Functional morphology of suction discs and attachment performance of the Mediterranean medicinal leech (Hirudo verbana Carena)

Abstract

Medicinal leeches use their suction discs for locomotion, adhesion to the host and, in the case of the anterior disc, also for blood ingestion. The biomechanics of their suction-based adhesion systems has been little understood until now. We investigated the functional morphology of the anterior and posterior suckers ofH irudo verbena by using light and scanning electron microscopy. Furthermore, we analysed the adhesion qualitatively and quantitatively by conducting behavioural and mechanical experiments. Our high-speed video analyses provide new insights into the attachment and detachment processes and we present a detailed description of the leech locomotion cycle. Pull-off force measurements of the anterior and posterior suction organs on seven different substrates under both aerial and water-submersed conditions reveal a significant influence of the surrounding medium, the substrate surface roughness and the tested organ on attachment forces and tenacities.

Keywords: attachment force; biomechanics; functional morphology; medicinal leech (Hirudo verbana); suction; surface roughness.

Figure 1.

Morphology of medicinal leech suction discs. The figure shows light microscopy (LM) and scanning electron microscopy (SEM) images of the anterior (a,c,d) and posterior (b,e,f) suction discs of H. verbana. (a,b) LM images of attached anterior and posterior suckers. The retracted velum of the anterior sucker exposes the three jaws and the oral cavity. (c–f) SEM images of the inner surfaces of both suction organs showing furrows (black arrows) and gland-like structures (white arrows). (c,e) Marginal regions of inner sucker surfaces. (d,f) Central regions of inner sucker surfaces. (Online version in colour.)

Figure 2.

CLSM images of the seven replicated test substrates including surface profiles showing their different texturing. (a–d) Surface replicas of natural substrates (Nymphaea sp., C. palustris, human skin, slate rock). (e–g) Surface replicas of artificial substrates (fine and rough sandpaper, acrylic glass). (h) Surface profiles of all seven test surfaces (R1–R7).

Figure 3.

Suction disc kinematics. Anterior attachment (a–e, frontal; f–g, lateral) and detachment (h–l, frontal; m–n, lateral) processes. Arrows mark the upper (black) and lower (white) margins of the suction organ. Posterior attachment (o–s, lateral; t–u, frontal) and detachment (v–z, lateral; zi–zii, frontal) processes. Images are frames from the electronic supplementary material, Videos S1–S8. Original brightness and contrast have been increased to improve the image clarity.

Figure 4.

Locomotion cycle of H. verbana. (a) Starting position. (b) Searching (including elongation wave propagation). (c) Start of anterior attachment process. (d) Start of posterior detachment and end of anterior attachment processes. (e) Posteroanterior shift of the body's centre of gravity and end of posterior detachment process (including contraction wave propagation). (f) Loop formation (exclusive for inchworm crawling). (g) Start of posterior attachment and anterior detachment processes. (h) Anteroposterior shift of the body's centre of gravity and end of posterior attachment process. (i) End of anterior detachment process and completion of locomotion cycle. The asterisk marks the body's centre of gravity. Dotted lines subdivide the leech body in head unit, 21 body segment units and tail unit. The central compass indicates the anterior (A), dorsal (D), posterior (P) and ventral (V) directions.

Figure 5.

Intensive mucus production during leech attachment. (a) The picture shows H. verbana during upside-down crawling along a metal wire mesh. Intensive mucus production is visible in the head region (mucus thread) and on the leech back (drop-like mucus accumulation). The beads-on-a-string configuration of the thread suggests visco-elastic properties of the leech mucus (cf. [35]). (b,c) Scanning electron microscopy images showing secretory glands and mucus residues on the inner sucker surfaces of the anterior (scale bar is 90 µm) and posterior attachment organs (scale bar is 60 µm), respectively.

Figure 6.

Parameters influencing the attachment performance of H. verbana. Increasing surface contact areas favour leech attachment, whereas increasing surface roughness as well as increasing interspaces between surface irregularities lower attachment performance.