Species identified:
Isthmia nervosa Kützing 1844.
Genus: Isthmia Agardh, C.A. (1832).
Taxonomic classification:
Phylum: Bacillariophyta,
Subphylum: Bacillariophytina,
Class: Mediophyceae,
Subclass: Chaetocerotophycidae,
Order: Hemiaulales,
Family: Isthmiaceae,
Genus: Isthmia (Guiry & Guiry, 2020).
Samples from the Salish Sea as part of the Salish Sea Diatom Project (imerss.org)
Genus and species Description for both I. enervis and I. nervosa:
The genus is heteropolar, where one valve is more elevated than the other. Isthmia species are found in both trapezoidal and rombhoidal forms. Valves are oval with no distinct transition from valve to mantle. Isthmia nervosa apical axis 170-240 µm long (Cupp 1943: 166). Both I. nervosa and I. enervis have only one pseudocellus per cell, a bulbous extension in which a cluster of small areolae are seen. The areolae cluster is far more pronounced in I. nervosa than I. enervis: the areolae being smaller in size in I. enervis, and the areolae suddenly reduces in size at the pole of the pseudocellus of I. nervosa. It is proposed that the pores behind the apex of the pseudocellus secrete a mucilage pad by which the cell attaches to the host, though areolae of the pseudocellus may also be involved. The costae (rib-like thickenings) are the distinctive “nerves” of I. nervosa. The cribra (plates with pores) in I. nervosa vary over the valve face unlike I. enervis where the cribra are more regular.
Both species have a U-shaped notch on the distal end of the girdle band (copula). However, it is far less obvious in I. nervosa, but usually a distinctive feature of the epivalve of trapezoidal cells of I. enervis. Round (1984) notes that on some cells of I. enervis the girdle band notch could not be detected. I. nervosa has very prominent struts supporting the girdle diaphragm, less so for I. enervis. Isthmia, a small genus, and a coastal marine diatom are colonial, and epiphytic on seaweeds. It is both highly silicified and large relative to other diatoms. Many diatom taxonomists have placed Isthmia in the Biddulphiaceae as there are complexities to its characteristics and hard to place in the centric group (Kützing, 1844; W.Smith 1856: Plate 47 Fig. F.; Van Heurck 1880-1881: XCVI, fig. 1-3.; Hendey 1964; Round 1984; Round et al. 1990; Sims (1996); Guiry and Guiry 2020).
Salish Sea specimens. Images: Isthmia nervosa-Gen Plank Box 009 SHW-Dec 20-2010_TM4000-H-H2O2-Aug 24-2022_Stb 5a MW_22(x250)_3.tif; SHW-Isthmia nervosa-3495-Dec 26-2010.jpg; SHW-Isthmia nervosa-3506-Dec 26-2010.jpg; SHW-Isthmia nervosa-3509_2-Dec 26-2010.jpg; SHW-Isthmia nervosa-9942-July 20-08.jpg; SHW-Isthmia nervosa-9945-July 20-08.jpg; SHW-Isthmia nervosa-9962-July 20-08.jpg.
Morphometric data:
Frustules are heteropolar, rhomboidal or trapezodial in girdle view. Valves are ovoid without distinction or transition from valve through mantle. Apical axis 164.9-202.2 µm long, intact frustule length 251.0-333.3 µm (SHW SEM and LM specimens). Only one pseudocellus per cell with a distinctive cluster of areolae and the areolae suddenly reduce in size at the pole of the pseudocellus. Valve areolae are round, elliptical to polygonal and have complex thick cribra and simple pores penetrate the framework in various locations. Costae are robust and distinctive leading to the name of the species “nerves”. The U-shaped notch on the distal end of the girdle band is hard to detect in I. nervosa. I. nervosa has very prominent struts supporting the girdle diaphragm. Areolae in the girdle bands of I. nervosa have cribra. Forms branching colonies with large mucilage pads. Discoid chloroplasts.
