Video observation of surface exploration in cyprids of <i>Balanus amphitrite</i>: the movements of antennular sensory setae

Video microscopy of cyprids of Balanus amphitrite was used to monitor the action of antennular setae during the exploratory behaviour prior to attachment. In addition, SEM was used to provide a revised description of all antennular setae for that species. The videos describe if a particular seta touches the substratum and the area it can cover during surface exploration. On the fourth segment, the plumose terminal setae A and B are never in contact with the substratum, lack a terminal pore and it is argued that they sense hydrodynamic forces. The aesthetasc-like terminal seta D is likewise held free in the water at all times and it is speculated that it senses dissolved substances, but, since it contains a scolopale rod, it must also have a mechano-receptive function. All remaining antennular setae on the second, third and fourth segments have a terminal pore and it is argued that these are bimodal receptors with both chemo- and mechano-receptive modalities. These setae are also at one time or another in contact with the substratum, except perhaps for the small preaxial seta 2 and terminal seta C. The first seta to contact the surface during a tentative step is radial seta 5, which is longer than all other radial setae. All other setae on the second and third segment are only in contact after a step is completed. When the attachment disc touches the surface (=a step completed) the long and curved postaxial seta 2 (on the second segment) and postaxial seta 3 on the third segment are both flexed to either side of the antennule. This lateral displacement ensures that these two setae can touch large surface areas to either side of the appendage. The four subterminal setae on the fourth segment contact the surface both immediately before and after a step has been completed, and the constant flicking of the segment significantly increases the surface area tested by both these chemoreceptors and by terminal seta E, which can sweep up to 60 μm laterally from the attachment disc. The flicking of the fourth segment may also serve to dilute the boundary layer of chemoreceptors on the fourth segment such as the aesthetasc-like terminal seta D and thus facilitate the detection of new stimuli.本研究采用视频显微术（Video microscopy）对纹藤壶（Balanus amphitrite）腺介幼体（cyprids）进行观测，以监测其附着前探索行为过程中触角刚毛（antennular setae）的运动状态。此外，本研究借助扫描电子显微镜（SEM）对该物种的全部触角刚毛进行了修订版形态描述。视频观测可记录单根刚毛是否接触基底，以及其在表面探索过程中可覆盖的区域范围。 在第四体节上，羽状端刚毛A与B始终未与基底接触，且不具备端孔（terminal pore），据此推测其可感知流体动力（hydrodynamic forces）信号。嗅毛样端刚毛D同样始终游离于水体中，虽有推测认为其可感知溶解态物质，但因其内部含有感杆（scolopale rod），故必然同时具备机械感受功能。 第二、第三及第四体节上其余所有触角刚毛均带有端孔，据此认为这类刚毛属于双模态感受器，兼具化学感受与机械感受功能。上述刚毛均会在不同时刻与基底接触，仅第二体节上的小型前轴刚毛2与端刚毛C可能例外。 在试探性迈步过程中，最先接触表面的刚毛为放射状刚毛5，其长度长于其余所有放射状刚毛。第二与第三体节上的其余刚毛仅会在迈步完成后与基底接触。 当附着盘接触表面（即迈步完成）时，位于第二体节的长而弯曲的后轴刚毛2，以及第三体节的后轴刚毛3均会向触角两侧弯曲。这种侧向位移可确保这两根刚毛可触及附肢两侧的大面积区域。 第四体节上的四根亚端刚毛会在迈步完成前后均接触表面，而该体节持续的摆动动作可显著提升两类化学感受器的探测表面积：其一为上述亚端刚毛，其二为端刚毛E——该刚毛可从附着盘侧向扫过最大60μm的范围。 第四体节的摆动动作还可稀释第四体节上化学感受器（如嗅毛样端刚毛D）的边界层，从而助力新刺激的检测。