Contributions to Zoology, 86 (2) – 2017Nikolai Y. Neretin; Anna E. Zhadan; Alexander B. Tzetlin: Aspects of mast building and the fine structure of “amphipod silk” glands in Dyopedos bispinis (Amphipoda, Dulichiidae)
Results

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Substrata for mast building and formation of the basal section

Dyopedos bispinis masts can be attached to solid substrata (stones, shells of dead or living molluscs and brachiopods) or to other fouling organisms including hydroids, bryozoans, ascidians and sponges; masts rarely occur on soft ground (sand and mud). Examination of the collected masts showed that the most frequent substrata are other organisms, particularly hydroids (Gonothyraea loveni (Allman, 1859), Ectopleura larynx (Ellis & Solander, 1786), Eudendrium sp., Sertularia mirabilis (Verrill, 1873)), bryozoans (Eucratea loricata (Linnaeus, 1758), Flustra sp., and Scrupocellaria sp.), sponges (presumably Leucosolenia complicata (Montagu, 1814)) and, occasionally, amphipod tubes (Crassicorophium bonellii).

On solid substrates, the bases of the masts are disk-like or weakly thickened, but on bush-like hydroid and bryozoan colonies, the masts are attached to the tips of branches (Fig. 2A-C). These masts typically coat a section of the colonies heading downward (H+M), often reaching the base, so that they appear to have been built directly on the solid substrate, without the hydroid or bryozoan.

When more than one mast attaches to a hydroid colony, a ramified mast occasionally results (arrows, Fig. 2A, D). Large masts typically have several supports; i.e., they are composed of several independent hydroid or bryozoan branches combined or even several smaller masts (m, m1, m2, m3) that are attached to different individual branches (Fig. 2A, B).

Some hydroids coated with mast remain alive, and their hydranths are free of masts and have tentacles (Fig. 2A, C). Other branches are completely immured (3, Fig. 2A).

FIG2

Fig. 1. Internal structure of a Dyopedos bispinis mast. A – drawing of a Dyopedos bispinis mast. B1, C1 and D1 - scanning electron microscopy (SEM) micrographs of mast cross sections at different levels; B2, C2 and D2 – their schematic representations. E and F – SEM micrographs of masts with damaged cortexes, showing that the cortex always comprises amphipod silk layers and, sometimes, detritus layers. Abbreviations: ASL, ASL-1, and ASL-2 – silk layers of the laminated cortex; C – central cylinder of the mast; C-1 m – the main cylinder; C-2, C-3, C-4 – additional cylinders; DL-1 – detritus layer of the laminated cortex; f – Dyopedos bispinis female; m – Dyopedos bispinis male; P – laminated cortex of the mast; P-1 – laminated cortex of the main cylinder; P-2 – laminated cortex of an additional cylinder; P-c – laminated cortex covering all cylinders.

FIG2

Fig. 2. Attachment of Dyopedos bispinis masts to hydroids and bryozoans, multi-supporting and branching masts. А – schematic drawing of two masts attached to a hydroid colony, showing that hydroid branches can be immured by Dyopedos bispinis to varying degrees; B and C – light microscopy (LM) photos showing mast attachment to bryozoans (B) and hydroids (C); D - underwater photo of a branching mast. Abbreviations: 1 – free hydroid branch; 2 – partially immured hydroid branch continuing as a single-supporting mast; 3 – completely immured hydroid branch continuing as a multi-supporting mast; arrows – branching sites of masts; arrowhead – filamentous algae included in mast cortex; fh – free hydranth; H – free section of hydroid branch; ih – partially immured hydranth; m, m1, m2 and m3 – basal masts forming multi-supporting mast; M – mast itself; H+M – basal section of mast built around a hydroid branch; bry – bryozoan branches.

FIG2

Fig. 3. Internal structure of a Dyopedos bispinis mast including diatoms and multicellular algae. А – drawing of a semi-thin mast cross-section; B – enlarged detail of A, LM micrograph; C and D – LM micrographs of semi-thin sections illustrating algae included in the central mast cylinder; E and F – SEM micrographs of mast surface showing the diatom Thalassionema nitzschioides; G – SEM micrograph of the inside of the mast laminated cortex showing the presence of numerous pelagic and benthic diatoms. Arrowheads – mucous pads connecting T. nitzschioides cells; as – silk threads; bda – likely benthic diatoms; C – central cylinder of the mast; da – diatoms; ma – multicellular algae; P – laminated cortex of the mast; pda – likely pelagic diatoms; Tn – diatom Thalassionema nitzschioides.