Sampling Statistical Analysis Stat View 5.0

Map of samples collection sites on the Ryukyu Islands, Japan.

Overview of Shira mangroves on Iriomote Island, Okinawa, Japan

Shira River on Iriomote Island, during low tide

Shira River on Iriomote Island, during high tide

Sampling station of Shira mangroves, during low tide

Sampling station of Shira mangroves, during high tide

Study species, the mangrove sesarmid crab Neosarmatium trispinosum Davie

Three different types of Bruguiera gymnorrhiza mangrove leaves used for feeding experiments, C/N ratio and nutritional analysis. Scale = 5 cm. Green leavesYellow leaves Brown leaves

Stomach fullness (%) SeasonsSexesIslands Stomach fullness of N. trispinosum. SS, Miyako Island; SR, Iriomote Island; NR, Okinawa Island. Bars with different letters are significantly different (P<0.0001, Statview ANOVA).

Percentage occurrence (%) Food categories Percentage occurrence (%) Stomach content analysis of N. trispinosum. NF, not found; SS, Shimajiri swamp on Miyako Island; SR, Shira River on Iriomote Island; NR, Nuha River on Okinawa Island. Islands Seasons Sexes

Supplied mangrove leaves Consumption rate (%) Growth by body weight (%) Consumption rate and growth by body weight of N. trispinosum. Data indicate mean (±SD), N=24. Bars with different letters are significantly different (P<0.0001, Fisher’s PLSD).

Changes in appearance of B. gymnorrhiza mangrove leaves by N. trispinosum. Mode of Processing and Consumption Supplied separately Supplied together

Preference of B. gymnorrhiza mangrove leaves by N. trispinosum. Leaf Preference Based on Colors and Quality First choice Second choice Third choice

TC (mg/g dry wt.) TOM (mg/g dry wt.) TN (mg/g dry wt.) C/N ratios Burrow leaves TC, TN and TOM contents, and C/N ratio in burrow leaves. Bars with different letters are significantly different (P<0.0001, Fisher’s PLSD). NR=Nuha River SR=Shira River

TC (mg/g dry wt.) TOM (mg/g dry wt.) TN (mg/g dry wt.) C/N ratios Burrow sediments TC, TN and TOM contents, and C/N ratio in burrow sediments. Bars with different letters are significantly different (P<0.0001, Fisher’s PLSD). NR=Nuha River SR=Shira River

Comparison of C/N ratio in burrow leaves and sediments. NR=Nuha RiverSR=Shira River Leaves C/N ratios NR SR C/N ratios Sediments NR SR C/N = 22C/N = 52 C/N = 17-20, a value suggested as a general maximum for sustainable animal nutrition by Russel-Hunter (1970).

C/N ratios Leaf processing C/N ratio in mangrove leaves, processed by N. trispinosum. Bars with different letters are significantly different (P<0.0001, Fisher’s PLSD).

C/N ratios Crab faeces C/N ratio in faeces of N. trispinosum. Bars with different letters are significantly different (P<0.0001, Fisher’s PLSD).

Present study and literature records of C/N ratio in leaves and sediments from mangroves- a habitat of sesarmid crabs. Materials C/N ratio LocationReference Leaves74 South Africa Steinke et al. (1983) 92Australia Micheli (1993) 115Japan Present study Sediments19Malaysia Tanaka and Choo (2000) 20Tanzania Skov and Hartnoll (2002) 22Japan Present study

Nutritional composition of mangrove leaves at wet conditions (g/100g). Carbohydrate (g/100g) Energy (kcal/100g)

CONCLUSIONS Stomach content showed that the diet of Neosarmatium trispinosum mainly consisted of mangrove leaves, complemented with little amount of sediments, indicating as a primarily detritivorous species. Consumption rates of the brown leaves was significantly higher than those of yellow or green leaves, indicating that brown leaves are highly nutritious for the mangrove sesarmid crabs. Burrow sediments of N. trispinosum had C/N ratio 2-3 times lower than that of burrow leaves, indicating that the sediments could have higher nutritional value than the burrow leaves.

HYPOTHESES Why do mangrove crabs eat brown (aged) leaves? Mangrove leaves are unlikely to fulfill the N requirements of crabs, which they plaster onto burrow wall to increase the edibility, where N content increase and C/N ratio decrease through microbial activities. Thus by not eating leaves immediately, leaving them to age on burrow wall. Therefore, crabs might not only improve the digestibility of leaves, but also decrease the C/N ratio in their diets. Mangrove leaves are unlikely to fulfill the N requirements of crabs, which they plaster onto burrow wall to increase the edibility, where N content increase and C/N ratio decrease through microbial activities. Thus by not eating leaves immediately, leaving them to age on burrow wall. Therefore, crabs might not only improve the digestibility of leaves, but also decrease the C/N ratio in their diets. Might sediment be an important source of N for crabs? Mangrove sediment detritus could be a richer source of N, as shown by lower C/N ratio, and regular ingestion of crabs. Mangrove sediment detritus had C/N ratio 2-3 times lower than mangrove leaves, indicating higher nutritional value than leaves. Bacteria may certainly reach high densities in mangrove mud and are highly digestible by mangrove sesarmid crabs. Mangrove sediment detritus could be a richer source of N, as shown by lower C/N ratio, and regular ingestion of crabs. Mangrove sediment detritus had C/N ratio 2-3 times lower than mangrove leaves, indicating higher nutritional value than leaves. Bacteria may certainly reach high densities in mangrove mud and are highly digestible by mangrove sesarmid crabs.

THANK YOU ALL FOR KIND ATTENTION We are greatly acknowledged to: 1. the Japanese Society for the Promotion of Science for financial support 2. the University of the Ryukyus, Okinawa, Japan for technological support 3. Prof. Gustav Paulay, University of Florida, USA, for scientific support 4. Prof. Chang-Po Chen, Institute of Zoology, Taiwan, encourage to join ISISA 5. the Organizing Committee of ISISA-2004 in Taiwan for inviting us 6. Mr. Hiratsuka, Mr. Obuchi and Mr. NI Khan, University of Ryukyus for technical support