Consumption of mangrove litter by the crabs Sesarma messa and Sesarma smithii in north eastern Australia

Leaf characteristics, which may be relevant to food preferences, were determined using 10 senescent leaves from each of the species to be used in experiments (Avicennia marina, Rhizophora stylosa, Bruguiera exaristata and Ceriops tagal). Leaves were weighed and oven dried for 5 days at 65°C and reweighed to calculate dry weight/wet weight regressions, which would be used in later experiments. Three dried leaves from each species were then ground and analysed for percent organic matter, percent carbon and nitrogen and tannin content. An index of relative toughness of leaves was determined from 18 leaves of each species by estimating the force required to pierce the leaves with a metal point. Carbon and nitrogen content and leaf toughness were also determined for decayed leaves of Rhizophora stylosa, Bruguiera exaristata and Ceriops tagal, produced by placing senescent leaves in mesh bags and leaving them in the field for 17 days.The feeding preferences of 47 Sesarma messa and 10 Sesarma smithii, captured at Cape Ferguson, were tested in the laboratory using senescent leaves of Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal and Avicennia marina from the same area. Leaves of each species were cut in half and after weighing, each crab was offered half a leaf of each species. The other half was retained as a control for natural weight loss. Leaf remains from experimental and control containers were removed after 24 hours, dried and weighed. Ingestion rates were calculated as mg dry weight of leaf/g weight of crab /24 hours. A second similar experiment was carried out with 24 Sesarma messa, which were offered both senescent and decayed leaves of Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal simultaneously.Field experiments were carried out during the dry season, April to November 1989, at 4 sites at Cape Ferguson and 1 site at Coral Creek, Missionary Bay, Hinchinbrook Island.Leaf choice experiments were carried out at all field sites using leaves from Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal and Avicennia marina. Twenty senescent leaves of each species were weighed, marked and tied by the petiole to lengths of twine, which were then tied to a prop root or a tree. Leaves were deployed haphazardly, retrieved after 24 hours and the number taken down burrows counted. Burrows were excavated and all litter from the burrow walls collected. All uneaten leaf material was rinsed, dried and weighed. Leaf consumption was calculated as percent dry weight loss.An experiment was carried out at three sites at Cape Ferguson, to test if crabs allowed leaves to decompose in their burrows before consumption. The sites were located along an intertidal gradient. Sixty leaves of the most abundant tree species at each site were weighed, marked, tied and deployed. Twenty leaves were retrieved 24 hours, 8 days and 17 days after deployment and burrows excavated to retrieve leaf fragments. Leaf remains were dried and weighed. Controls, inside mesh bags, were deployed on the mud surface and down burrows and retrieved on the same dates. These leaves were dried, reweighed to determine weight loss, ground to a powder and analysed for carbon and nitrogen content.The feeding behaviour of 30 Sesarma messa was monitored at Cape Ferguson during October and November 1989. Each 30 minute observation commenced when the crab left the burrow and the type and duration of feeding and other activities recorded.
Field and laboratory experiments were undertaken to determine:1. whether sesarmid crabs show preferences for particular ages or species of leaves2. whether the two species of crabs chosen for the study have different food preferences3. whether crabs store litter in their burrows for prolonged periods before consumption4. whether food items other than leaf litter play an important part in the diet of mangrove crabs

为明确与食物偏好相关的叶片性状，本研究采集实验所用各物种的10片衰老叶片（白骨壤（Avicennia marina）、红海榄（Rhizophora stylosa）、木榄（Bruguiera exaristata）以及角果木（Ceriops tagal））开展测定。将叶片称重后置于65℃烘箱中干燥5天，再次称重以计算干重/湿重回归方程，供后续实验使用。随后取各物种的3片干燥叶片研磨，测定其有机质占比、碳氮占比及单宁含量。通过以金属尖穿刺叶片所需的力，对各物种的18片叶片进行测定，得到叶片相对韧性指数。将红海榄、木榄及角果木的衰老叶片装入网袋置于野外17天以获得腐解叶片，同时测定这些腐解叶片的碳氮含量与叶片韧性。

在实验室中，以同区域采集的红海榄、木榄、角果木及白骨壤的衰老叶片为饵料，对47只梅萨厚纹蟹（Sesarma messa）和10只史氏厚纹蟹（Sesarma smithii）的取食偏好进行测试。这些螃蟹均采自弗格森角（Cape Ferguson）。将各物种的叶片对半切开，称重后向每只螃蟹投喂各物种的半片叶片，剩余半片叶片作为对照以校正自然重量损耗。实验与对照容器内的叶片残体于24小时后取出，干燥称重。取食速率以每24小时内每克螃蟹体重对应的叶片干重毫克数计算。

针对24只梅萨厚纹蟹开展第二轮类似实验，同时向其提供红海榄、木榄、角果木的衰老叶片与腐解叶片。

野外实验于1989年旱季（4月至11月）开展，共设置5个采样点：弗格森角4个，欣钦布鲁克岛传教湾珊瑚溪（Coral Creek, Missionary Bay, Hinchinbrook Island）1个。

在所有野外采样点开展叶片选择实验，所用材料为红海榄、木榄、角果木及白骨壤的叶片。将各物种的20片衰老叶片称重、标记后，通过叶柄系于细绳，再将细绳绑定至支柱根或树干上。随机布设叶片，24小时后取回并统计被拖入洞穴的叶片数量。挖掘洞穴并收集洞壁上的所有凋落物，将所有未被取食的叶片残体冲洗、干燥后称重，以干重损失百分比计算叶片消耗量。

在弗格森角的3个沿潮间带梯度布设的采样点开展实验，以探究螃蟹是否会将叶片置于洞穴中腐解后再取食。在每个采样点采集60片该区域优势树种的叶片，称重、标记、系缚后布设。分别于布设后24小时、8天及17天取回20片叶片，并挖掘洞穴以获取叶片残片，将残体干燥后称重。设置对照实验：将叶片装入网袋分别置于泥面与洞穴内，于相同时间点取回。将对照叶片干燥后重新称重以计算重量损失，随后研磨成粉并测定其碳氮含量。

1989年10月至11月，在弗格森角对30只梅萨厚纹蟹的取食行为进行监测。每次30分钟的观测始于螃蟹离开洞穴之时，记录其取食及其他活动的类型与持续时长。

本研究通过野外与室内实验，旨在明确以下四个科学问题：
1. 相手蟹科（Sesarmidae）螃蟹是否对特定叶龄或物种的叶片存在取食偏好；
2. 本研究所选用的两种螃蟹是否具有不同的取食偏好；
3. 螃蟹是否会将凋落物储存在洞穴中较长时间后再行取食；
4. 除叶片凋落物外，其他食物是否在红树林螃蟹的食谱中占据重要地位。