Mast formnext section
Mast building by Dyopedos bispinis on hydroids occasionally leads to the appearance of branching masts (Fig. 2D), and this phenomenon is unknown in other dulichiids. Dyopedos bispinis are apparently territorial, and it remains unknown how these organisms share areas of conflict on the branching masts. Masts with several supports (Fig. 2A-B) have also not been previously described, and whether each individual builds additional masts or utilizes abandoned masts remains unknown.
Internal structure and growth of the masts
The mast of Dyopedos bispinis comprises a homogeneous central cylinder and a laminated cortex (Fig. 1A-D and Fig. 3A-B), and the mast surface is covered with a silk sheath (Fig. 1F and Fig. 4), confirming the observation of Mattson and Cedhagen (1989), who reported that mast construction begins with the formation of the detrital central region and continues through the production of silk layers. However, in addition to silk, the laminated cortex includes a large amount of detritus (Fig. 1C-D and Fig. 3A-B), and it is likely that detritus layers alternate with silk layers (Fig. 1F). Moreover, the thickness of the cortex is occasionally comparable to the radius of the central cylinder (Fig. 1C-D).
Thus, we propose that there are different methods for developing the mast for different purposes: (A) mast reinforcement with only silk threads (described by Mattson and Cedhagen (1989)) and (B) mast thickening with detritus and silk. It remains unknown whether thickening is realized by active amphipod activities or by passive detritus settlement and gluing.
Detritus for mast building and maintenance can likely be collected from both the water column and from the bottom because both planktonic and benthic diatoms are present in the mast (bda, pda, Tn in Fig. 3E-G). Both methods were described for Dyopedos monacantha (Mattson and Cedhagen, 1989).
Mast thickening is likely necessary as mast size increases with amphipod growth. The design of the mast facilitates growth without structural disturbance. In contrast, tube-building amphipods, according to the observed data, occasionally (a) leave old dwellings and build new, more spacious tubes, (b) stretch existing tubes, and (c) break old tubes into pieces and build new tubes utilizing these pieces during body growth (Goodhart, 1939; Barnard et al., 1988). Thus, gradual build up is at least occasionally not an option for tube-building amphipods, unlike mast builders.
The surface of a Dyopedos bispinis mast is covered with silk threads that are oriented in various directions and in many layers (Fig. 3E, and Fig. 4A-D). For Dyopedos monacantha, it was observed that under conditions with strong currents, the silk threads are placed at different angles to increase mechanical strength. We collected Dyopedos bispinis masts from straits with strong tidal currents, and there was no prevailing silk direction in these conditions.
When covering the mast with silk, each Dyopedos monacantha pereopod 3-4 tip moves back and forth “along an ample half of the circumference of the mast” (Mattson and Cedhagen, 1989), and this spinning procedure gradually proceeds along the mast (Mattson and Cedhagen, 1989). In Dyopedos bispinis, regular parallel thread deposition was sometimes observed, and remarkably, a nearly rectangular, meshy structure was occasionally observed (Fig. 4B-C). A similar structure was identified by Moore and Earll (1985) in SEM photos of Dyopedos porrectus masts, and according to these authors, “the resultant meshwork is reminiscent of geodetic construction of aircraft fuselages and would confer relative strength together with flexibility”. Similar silk structures were also observed in tube-building species, such as Crassicorophium bonellii and Peramphithoe femorata (Cerda et al., 2010; Kronenberger et al., 2012a).