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47

Observation of an exothecal coccolith associated with Syracosphaera halldalii: evidence for dithecatism?
Pauhla B. McGrane
Galway-Mayo Institute of Technology. Dublin Road, Galway, Ireland; pmcgrane@gmit.ie

Abstract: A specimen of the coccolithophore Syracosphaera halldalii collected in Irish Shelf Waters is observed inScanning Electron Micrographs (SEM) associated with a single exothecal coccolith. The exothecal coccolith is situated
LQWKHFLUFXPÁDJHOODUDUHDRIDFROODSVHGFRFFRVSKHUHDQGLVDWKLFNHOOLSWLFDOSODQROLWKFRQFDYHO\EHQWDQGZLWKDQopen centre. S. halldalii has been regarded as monothecate although this single observation may indicate dithecatism inWKHVSHFLHVDQGZLWKIXUWKHUREVHUYDWLRQVFRXOGQHFHVVLWDWHUHFODVVLÀFDWLRQZLWKLQSyracosphaeraKeywords: Syracosphaera, exothecal coccoliths, taxonomy
1. Introduction
Syracosphaera halldalii is a common coccolithophore
species that has been widely observed in the plankton andpreviously recorded in the eastern North Atlantic Ocean
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the Mediterranean Sea (Cros and Fortuño, 2002; Triantaphyllou et al., 2004) and the northern Red Sea (Kleijne,1991; Kleijne 1993; Young et al. 2003). The species differsfrom most of the Syracosphaeraceae in that exothecal
coccoliths are absent and it is therefore considered monothecate (Young et al. 2003). Water samples collected forcoccolithophore analysis in Irish shelf waters in 2002
revealed relatively high abundances (>9 x 103 cells l-1) ofS. halldalii, a single specimen of which was associatedwith a previously undescribed, morphologically distinct
exothecal coccolith (Figs 1-2).

 PP GLDPHWHU SRO\FDUERQDWH PHPEUDQH ÀOWHUV ZLWK Dpore size of 0.8 µm using a vacuum pump and according
to the methodology outlined in McGrane, 2007. Filters
were mounted with carbon tape onto a 25 mm aluminium
stub aluminium stubs and placed in a Emitech K550
VSXWWHUFRDWHUDQGFRDWHGZLWKDJROGSDOODGLXPÀOPRInm thick at 25 mA for 2 minutes. Coccolithophore examiQDWLRQZDVFDUULHGRXWXVLQJD+LWDFKLVFROGÀHOGemission Scanning Electron Microscope equipped with acomputer-controlled stage.

3. Results and discussion
Amongst coccolithophores, the development of a discreteouter layer of coccoliths or exotheca, morphology
different to the coccoliths of the inner layer or endotheca, is exclusively the preserve of the Syracosphaeraceae (Cros 2000; Cros and Fortuno 2002; Young et al.
2. Materials and methods
2003). Dithecatisim, the possession of both an endotheca
7KHVSHFLPHQÀJXUHGZDVFROOHFWHGDVSDUWRIWKH&9and an exothecal layer, has been observed in all Syracosurvey of the biogeographic distribution of planktonic
sphaera species with the exception of S, halldalii and S.
coccolithophores in Irish Shelf Seas aboard the RV Celtic
protrudens (Kleijne and Cros, 2009).
Voyager on 28th July 2002 at station 3601 (55°49.98 N,In addition to the morphological characteristics of-7°51.27 W) from 30m depth (Raine and McGrane, 2002).
coccospheres and coccoliths, the diverse morphology ofDiscrete water samples were collected using a 5 ltr
exothecal coccoliths and the degrees of differentiation
Niskin water sampling bottle mounted on a Sea-Bird SBE
between species, are an important diagnostic tool used to&7'URVHWWH8SWROWURIVDPSOHZDVÀOWHUHGRQWRFigure 1-2: Syracosphaera halldalii Gaarder, in Gaarder & Hasle 1971 ex. Jordan and Green, 1994. Scale bar = 1 µm.
Fig. 1: Collapsed coccosphere of S. halldaliiZLWKDVLQJOHH[RWKHFDOFRFFROLWKVLWXDWHGDWWKHÁDJHOODURSHQLQJDORQJVLGHFLUFXPÁDJHOODUFDQHROLWKVdisplaying a tall blunt spine. Body coccoliths show variation in size (length 2.2-3.2 µm, width 1.5-2.0 µP ZLWKZLGHVPRRWKGLVWDOÁDQJHDQGWRRWKOLNHSURMHFWLRQVKRUL]RQWDOO\WRZDUGVWKHODWKÀOOHGFHQWUDODUHD)LJ'LVWDOYLHZRIH[RWKHFDOFRFFROLWKVKRZLQJUREXVWHOOLSWLFDOFRQFDYHSODQROLWKZLWKVHUUDWHGGLVWDOÁDQJHPDUJLQH[RWKHFDOFRFFROLWKOHQJWK3.2 µm; exothecal coccolith width ~2 µm; central open area ~1.6 µm;

