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)
Discussion

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Social behaviour and mast-building in Dyopedos bispinis

Social structure on masts

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Dyopedos bispinis masts are typically inhabited by no more than one adult female and no more than one adult male, indicating that the territorial behaviour of Dyopedos bispinis is similar to that of other Dyopedos species (Mattson and Cedhagen, 1989; Thiel, 1997). Each large mast of Dyopedos monacantha or Dyopedos porrectus is a territory belonging to a single adult female. Only one male can enter, and female descendants can remain for some time on the maternal mast.

Adult male Dyopedos bispinis are occasionally observed on individual masts, but it is unknown whether these males, such as those of Dyopedos porrectus (Mattson and Cedhagen, 1989), can construct their own masts or whether, similar to male Dyopedos monacantha, they use abandoned masts, which are numerous in sea bottom landscapes.

Extended maternal care has been reported in some dulichiid species (McCloskey, 1970; Mattson and Cedhagen, 1989; Thiel 1997). Apparently, Dyopedos bispinis is no exception as numerous juveniles have been observed together with adult females on large masts.

Collective masts

In addition to the masts of Dyopedos bispinis being typical of species from the genus Dyopedos, in that they are usually inhabited by a single female, masts with two and more adult, and sometimes even ovigerous, females were observed. Mattson and Cedhagen (1989) reported Dyopedos monacantha masts with one large male and two smaller females (although not two large females), and McCloskey (1970) also observed masts with three nonbreeding adult Dulichia rhabdoplastis. However, among other building corophiids, the coexistence of several adult females in one dwelling is unknown (Thiel, 2007; Moore and Eastman, 2015). However, some leucothoid amphipods (Thiel, 2000) and other symbiotic malacostraca (Duffy 2002, 2007) have been described inhabiting the inner canals of sponges. The corophiid tube has only one or two openings for outside feeding and only a single place for intratubular filtration (Dixon and Moore, 1997). In contrast, if the mast is long enough, then multiple individuals could probably feed without hindering each other because Dyopedos spp. utilize the water current perpendicular to the mast axis for feeding (Mattson and Cedhagen, 1989). However, it is not clear why collective Dyopedos masts are not widespread. Some possible explanations include mechanical factors (current, waves, substrate quality) or fish-predation pressure, as reported for Dyopedos monacantha, (Mattson and Cedhagen, 1989) which could, in theory, increase with mast elongation.

It is also unknown whether the adult amphipods inhabiting one mast are relatives (is it a parent-offspring or conspecific association? See Thiel, 2011) and whether these crustaceans defend the mast from interspecific competitors. Most likely, these congested masts reflect the development of extended parental care, which has been described in Dyopedos spp. (Mattson and Cedhagen, 1989; Thiel, 1997). However, increasing levels of interspecific aggression typically force juveniles to leave the maternal mast (Mattson and Cedhagen, 1989), and the number of juveniles on the mast rapidly declines with growth (Thiel, 1997). It is possible that a certain aggression suppression mechanism could be involved that would allow the brood to remain on the mast. It is also unknown whether the mother of the brood remains on the mast and, consequently, whether adult amphipods from different generations can coexist on the mast. Parent-offspring groups, iteroparity and the coexistence of several offspring cohorts were observed in many percarids (Thiel, 2007), but overlapping generations (offspring that begin to reproduce in the presence of reproductive parents) and coexisting reproducing adult offspring in the maternal dwelling has not been reported (Thiel, 2007; Thiel, 2011). Although much remains to be discovered, Dyopedos bispinis exhibits the movement from a typical corophiid small parent-offspring group to more complex groups with different social structures.

Cooperative mast building

“Collective” masts are typically much longer than individual masts, likely reflecting cooperative mast maintenance. Indeed, all adults have pereopod glands, so it is difficult to identify a single builder. However, “individual” masts might also be considered a product of cooperative building; for example, Dyopedos monacantha males share the maintenance of female masts, using pereopods 3-4 (Mattson and Cedhagen, 1989). Dyopedos bispinis males have developed pereopod glands, and it is quite likely that they also use these structures. Moreover, these glands are developed in the youngest Dyopedos bispinis juveniles. Silk threads of different diameters are found on the mast surface, which could also be viewed as a result of cooperative building (for more detail, see Discussion section 4.1. on “nonpereopodal secretions”). Juveniles of Dyopedos monacantha leave the maternal mast to build their own masts at different ages (Thiel, 1997), suggesting that there is a time period in which young amphipods are capable of building but inhabit the maternal mast. Thus, although we do not have direct evidence that juveniles participate in maternal mast maintenance, we suggest that, in some cases, juveniles begin to strengthen (build up) and elongate the maternal mast instead of leaving it, leading to the appearance of collective masts.