The unadorned developmental phases of Maruzzo et al. and Høeg et al. (2012): artifacts?

Abstract

In an ecologically oriented paper, Maruzzo et al. (2012) illustrate six developmental phases from the freshly settled cyprid larvae to the sessile juvenile in the barnacle, Amphibalanus amphitrite (Darwin, 1854), and in an evolutionarily oriented paper Hoeg et al. (2012) do likewise for Megabalanus rosa (Pilsbry, 1916). Both video-based studies illustrate 5 unadorned increasingly sac-like phases leading up to the sessile juvenile (Phase 6). In both species, Phase 5 is not only illustrated as sac-like, it resides outside the protective canopy of the cyprid shell, and this is reminiscent of the situation described by Bernard and Lane (1962, Fig. 24A). However, it has been long known that not only is the metamorphosing balanoid larva within the cyprid carapace divided into a capitulum and peduncle, but the anlagen of plates can seen with light microscopy through the largely transparent cyprid carapace (Runnstrom, 1925; Walley, 1969; Gusenbauer, 2003), and primordial plates appear in SEMs in Phase 5, after the carapace has been shed (Glenner and Hoeg, 1993; Gusenbauer, 2003). Maruzzo et al. and Hoeg et al. neither recognize such plates nor do they recognize the peduncle in any of the phases. Yet it is largely the peduncle that is involved in the struggle to shed the cyprid carapace in the transition to semi-sessile Phase 5, which they describe and illustrate as a “bag-like body shape.” Maruzzo et al. claim this phase persists for as much as 24 hours before becoming fully sessile (Phase 6), and they call for studies to assess its contribution to differential mortality. But Phase 5 has generally been observed to persist for less than three hours. Thus, the significantly longer duration being reported is either an artifact resulting from in vitro conditions and manipulations and/or failure to determine when the wall plates they illustrate in Phase 6 began to form. On the other hand, SEMs of Glenner and Hoeg, and more recently Gusenbauer, show that semi-sessile Phase 5 is by and large comparable to that illustrated by Runnstrom (1925) and, therefore, an unadorned sac-like form surely does not exist. Thus, Phase 5 of Maruzzo et al. and Hoeg et al., like that of Bernard and Lane (Fig. 14A), is most certainly an artifact.