This study has determined the diet of the demersal Knifejaw Oplegnathus woodwardi in depths of 50–200 m off the southern coast of Western Australia, where the benthic invertebrate epifauna is overwhelmingly dominated by sponges. The many fused teeth that form the parrot-like beak of O. woodwardi are used in conjunction with strong muscular plate-like jaws to shear/chop off pieces of sponge and crush the spicule-containing skeleton. Despite the potentially formidable physical and chemical defences (siliceous spicules and secondary metabolites) of sponges, these invertebrates constitute the main prey of O. woodwardi. Sponges were thus ingested by 44% of O. woodwardi and contributed 38% to the volume of the stomach contents across a wide length range of fish. The volumetric contribution of sponges was, however, far less in the stomachs than intestines, whereas the reverse was true for teleosts. This is presumably due to sponges undergoing less digestion than the externally soft-bodied teleosts as food passes through the gut, which is consistent with the large numbers of mainly intact spicules present in the intestine. Since the poriferan prey consisted almost exclusively of species of the Tetractinellida, even though there are three other speciose sponge orders in the region where O. woodwardi feeds in southern Western Australia, this predator apparently selects the sponge taxa it ingests. The length of the siliceous spicules of the Tetractinellida often exceeds 2000 μm, a value nearly ten times that which acts as a deterrent in another fish predator-sponge interaction. Thus, despite possessing formidable defence mechanisms, tetractinellinids are palatable to O. woodwardi. The next most important prey after sponges was teleost fish (among which clupeids were relatively abundant), being ingested by 35% of individuals and comprising 28% of the stomach contents. Other major invertebrate prey comprised hard-bodied crustaceans (mainly decapod crabs), echinoderms, bivalves and cephalopods (with their hard beaks and internal shells), all of which were probably associated with the particular habitat occupied by tetractinellid sponges. These hard-bodied prey are processed by O. woodwardi through the crushing action of its strong jaw mechanism. It is beneficial for O. woodwardi to feed on tetractinellid sponges because they are sessile, palatable and very abundant in the environment of this predator and thus do not incur a high energetic cost of foraging.

中文翻译：

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温带中光水域中Knifejaw Oplegnathus woodwardi (Waite) 中海绵动物的特征和意义

这项研究确定了西澳大利亚南部海岸 50-200 m 深处的底层 Knifejaw Oplegnathus woodwardi 的饮食，那里的底栖无脊椎动物附生动物群以海绵为主。形成 O. woodwardi 鹦鹉状喙的许多融合牙齿与强壮的肌肉板状颚结合使用，以剪切/切碎海绵碎片并压碎含有骨针的骨骼。尽管海绵具有潜在的强大物理和化学防御（硅质针状体和次生代谢物），但这些无脊椎动物构成了 O. woodwardi 的主要猎物。海绵因此被 44% 的 O. woodwardi 摄入，并在很宽的长度范围内对胃内容物的体积贡献了 38%。然而，海绵的体积贡献是 在胃中的含量远低于肠道，而硬骨鱼则正好相反。这可能是由于当食物通过肠道时，海绵比外部软体硬骨鱼消化得更少，这与肠道中存在的大量主要完整的针刺一致。由于 poriferan 的猎物几乎完全由 Tetractinellida 的物种组成，即使在 O. woodwardi 在西澳大利亚南部觅食的地区还有其他三种特殊的海绵目，这种捕食者显然会选择它摄取的海绵类群。Tetractinellida 的硅质针状体的长度通常超过 2000 μm，该值几乎是在另一种鱼类捕食者-海绵相互作用中起到威慑作用的值的十倍。因此，尽管拥有强大的防御机制，tetractinellinids 对 O. woodwardi 来说是可口的。仅次于海绵的下一个最重要的猎物是硬骨鱼（其中鲽科相对丰富），被 35% 的个体摄取，占胃内容物的 28%。其他主要的无脊椎动物猎物包括坚硬的甲壳类动物（主要是十足目螃蟹）、棘皮动物、双壳类动物和头足类动物（带有坚硬的喙和内壳），所有这些都可能与四肢海绵动物占据的特定栖息地有关。这些坚硬的猎物由 O. woodwardi 通过其强大的颚机构的粉碎作用进行加工。O. woodwardi 以 tetractinellid 海绵为食是有益的，因为它们无柄、可口且在这种捕食者的环境中非常丰富，因此不会产生高能量的觅食成本。海绵之后的下一个最重要的猎物是硬骨鱼（其中鲽鱼相对丰富），被 35% 的个体摄取，占胃内容物的 28%。其他主要的无脊椎动物猎物包括坚硬的甲壳类动物（主要是十足目螃蟹）、棘皮动物、双壳类动物和头足类动物（带有坚硬的喙和内壳），所有这些都可能与四肢海绵动物占据的特定栖息地有关。这些坚硬的猎物由 O. woodwardi 通过其强大的颚机构的粉碎作用进行加工。O. woodwardi 以 tetractinellid 海绵为食是有益的，因为它们无柄、可口且在这种捕食者的环境中非常丰富，因此不会产生高能量的觅食成本。海绵之后的下一个最重要的猎物是硬骨鱼（其中鲽鱼相对丰富），被 35% 的个体摄取，占胃内容物的 28%。其他主要的无脊椎动物猎物包括坚硬的甲壳类动物（主要是十足目螃蟹）、棘皮动物、双壳类动物和头足类动物（具有坚硬的喙和内壳），所有这些都可能与四肢海绵占据的特定栖息地有关。这些坚硬的猎物由 O. woodwardi 通过其强大的颚机构的粉碎作用进行加工。O. woodwardi 以 tetractinellid 海绵为食是有益的，因为它们无柄、可口且在这种捕食者的环境中非常丰富，因此不会产生高能量的觅食成本。