Meyer Christian, André Thomas, Purschke Günter
Department of Biology and Chemistry, Zoology and Developmental Biology, University of Osnabrück, Barbarastr. 11, 49069, Osnabrück, Germany.
BMC Zool. 2021 Mar 9;6(1):5. doi: 10.1186/s40850-021-00068-8.
The sedentary polychaete Sabellaria alveolata, the sandcastle or honeycomb worm, possesses four different kinds of appendages besides the parapodia: opercular papillae, tentacular filaments, palps, and branchiae. It exhibits a highly specialized anterior end, the operculum, formed by the prostomium, peristomium, and two anterior segments. The operculum comprises opercular papillae, tentacular filaments, and palps. Paired branchiae are present from the second thoracic chaetiger onwards on the posteriorly following segments except for the last ones. Ultrastructural data on these appendages are either scanty, incomplete, or even lacking in Sabellariidae. In order to analyze their functional morphology, to bridge the data gap, and providing data for future phylogenetic and evolutionary analyses, we investigated the appendages of S. alveolata by applying light microscopy, confocal laser scanning microscopy, scanning, and transmission electron microscopy.
In S. alveolata the entire body is covered by a thin cuticle characterized by the absence of layers of parallel collagen fibers with no differentiation between the various body regions including the branchiae. The opercular papillae bear numerous tufts of receptor cells and lack motile cilia. The tentacular filaments show a distinctive pattern of motile cilia. Their most conspicuous morphological feature is a cell-free cartilaginous endoskeletal structure enclosed by ECM. Besides musculature the filaments include a single coelomic cavity but blood vessels are absent. The palps are ciliated and possess two coelomic cavities and a single blind-ending internal blood vessel. Besides external ciliation and receptor cells, the coelomate branchiae are highly vascularized and equipped with numerous blood spaces extending deep between the epidermal cells resulting in low diffusion distances.
All appendages, including the branchiae, bear receptor cells and, as such, are sensory. The opercular papillae resemble typical parapodial cirri. In contrast, the tentacular filaments have a triple function: sensing, collecting and transporting particles. A similarity to branchiae can be excluded. The palps are typical grooved palps. A revised classification of polychaete branchiae is suggested; thereby, the branchiae of S. alveolata belong to the most common type comprising coelom, musculature, and blood vessels. The results indicate that diffusion distances between blood and environment have been underestimated in many cases.
固着生活的多毛纲动物蜂窝沙蚕，即沙堡虫或蜂窝虫，除了疣足外还拥有四种不同类型的附肢：鳃盖乳头、触手丝、触须和鳃。它具有一个高度特化的前端，即鳃盖，由口前叶、围口节和两个前部体节组成。鳃盖包括鳃盖乳头、触手丝和触须。从第二胸节体节开始，除最后几个体节外，后续体节上都有成对的鳃。关于这些附肢的超微结构数据在沙蚕科中要么稀少、不完整，甚至完全缺失。为了分析它们的功能形态，填补数据空白，并为未来的系统发育和进化分析提供数据，我们应用光学显微镜、共聚焦激光扫描显微镜、扫描电子显微镜和透射电子显微镜对蜂窝沙蚕的附肢进行了研究。
在蜂窝沙蚕中，整个身体被一层薄的角质层覆盖，其特征是没有平行胶原纤维层，身体各部位包括鳃之间没有分化。鳃盖乳头有许多成簇的感受器细胞，没有能动纤毛。触手丝显示出独特的能动纤毛模式。它们最显著的形态特征是由细胞外基质包围的无细胞软骨内骨骼结构。除了肌肉组织外，触手丝还包括一个单一的体腔，但没有血管。触须有纤毛，有两个体腔和一条单一的盲端内部血管。除了外部纤毛和感受器细胞外，有体腔的鳃血管高度丰富，有许多血腔深入表皮细胞之间，导致扩散距离较短。
所有附肢，包括鳃，都有成簇的感受器细胞，因此都是感觉器官。鳃盖乳头类似于典型的疣足触须。相比之下，触手丝具有三重功能：感知、收集和运输颗粒。可以排除与鳃的相似性。触须是典型的有沟触须。建议对多毛纲动物的鳃进行修订分类；因此，蜂窝沙蚕的鳃属于最常见的类型，包括体腔、肌肉组织和血管。结果表明，在许多情况下，血液与环境之间的扩散距离被低估了。
