Diatom of the Month - June 2018: Achnanthes felinophila

by Rex Lowe*

The freshwater diatom flora of Hawaii has long been thought to be composed of cosmopolitan species, as demonstrated in the checklist produced by Sherwood (2004).  We found it curious that remote islands, 1 to 5 million years old surrounded by a saltwater ocean would have an algal flora composed largely of European species. More recently the flora is being shown to contain many unique species, especially diatoms (Lowe et al. 2009; Lowe & Sherwood 2010; Kociolek et al. 2010; Graeff & Kociolek 2011). Perhaps the most distinctive species of freshwater diatom described from Hawaii is Diprora hanaensis Main, a monotypic genus described from a cave on the island of Kauai (Main 2003).

Figure 1. The Hawaii islands: Kauai is the first island from the top left.

The diatom genus Achnanthes, in the broad sense, included many species with one valve possessing a raphe and the other valve without a raphe (i.e. heterovalvar), Patrick and Reimer 1966). Achnanthes was distinguished from Cocconeis by the absence of a marginal hyaline (i.e. unornamented) area on the raphe valve and secondarily by its more elongate rather than coccoid shape. As the genus Achnanthes came under increased scrutiny, it was fragmented into several genera based largely on morphological details. Those species most closely related to the type for the genus (Achnanthes adnata Bory) remained in the genus. Although Achnanthes sensu stricto is most common in marine habitats, there are several exceptions (Round et al 1990). Achnanthes coarctata (Bréb. in W. Sm.) Grun. has been reported from numerous subaerial florae.

Figure 2. Light microscope image of Achnanthes felinophila specimens: raphe valve (a) and rapheless valve (b) (for more details, see Lowe et al., 2009).

While exploring the subaerial algal communities of the Hawaiian Islands, we discovered several populations of Achnanthes, the majority of which were undescribed.

Achnanthes felinophila is one of five species of the genus that is endemic to Hawai’i.  The valves are distinctly triundulate in large specimens but become less triundulate and more lanceolate in smaller valves with narrowly rounded to cuneate ends, 11-46 μm long, 6-10 μm wide. The raphe valve has a narrow axial area and a rectangular central area reaching the valve margins. The raphe is externally sinuous, with proximal ends deflected slightly in one direction and with distal ends curved strongly towards mantle in opposite direction. The rapheless valve has an axial area near the valve margin. Striae are punctate and radiate throughout on the raphe valve on the rapheless valve striae are parallel in the center becoming radiate with respect to the axial area and curved at the ends. The internal costae separating striae have oblique ridges in the rapheless valve forming a unique spiral pattern visible by scanning electron microscope (SEM).

Figure 3. Scanning electron microscope image of a specimen of Achnanthes felinophila (scalebar: 10µm).

The type locality for this taxon was observed in a sample from a feeding station for feral cats on a roadside cliff on Iao Valley on the island of Maui; dozens of cats were seen while the collection was made. Achnanthes felinophila was named for this unique association. This species was present and common in only one collection from this relatively dry volcanic-rock wall in Iao Valley on Maui. Achnanthes felinophila was discovered as a result of a 3-year study of the algal flora of the five major Hawai'ian Islands.  This study illustrated that diatoms have their own biogeographical tales to tell and that although diatoms are abundant worldwide isolated habitats support unique florae.

*Emeritus Professor of Biology, Bowling Green State University (Ohio)

References

Lowe  R.L. & Sherwood A.R. 2010. Distribution and morphological variability of Cosmioneis (Bacillariophyceae) in Hawaii. Proceedings of the Academy of Natural Sciences of Philadelphia 160: 21-28.

Kociolek, J. P., Graeff, C. L. and Lowe, R. L.  2010. A new freshwater Gyrosigma (Bacillariophyceae) species from Hawaii. Polish Botanical Journal 55:65-71.

Lowe R.L., Sherwood A.R. & Ress J.R. 2009.  Freshwater species of Achnanthes Bory from Hawaii.  Diatom Research 24: 327-340.

Main S.P. 2003. Diprora haenaensis gen. et sp. nov., a filamentous, pseudoaerial, araphid from Kauai (Hawaiian Islands) (Bacillariophyta). Diatom Research 18: 259–272.

Patrick R.M. & Reimer C.W.  1966.  The Diatoms of the United States.  Vol. 1.. Monograph 13, Academy of Natural Sciences of Philadelphia. 688p.

Round F.E., Crawford R.M. & Mann D.G.  1990.  The Diatoms.  Biology and Morphology of the Genera.  747p. Cambridge University Press, Cambridge.

Sherwood A.R.  2004.  Bibliographic checklist of the nonmarine algae of the Hawaiian Islands.  Bishop Museum Occasional Papers 80: 1-23.