Structure of masts
Mast formsnext section
Dyopedos bispinis structures are representative of the masts of typical dulichiids (Fig. 1A); they are usually 2-6 cm long and rarely reach 16-20 cm. Except for the smallest, masts have variable diameters but reach a maximum at the base and a minimum nearer the tip. The diameter in the middle of the masts varies between 0.02 (smallest) and 0.1 cm.
Internal mast structure
Masts comprise a relatively homogeneous central cylinder (C, diameter 70-460 µm) and a laminated cortex (P, from 440 µm to 20 µm thick and thinner), but the boundary between these structures is occasionally fuzzy (Fig. 1A-D and Fig. 3A). The size of the cortex varies greatly and might account for approximately 0 to 70% of the mast diameter. Along the mast length, the diameter of the cylinder varies only slightly (190-270 µm), but the cortex gradually thins toward the tip of the mast (Fig. 1A-D).
The lower portion of the mast often contains several central cylinders (several smaller masts, C1-C4, Fig. 1A, D), and one cylinder is typically thicker than the others (C-1 m). Each cylinder is coated with its own cortex (P-1, P-2), but together they share one common coat (P-c).
The central cylinder of the mast is composed of closely packed detritus; we did not detect any cementing mucus. The cortex of the masts contains amphipod silk (as) and detritus. A thin cortex (up to 20 µm) might be almost entirely composed of silk (Fig. 1A-B, Fig., 4A), but when the cortex is even slightly thicker, the proportion of detritus is much larger than that of silk (Fig. 1A, C, F). The detritus is represented by isolated or groups of particles surrounded by silk (Fig. 3A-B) or detritus layers (DL) alternating with silk layers (ASL, Fig. 1F). Under mechanical impact, the cortex of the dried mast exfoliates and becomes separate layers of approximately 5 µm in thickness (Fig. 1E).
Numerous diatoms are present within masts (da, bda, pda, Tn, Fig. 3D-E). Some are pelagic (pda, Tn), and others are large and likely benthic species (bda). Diatoms have not been observed on the mast surface outside the silk layer.
Fine mast structure
The mast surface is covered with a layer of silk threads, and the thickness of the silk layer varies (Fig. 4A-C). The silk threads are oriented in all directions and are sometimes almost parallel to the mast axis. Frequently, regular shifts in the parallel threads (approximately 5 µm) are observed (arrowheads, Fig. 4B-C). Furthermore, TEM photos suggest that the silk layer contains sublayers (L1-L6) that likely differ based on their angle of orientation (various directions or perpendicular to the mast axis), thickness and density (Fig. 4E).
Silk threads are 0.1-0.3 µm in diameter, and threads of varying widths (t1, t2) might be observed on the same mast (Fig. 4D). However, we did not observe threads with changing diameters in the photos. The surfaces of the analysed mast tips were covered with thinner threads (0.1 µm) (arrows, Fig. 4A), whereas the remainder of the mast was coated with 0.3-µm threads (arrowheads, Fig. 4B-C).