THE MORPHOLOGY OF THE CETACEAN DIATOM
GENUS PLUMOSIGMA NEMOTOSACHIKO NAGASAWA 1), ROBERT W. HOLMES 2)
AND
TAKAHISA NEMOT0 1H
ABSTRACTThis report concerns the morphology of the cetacean diatom genus
Plumosigma which may be obligately associated with sperm whales (Physetercatodon). Diatom colonies on sperm whales collected off the Pacific coast ofHokkaido, Japan and off San Francisco, U.S.A. were cleaned using sulfuric
acid/potassium permanganate/oxalic acid. The cleaned materials wereexamined with a light microscope, a scanning electron microscope and atransmission electron microscope. The morphology of the colonies was described as well as the valve morphology of P. hustedti and P. rimosum,
although it was impossible to observe internal valve structure of the latter
due to lack of specimens. Additional material of P. rimosum needs to be
examined to characterize its morphology and ultrastructure.
Key words: cetacean diatom, Plumosigma, valve morphology, sperm whale

INTRODUCTION

Diatoms observed on the skin of cetaceans have been studied by Bennett(1920), Hart (1935), Karcher (1940), Usachev (1940), Omura (1950), Hustedt(1952), Okuno (1954), Nemoto (1956, 1958), Kliashtorin (1962), Best (1969),
Nemoto, Brownell and Ishimaru (1977), Nemoto, Best, Ishimaru and Takano(1980) and Holmes (1985). Except for some planktonic species found on theskin, which may have been attached accidentally, genuine cetacean diatoms are
few and comprise only 4 genera, Bennette/la, Epipellis, Stauroneis, Plumosigma,
and a small number of species.
Cocconeis ceticola, which Nelson described and named in Bennett's (1920)

Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-Ku, Tokyo, 164 Japan
Died August 1990.
2) Department of Biological Sciences, University of California, Santa Barbara, California, 93106 U.S.A.
1)

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NAGASAWA, HOLMES AND NEMOTO

paper, was recently transferred to the new genus Bennettella after careful examination of diatoms assigned to the genus Cocconeis (Holmes, 1985). Bennettella
includes 3 species (Holmes, 1985). Epipellis, a monotypic genus, was established to accommodate the invalid Cocconeis orcii Kliashtorin (Holmes, 1985).
We report here morphological studies on Plumosigma Nemoto, a group not
examined since Nemoto established this genus and its two species (P. hustedtiand P. rimosum) in 1956.
MATERIALS AND METHODS

Diatoms examined in this study came from two sources:
1) Skin samples of sperm whales (Physeter catodon) with diatom patches
collected by T. Nemoto off the Pacific coast of Hokkaido, Japan in 1957.
2) Skin samples of sperm whales with diatom colonies collected west of San
Francisco in 1970 by Dr Judith Hansen.
All samples were rinsed in distilled water, treated with sulfuric acidpotassium permanganate and decolorized with oxalic acid. The cleaned material
was mounted in Naphrax for light microscopic examination. SEM mounts wereprepared by air-drying cleaned material directly on stubs or on cover glasses,affixing the cover glasses to aluminium stubs, and coating the material with goldor gold/palladium. Three different scanning electron microscopes were used: aHitachi S-415A at University of California, Santa Barbara; a JSM-35 at Ocean
Research Institute, University of Tokyo; and a JSM-25 at Otsuchi Marine
Research Center, University of Tokyo. For TEM, aqueous suspensions ofcleaned cells were air-dried on formvar-coated copper grids. A JEM-lOOCX
located at the Ocean Research Institute, University of Tokyo, was used fortransmission electron microscopy.
RESULTS

Plumosigma typically forms circular patches of varying dimensions on the skinof sperm whales. Individual diatoms within the patches attach adnately to theskin (Plate I, Fig. 1). This taxon usually co-occurred with Stauroneis aleutica, S.
olympica, Bennettella ceticola and B. constricta (Plate I, Fig. 2). Although thefirst three of these diatoms also occur on the Dall's porpoise Phocoenoides dalli(Holmes, unpublished) only one specimen of Plumosigma has been observed in
the many hundreds of Dall's porpoise diatom colonies examined by us. Provisionally, we conclude that Plumosigma is obligately associated with spermwhales.
In the samples examined by us, P. hustedti (Plate I, Figs 3, 4, 5, 7 and 8;
Plate II, Fig. 3) was much more abundant than P. rimosum (Plate I, Fig. 6;
Plate II, Figs 1 and 2). Nemoto (1956) reported that in his study P. rimosumSci. Rep. Cetacean Res.,No. I, 1990