48&RFFROLWKRSKRUHV$EVWUDFWVclassify and group Syracosphaera species (Kleijne andCros, 2009). ,Q D UHYLVHG FODVVLÀFDWLRQ V\VWHP IRU WKHgenus Kleijne and Cros (2009) distinguished four maingroups and 15 subgroups of Syracosphaera and placed
the two monothecate, dimorphic species in the halldalii
subgroup, of the S. molischii group, characterised by
YHUWLFDO FRFFROLWK ZDOOV D ÁDW GLVWDO ÁDQJH LQ DGGLWLRQWR D SUR[LPDO ÁDQJH DQG VSLQHEHDULQJ FLUFXPÁDJHOODUcoccoliths,The exothecal coccolith observed in association witha specimen of S. halldalii is a thick, elliptical, concavely
bent planolith (3.2 µPLQOHQJWK ZLWKDZLGHGLVWDOÁDQJH
bearing a serrated margin and a relatively large open
central area (1.6 µm in length) which has a tooth-likestructure similar to the central area of the halldalii muroliths. It is possible that some corrosion of the coccolith
has taken place, or that it may be allochthonous in origin,
i.e. it might be a coccolith of another species which has
accidently fallen on the coccosphere. Alternatively it may
represent evidence for dithecatism in S. halldalii.
The dithecatism interpretation is supported by a numberof factors. First, the position of the exothecal coccolith,
VLWXDWHGLQWKHFLUFXPÁDJHOODUDUHDRIWKHFRFFRVSKHUHLVsimilar to that of other members of the S. molischii group.
6HFRQG WKH H[RWKHFDO FRFFROLWK ÀJXUHG LV D WKLFN HOOLStical planolith, concavely bent and with an open centralarea. These characteristics resemble those of S. andruleitii (Kleijne and Cros, 2009) but it is larger in size (3.2µPOHQJWK SUHVHQWVDZLGHUGLVWDOÁDQJHZLWKDVHUUDWHGmargin and a larger central opening (1.6 µm width), andtherefore appears morphologically distinct from otherexothecal coccoliths recorded previously (Cros, 2000;Young et al. 2003; Kleijne and Cros, 2009). Third, it does
not obviously resemble any other known heterococcolith
although this might be partially due to corrosion. So wesuspect it is an exothecal coccolith but a single observation is inconclusive and further observations are required
before we can conclude that S. halldalii has exothecal
coccoliths and exhibits dithecatism.
Acknowledgements
I would like to thank J. Young, M. Triantaphylou, K.
Hagino and L. Cros who kindly reviewed this manuscript
and A. McGrane for processing of images. This research
was supported by the Higher Education Authority ofIreland under PRTLI Cycle II with shiptime provided bythe Marine Institute of Ireland. Access to the SEM wasprovided by the Centre for Microscopy and Imaging, NUI
Galway.

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