Silk and mucus production in tube- and mast-building species
Nonpereopodal secretionsnext section
The amphipod silk produced by pereopods 3-4 is a very important component of tubes and masts. However, in some cases, additional secretions produced by other appendages are exploited for mast or tube building (Mattson and Cedhagen, 1989; Goodhart, 1939), and these secretions are likely presented as glues. Their importance may be underestimated, especially as mouthparts and gnathopods are very often involved in material collection (Skutch, 1926; Meadows and Reid, 1966; Dixon and Moore, 1997). However, at least in Crassicorophium bonellii and Lembos websteri, the pereopodal silk also includes a mucopolysaccharide adhesive component (Kronenberger et al, 2012a,b). It remains unclear how widespread is the use of additional secretions among tube- and mast-building corophiids.
The tip of one of the examined masts was covered with threads (Fig. 4A) that were thinner than those in the remaining mast surface (Fig. 4B-C). They could be considered mucus threads, but according Mattson and Cedhagen, 1989, the mucus threads of Dyopedos monacantha are more than 10 times thicker than silk threads, and the mouthpart origin of the thin threads is doubtful. Because pereopods 3 and 4 are practically identical and likely to produce threads with similar diameters, we can offer two hypothesizes: threads of different diameters reflect (A) changes in thread diameter (for example, depending on the tension) or (B) shared building activity between several amphipods of different size. We did not detect threads with changing diameters, which supports the second hypothesis.
The mast SEM photos (Fig. 3E-G, and Fig. 4A-D) showed various detritus and silk threads, but we did not observe anything that could be accurately interpreted as cement or glue (amorphous mucus). We proposed that the mucus quantity is not great or that it is washed out during experimental fixation and treatment procedures. However, the fixed masts maintained such characteristics as form and flexibility. Therefore, one possibility is that mucus is important during the first stages of mast building, but silk threads subsequently provide the strength.
Pereopod glandular complex composition
Compositions of the pereopod 3-4 glandular complex are similar in Dyopedos bispinis and tube-building species. All studied corophiid species have two gland groups, proximal and distal, a common cuticular chamber and a single excretory opening (Nebeski, 1880; Kronenberger et al; 2012b; Neretin, 2016, and Fig. 5A). Dyopedos bispinis proximal glands (D1) are multicellular and have a strongly elongated form (i.e., “pseudotubular” glands, see discussion below in sections 5.4-5.6 and Fig. 9D-H), as has been observed in most other corophiid species (Nebeski, 1880; Neretin, 2016), likely with the exception of Crassicorophium bonellii and Lembos websteri (Kronenberger et al., 2012b). It is not clear whether the distal glands (D2) or Dyopedos bispinis are multicellular like the proximal glands (D1) or whether each D2 secretory cell has an individual duct. Both variants have been identified in other amphipods: multicellular distal glands in Ampithoe spp. (Nebeski, 1880; Neretin, 2016) and glands including a single secretory cell in J. falcata (Nebeski, 1880).
According to Kronenberger et al. (2012b), the Crassicorophium bonellii and Lembos websteri glandular complexes comprise both rosette (as in Fig. 9C) and lobed glands. According to our findings, Dyopedos bispinis and Ampithoe rubricata glands are not typical rosette or lobed but have some common features with both these gland types (facultative binuclearity, presence of deep cell membrane invaginations and cellular composition, see discussion section 5 and Fig. 9). Considering that another species of Crassicorophium (Crassicorophium crassicorne) has elongated glands typical of other amphipods (Nebeski, 1880), these differences most likely reflect differences in terminology. On the other hand, the schematics by Kronenberger et al., (2012b) show typical rosette glands. Thus, it remains in question whether the structure of the pereopodal glands of Crassicorophium bonellii and Lembos websteri is unique among other corophiids.
Fig. 9. Comparative schematic drawings of Dyopedos bispinis proximal glands (D1) and other crustacean tegumental glands. A, B and C – several types of crustacean tegumental glands modified from Talbot and Demers, 1993 and Rieder, 1977: A – bicellular glands, B – tricellular glands, C – rosette glands; D, E and F – pseudotubular silk glands of Amphipoda: D – A. rubricata (after Neretin, 2016), E – J. falcata (modified from Nebeski, 1880), F – Dyopedos bispinis; G and H – tube-building glands of the tanaid H. oerstedii (Kroyer 1842) (after Blanc, 1884), also likely pseudotubular; I – cirripedian cement glands (modified from Lacombe and Liguori, 1969). Abbreviations: as – accumulation site; c – cuticle; CC – central cell; DC – duct cell; hyp – hypoderm; IC – intermediary cell; LD – lateral duct; MD – main duct; nu, nu1 and nu2 – nucleuses of secretory cells; SC – secretory cell; ySC – young secretory cells.
Volume of pereopodal glandular complex
Although the Dyopedos bispinis masts are large structures compared with the length of the amphipod body, we have not observed Dyopedos bispinis glands to be strongly enlarged compared with tube-building species. The number of secretory cells (approximately 70 in each Dyopedos bispinis pereopod, Fig. 5B, 8A) is lower than in other species (about 130 in Crassicorophium bonellii, based on figures in Kronenberger et al., 2012b, and more than 400 in Ampithoe rubricata, N. Neretin, unpublished data), and the cell size is also minimal in Dyopedos bispinis. These facts might reflect small body size of the White Sea Dyopedos bispinis compared with other studied amphipods.
The secretory granule ultrastructure is similar in the studied species (Dyopedos bispinis, Ampithoe rubricata, and Crassicorophium bonellii, Kronenberger et al., 2012b; Neretin, 2016); granules of proximal gland groups are electron dense and have a normal, round shape (Fig. 6D, H, and Fig. 7C-D), unlike distal group granules (Fig. 8B-E).
The distal glands of Dyopedos bispinis may produce several different substances because D2 secretory cells contain different types of granules (Fig. 8D). Two morphologically different types of granules are presented in Ampithoe rubricata distal glands (Neretin, 2016), and only uniform granules are found in Crassicorophium bonellii distal glands (Kronenberger et al., 2012b).
Secretion for tube and mast building summary
In general, we did not detect any crucial differences in the glandular complex structure and forms of secretions in tube-building species and the mast-builder Dyopedos bispinis. The variability in the dwellings most likely reflects behavioural adaptations.