Previously reported by Shim (1976) in the Strait of Georgia, Salish Sea and Cupp (1943) for the Pacific West Coast, from San Francisco to Alaska.
Methods:
Net collection from Spanish Hills Wharf, Trincomali Channel, North Galiano Island, Southern Gulf Islands, British Columbia, Canada, July 20, 2008, December 20 and 26, 2010. Live cells imaged in 0.2 µm filtered marine water under a coverslip with either a Leitz Dialux microscope. Fixation by a final concentration of 4% formalin. Cells were prerinsed to remove salts, cleaned in 30% hydrogen peroxide at 100°C for 3 hours and rinsed many times in deionized, sterile and distilled lab grade water to a pH of 6.5-7. Cleaned frustules were pipetted onto aluminum SEM stubs and imaged with a Hitachi TM4000 Plus at the Advanced Microscope Facility (AMF), at University of Victoria (UVIC), B.C., Canada. Many thanks go to Elaine Humphrey of the Advanced Microscope Facility, University of Victoria. Collection, cleaning, preparation of stubs and slides, imaging, and taxonomy by Mark Webber (IMERSS).
References:
Agardh, C.A. (1832). Conspectus Criticus Diatomacearum. Part 4. pp. 49-66.Lundae [Lund]: Literis Berlingianus.
Baker, A.L. et al. 2012. Phycokey-- an image based key to Algae (PS Protista), Cyanobacteria, and other aquatic objects. University of New Hampshire Center for Freshwater Biology. (searched Feb 26, 2024)
http://cfb.unh.edu/phycokey/Choices/Bacillariophyceae/Centric/Centric_Unicells/ISTHMIA/Isthmia_key.html
Cupp, E. E. (1943). Marine Plankton Diatoms of the West Coast of North America. Bull. Scrips. Inst. Oceanography. 5: 1-238.
Dickman, M., Hodgkiss, J., Cheng, Z. and Gao, Y. Diatoms from the South China Sea. Searched July 4, 2020. http://121.192.179.196/diatom/diatomphoto/Centric%20diatoms/htmldir/0000026e.htm
Guiry, W. in Guiry, M.D. & Guiry, G.M. 2020. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 03 July 2020.
Hendey, N.I. (1964). An introductory account of the smaller algae of British coastal waters. Part V: Bacillariophyceae (diatoms). pp. [i]-xxii, 1-317. London: Ministry of Agriculture, Fisheries and Food, Fishery Investigations. Her Majesty’s Stationery Office.
Kützing, F.T. (1844). Die Kieselschaligen Bacillarien oder Diatomeen. pp. [i-vii], [1]-152, pls 1-30. Nordhausen: zu finden bei W. Köhne.
Round, F.E. (1984). Structure of the Cells, Cell Division and Colony Formation in the Diatoms Isthmia enervis Her. and I. nervosa Kütz. Annals of Botany 53, 457-468.
Round, F. E., Crawford, R. M. and Mann, D. G. (1990). The Diatoms, Biology & Morphology of the Genera, p. 142-142. Cambridge University Press, Cambridge, UK.
Shim, J. H. (1976). Distribution and Taxonomy of Planktonic Marine Diatoms in the Strait of Georgia, B.C. Phd. Thesis, UBC. P. 157. Plate XIX no. 1.
Sims, P.A. (ed.) (1996). An atlas of British diatoms arranged by B. Hartley based on illustrations by H.G. Barber and J.R. Carter. pp. [2], 1-601, incl. 290 pls. Bristol: Biopress Ltd.
Smith (1856). British Diatomaceae Vol.2
Guiry, W. in Guiry, M.D. & Guiry, G.M. 2020. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 03 July 2020.
Van Heurck, H. (1880-1881). Synopis des Diatomees de Belgiue. Edite par L’
Wolle, F. (1894). Diatomaceae. The Comenius Press. Bethlehem. PA.
