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

本研究测定了与食物偏好相关的叶片特征。实验所用的4个物种分别为白骨壤（*Avicennia marina*）、红海榄（*Rhizophora stylosa*）、木榄（*Bruguiera exaristata*）与角果木（*Ceriops tagal*），每个物种取10片衰老叶片开展测定。先对叶片称重，随后置于65℃烘箱中干燥5天，再次称重以建立干重-湿重回归关系，供后续实验使用。随后从每个物种中取3片干燥叶片研磨，分析其有机质占比、碳氮占比与单宁含量。针对每个物种的18片叶片，通过测定金属针穿刺叶片所需的力，计算得到叶片相对韧性指数。此外，本研究还测定了红海榄、木榄与角果木腐解叶片的碳氮含量与叶片韧性：将各物种的衰老叶片装入网袋后置于野外17天，以此获取腐解叶片样本。

本研究在实验室中，利用费格森角（Cape Ferguson）捕获的47只相手蟹（*Sesarma messa*）与10只斯密特相手蟹（*Sesarma smithii*），以及同一区域来源的上述4种植物的衰老叶片，开展取食偏好实验。将每个物种的叶片对半切开，称重后为每只螃蟹提供每种植物的半片叶片，剩余半片叶片作为对照，用于校正自然重量损失。24小时后移除实验与对照容器中的残留叶片，干燥后称重。取食率以‘每克蟹体重对应的24小时内摄入的叶片干重毫克数’计算。随后针对24只相手蟹开展第二组平行实验，同时为其提供上述3种植物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：费格森角的4个样点，以及欣钦布鲁克岛传教士湾珊瑚溪（Coral Creek, Missionary Bay, Hinchinbrook Island）的1个样点。在所有野外采样点开展叶片选择实验，所用叶片仍为上述4种植物的叶片。将每个物种的20片衰老叶片称重、标记后，通过叶柄将叶片系于细绳上，再将细绳绑定至支柱根或植株上。随机布置叶片，24小时后回收并统计被拖入洞穴的叶片数量。随后挖掘洞穴，收集洞穴壁上的所有凋落物。将所有未被取食的叶片材料冲洗、干燥后称重，以干重损失百分比计算叶片消耗量。

在费格森角的3个沿潮间带梯度分布的采样点开展预实验，以验证螃蟹是否会在取食前将叶片置于洞穴中腐解。针对每个采样点的优势树种，采集60片叶片，称重、标记、系绳后布置。分别在布置后24小时、8天与17天回收20片叶片，并挖掘洞穴以获取叶片残片，干燥后称重。同时设置网袋对照组：将叶片装入网袋后分别布置于泥滩表面与洞穴中，于相同日期回收，干燥后重新称重以计算重量损失；随后将其研磨成粉，测定碳氮含量。

1989年10月至11月，在费格森角对30只相手蟹（*Sesarma messa*）的取食行为展开监测。每一次30分钟的观测从螃蟹离开洞穴时开始，记录其取食与其他活动的类型与持续时长。

本研究通过野外与室内实验，旨在解答以下4个科学问题：
1. 相手蟹是否对特定树龄或物种的叶片存在取食偏好？
2. 本研究选取的2种相手蟹是否具有不同的食物偏好？
3. 螃蟹是否会将凋落物储存在洞穴中较长时间后再取食？
4. 除叶片凋落物外，其他食物是否在红树林蟹类的食谱中占据重要地位？