Smithsonian Marine Station at Fort Pierce

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Species Name: Falcula hyalina Takano
Common Name: None
Synonymy: None

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Protista Bacillariophyta Bacillariophyceae - - Falcula

    Species Description

    This is one of a small number of marine diatoms that live exclusively as epizoic symbionts on pelagic crustaceans, and was originally described from Japan (Takano 1983). In the IRL, copepods of the genus Acartia and Oithona, but also occasionally other copepod genera are the hosts. Falcula has somewhat lunate cells in valve view, normally with one convex and one concave side (Fig. 1, LM phase contrast), and transverse uniseriate rows of areolae forming the striae. A wide longitudinal hyaline area (septum) interrupts the striae, mostly toward the concave side of the valve (Fig. 2, valve apex, exterior, SEM). At each valve apex is a pore field composed of short slits (Fig. 3, valve apex, interior, SEM; arrowhead), from which short stalks of mucilage anchor the cell to its zooplankton host as individual cells or clusters. A rimoportula subtends this pore field at one end of the valve (Fig. 2 and 3, arrows). Cells have two elongate chloroplasts. The valves are 26-32µm long and 4-6µm wide, with 20-22 striae in 10µm. The striae are continuous around the mantle to the valve margin (Fig. 4, girdle view, SEM).

    This species is widespread in tropical and subtropical coastal regions, but rarely recognized. In addition to Japan, it is found in the Indian Ocean (Hiromi et al. 1985) and the Gulf of Mexico (Prasad 1989), among several other places

    Although its presence in the IRL has been noticed previously (Mahoney & Gibson 1983, under the name Falcula media), its potential for impact on food web dynamics has been overlooked. Larval copepods, with frequent molting through naupliar and copepodite stages, rarely have epizoic Falcula, but as many as 15% of the Acartia adult population in the IRL can be infested (Hargraves PE, personal observation). Sometimes copepods are infested with only a few Falcula cells (Fig. 5 Acartia lightly infested [white arrows] with numerous free-floating Falcula cells [black arrowheads]), but often with several hundred cells per animal on the antennae, prosome and urosome (Fig. 6, heavily infested adult Acartia). Apparently, effective infestation takes place only after molting is completed, but the initial infestation process is unknown. Zooplankton other than Acartia are infrequently infested; and phytoplankton tows at the time of maximum Acartia infestation are dominated by Falcula, presumably detached cells (Hargraves PE, personal observation). It is likely that heavy infestations are detrimental to the host, reducing the ability to acquire food, mate, and avoid predation. When epizoic infestation is abundant, free-floating cells are also abundant in the plankton, thus contributing to biomass measured as phytoplankton chlorophyll. The ecological consequences of such heavy infestation to food web dynamics are unknown.


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    Hiromi J, Kadota S, Takano H. 1985. Diatom infestation of marine copepods. Bull Tokai Reg Fish Lab 117: 37-46.

    Prasad AKSK, Livingston R, Ray G. 1989. The marine epizoic diatom Falcula hyalina from Choctawhatchee Bay, the northeastern Gulf of Mexico: frustule morphology and ecology. Diat Res 4: 119-129.

    Mahoney R, Gibson R. 1983. A checklist of the phytoplankton of the Indian River near Vero Beach, Florida. Fla Sci 46: 206-211.

    Takano H. 1983. New and rare diatoms from Japanese coastal waters. XI. Three new species epizoic on copepods. Bull Tokai Reg Fish Lab 111: 23-33.

Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat
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Page last updated: 22 September 2015

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