Little Discs of Doom

Okay, total hyperbole in the title, but I wanted to get your attention. Today I'll be looking at Pihiella liagoraciphila, a very distinctive member of the red algae that was only described a few years ago (Huisman et al., 2003).

Pihiella is an endo/epiphyte found on members of the red algal family Liagoraceae, but not a parasite as far as I can tell (red algae are notable for the range of associations between different taxa, most interestingly the occurrence of what is call 'adelphoparasitism', where parasitic species are closely related to their hosts - I'll have to write on that some day). It has a quite simple disc-shaped or subspherical morphology with rhizoids to attach it to the host and long hairs and trichogynes (hair-like appendages of the female carpogonia that catch the male gametes). Mature discs are very small, up to 400 μm in diametre and 150 μm thick, though the hairs can be up to 800 μm long. Specimens were first observed as long ago as 1858, but were interpreted as buds of the host plant. Authors thereafter disagreed as to whether the so-called 'monosporangial discs' were asexual reproductive organs of the host or an independent organism. All authors agreed that the discs were asexually reproductive.

Sexually reproductive organs on the discs weren't recorded until 2003, when Huisman et al. established that the discs were indeed a separate organism from the host. Pihiella seems to lack the obscenely complicated triphasic life cycles of other red algae (see here for an example). As already mentioned, the carpogonia (sexual organs) possess a long hair-like trichogyne, and Huisman et al. did observe examples with spermatia (the aflagellate male sex cells) attached. Nevertheless, Huisman et al. were unable to conclude whether the mature sporangia observed were asexually produced monosporangia, sexually produced zygotosporangia, or both (I feel the last option seems most likely, but what do I know?). No carposporophytes or tetrasporangia were observed (see the link above to find out what these are).

The morphology of Pihiella was too distinct from any other red alga to be phylogenetically informative, but Huisman et al. were able to assess the phylogeny molecularly. Pihiella turned out to be quite isolated from other red algae, enough that Huisman et al. established a new monotypic order for it. Interestingly, the trees recovered Pihiella as sister taxon to Ahnfeltia, another phylogenetically isolated taxon, with a high level of support. Morphologically, Ahnfeltia is very distinct from Pihiella, being a large cartilaginous plant with a triphasic life cycle found in cool waters (the host family of Pihiella, Liagoraceae, is a mostly warm-water group). Though Ahnfeltia and Pihiella are each other's closest relatives, the relationship is not close. Liagoraceae, in contrast, was in a quite distant part of the tree.

REFERENCES

Huisman, J. M., A. R. Sherwood & I. A. Abbott. 2003. Morphology, reproduction, and the 18S rRNA gene sequence of Pihiella liagoraciphila gen. et sp. nov. (Rhodophyta), the so-called 'monosporangial discs' associated with members of the Liagoraceae (Rhodophyta), and proposal of the Pihiellales ord. nov. Journal of Phycology 39: 978-987.