Stylet jaws of Chrysopetalidae (Annelida)

Micro-CT imaging elucidates jaw and pharynx structure in 14 nominal taxa belonging to Chrysopetalinae, Dysponetinae and Calamyzinae (Chrysopetalidae: Annelida). Systematic evaluation of chrysopetalid jaw form in each taxon is used to compare inter-generic relationships within and between each subfamily and with other polychaete families possessing lateral pairs of jaws. Jaw morphology proves diagnostic at all levels including agreement with current molecular phylogenetic analyses of the Chrysopetalidae. The greatest diversity of pharynx and jaw form, associated with sensory body characters, is found amongst taxa of the epibenthic Chrysopetalinae. It is hypothesized that the paired stylet jaws interlock to pierce prey tissue, and, in concert with a highly muscular pharynx, suck out prey fluids that pass down the internal groove of the stylets. Three generic groups are identified based on congruence of jaw form, and novel buccal structures are revealed in some taxa: a pharyngeal calcareous ‘ring’ and extensive pharyngeal glands. A continuum across free-living and facultative symbiont lifestyles is represented across all Chrysopetalinae. Dysponetinae comprises free-living, very small individuals that exhibit a meiofaunal lifestyle. All species possess one type of simple, tanned rod-like stylet jaws and certain simplified body structures. Calamyzinae species possess polyphyletic jaw forms and grades of radically simplified sensory morphologies. Free-living, bacteriovore calamyzins inhabit extreme chemosynthetic habitats and lack jaws, or possess modified jaws composed of an anterior platelet and posterior grooved jaw, present in the larvae and lost in the adult. New observations of an ectoparasitic Calamyzinae include details of a specialized mouth opening and stylet jaw present in a species that exhibits cryptic coloration, while grooved jaws are revealed for the first time in an obligate symbiont calamyzin. The grooved jaw form is considered a non-homoplasious synapomorphy that supports the monophyly of the family Chrysopetalidae.

显微计算机断层扫描（Micro-CT）成像技术阐明了隶属于环节动物门（Annelida）金扇虫科（Chrysopetalidae）下的金扇虫亚科（Chrysopetalinae）、Dysponetinae亚科（Dysponetinae）以及Calamyzinae亚科（Calamyzinae）的14个命名类群的颌骨与咽部结构。本研究通过对每个类群中金扇虫科颌骨形态的系统性评估，比较了各亚科内部、亚科之间，以及与其他具有侧生颌骨对的多毛类（Polychaeta）科之间的属间亲缘关系。颌骨形态在所有分类阶元上均具备鉴定价值，这与当前针对金扇虫科的分子系统发育分析结果一致。与躯体感官特征相关的咽部与颌骨形态最大多样性，见于底栖生活的金扇虫亚科（Chrysopetalinae）类群中。本研究推测，成对的针状颌骨（stylet jaws）会相互咬合以刺穿猎物组织，并与高度发达的肌肉质咽部协同作用，吸出经由针状颌骨内部沟槽输送的猎物体液。基于颌骨形态的一致性，本研究鉴定出三个类群，且部分类群中发现了全新的口部结构（buccal structures）：咽部钙质“环”以及广泛分布的咽部腺体。金扇虫亚科的所有类群均呈现出从自由生活（free-living）到兼性共生体（facultative symbiont）生活方式的连续谱系。Dysponetinae亚科包含营小型底栖生物（meiofaunal）生活方式的自由生活个体。该亚科所有物种均具有一种简单的、鞣化的杆状针状颌骨，以及部分简化的躯体结构。Calamyzinae亚科的类群具有多系起源的颌骨形态，以及一系列极度简化的感官形态特征。自由生活的噬菌性Calamyzinae类群栖息于极端化能合成生境（chemosynthetic habitats）中，既无颌骨，也拥有由前部骨板与后部沟槽状颌骨组成的特化颌骨，此类特化颌骨仅见于幼虫阶段，成虫阶段会丢失。本研究对一种外寄生性（ectoparasitic）Calamyzinae类群的新观察结果，详细描述了该物种特化的口部开口与针状颌骨，该物种具有隐蔽色（cryptic coloration）；同时首次在专性共生体（obligate symbiont）Calamyzinae类群中发现了沟槽状颌骨。沟槽状颌骨形态被视为非同塑性共源性状（non-homoplasious synapomorphy），这一特征支持金扇虫科的单系起源。