CETACEAN DIATOM PLUMOSIGMA

87

was more abundant than P. hustedti. We remain uncertain of the significance of
these contradictory observations.
The external valve surface of Plumosigma is concave. The raphe rises
above the basal siliceous layer forming what appears to be a ridge separating thetwo halves of the valve face. The valve outline is slightly sigmoid (Plate I, Figs
5, 6, 7 and 8; Plate II, Figs 1 and 3). The central raphe endings terminate in ahyaline area which differs in outline in the two species (Plate I, Figs 5, 6, 7, 8and 9; Plate II, Fig. 1). The valve mantle is easily deformed and often missing in
our photos (Plate I, Figs 7 and 8; Plate II, Figs 1, 6 and 7). Anomalous valvesindicated by the presence of two raphes and overlapped puncta were found in
our material (Plate II, Fig. 3). Valves somewhat similar to these valves werealso illustrated by Nemoto (1956).
The external distal raphe fissures at the poles are curved in opposite directions (Plate I, Figs 5 and 6). These distal raphe endings terminate in the interiorof the valve in a thickened U-shaped helictoglossa in P. hustedti (Plate I, Fig. 4;
Plate II, 5 and 6). The region between the central raphe endings in the interiorof the valve is thickened (Plate II, Fig. 5). The exterior surface of the valve
mantle is shown in Plate II, Fig. 4.
Nemoto's (1956) illustrations suggest that the puncta of the striae aremostly round in P. hustedti. By contrast, those close to the raphe and margin of
the valve are long and narrow in P. rimosum, although a few rows of round
puncta are sometimes present. Our observations confirm this morphology.
Morphometric data on the two species of Plumosigma have been published(Nemoto, 1956). Combining Nemoto's morphometric data with our own
measurements, the following characteristics may be summarized for P. hustedti:
length 11-22 µ,m, width 4.6-7.5 µ,m, length/width ratio 2.4-3.8, striae in the central region of the valve 31-40 in 10 µ,m, striae at the valve margin near the valve
ends 40-55 in 10 µ,m, puncta in the central region of the valve 23-30 in 10 µ,m,puncta at the margin of the valve near the valve ends 30-42 in 10 µ,m.
We do not have similar data on P. rimosum because of the limited numberof this species in our samples. According to Nemoto (1956), the striae of P.
rimosum are denser than those of P. hustedti.
DISCUSSION

We have obtained additional information on the valve morphology of Plumosigma hustedti using SEM and TEM. We have not found girdle views offrustules nor carried out examination of the internal structure of P. rimosum.
The distal raphe endings of P. hustedti have an unusual internal structure,
which looks rather like an anchor (Plate II, Figs 5 and 6). Unfortunately, it was
impossible to observe internal valve structure of P. rimosum due to lack ofspecimens.
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We were unable to document the structure of hymenate pore occlusions ineither taxon. These occlusions are very delicate and were absent in the sperm
whale samples examined, but one specimen of P. rimosum found in the sediments of Otsuchi Bay retained hymenate pore occlusions (Nagasawa, Holmesand Nemoto, 1989). The absence of hymenate pore occlusions in the present
material and the scarcity of specimens of P. rimosum have made it difficult to
describe the morphology of this species with any degree of certainty. However,transmission electron microscope photographs of the three valves of P. rimosum
available for observations suggest the possibility that this taxon represents astage in the dissolution of the siliceous valve structure of P. hustedti. The admixture of circular and elongate puncta along the axial area of P. rimosum and theirregular inner margin of the elongate puncta suggest the possibility that thewalls between puncta have dissolved, producting "elongate" puncta (Plate II,Figs 1 and 2). On the other hand, the central area is quite different from that ofP. hustedti. What appear to be very small poroids occur with some frequency
along the inter-striae in one of our photographs of P. rimosum (Plate II, Fig. 2).
Such poroids have not been observed in P. hustedti. Clearly, additional materialof P. rimosum needs to be examined to characterize its morphology and ultrastructure in more detail.
ACKNOWLEDGMENTS

We thank the staff of Otsuchi Marine Research Center, University of Tokyo, for
the use of their laboratory facilities and scanning electron microscope during
our visit. This study was supported by the Japan Society for Promotion ofScience, National Science Foundation and the Nissan Science Foundation.
REFERENCES
BENNETI, A. G. 1920. On the occurrence of diatoms on the skin of whales with an Appendix by E.W. Nelson.
Proc. R. Soc. Land. (B) 91: 352-357.
BEST, P. B. 1969. The sperm whale (Physeter catodon) off the west coast of South Africa. 4. Distribution andmovements. Invest. Rep. Div. Sea Fish. S. Afr. 78: 1-12.
HART, T.J. 1935. On the diatoms of the skin film of whales and their possible bearing on problems of whales'
movements. Discovery Rep. 10: 247-282.
HoLMES, R. W. 1985. The morphology of diatoms epizoic on cetaceans and their transfer from Cocconeis totwo new genera, Bennettella and Epipellis. Br. Phycol. J. 20: 43-57.
HusTEDT, F. 1952. Diatomeen aus der Lebensgemeinshaft des Buckelwals (Megaptera nodosa Bonn.). Arch.
Hydrobiol. 46: 286-298.
KARCHER, F. H. 1940. Uber den Algenbewuchs auf siidlichen Walen. Z. Fish. Hilfwiss. Beihefte l: 13-36.
KLIASHTORIN, L.B. 1962. The diatoms of the skin film of whales in the far-eastern seas (in Russian). Akad.
Nauk. SSSR Inst. Ocean. Trudy 58: 314-321.
NAGASAWA, S., HOLMES, R. W. and NEMOTO, T. 1989. Occurrence of cetacean diatoms in the sediments ofOtsuchi Bay, Iwate, Japan. Proc. Japan Acad. (B) 65: 80-83.
NEMOTO, T. 1956. On the diatom of the skin film of whales in the northern Pacific. Sci. Rep. Whales Res.
Inst., Tokyo 11: 97-132.

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NEMOTO, T. 1958. Cocconeis diatoms infected on whales in the Antarctic. Sci. Rep. Whales Res. Inst., Tokyo
13: 185-192.
NEMOTO, T., BROWNELL, R. L. and IsHIMARU, T. 1977. Cocconeis diatom on the skin of Franciscana. Sci. Rep.
Whales Res. Inst., Tokyo 29: 101-105.
NEMOTO, T., BEST, P.B., IsHIMARU, K. and TAKANO, H. 1980. Diatom films on whales in South Africanwaters. Sci. Rep. Whales Res. Inst., Tokyo 32: 97-103.
OKUNO, H. 1954. Electron microscopical study on Antarctic diatoms (6). Observations on Cocconeis ceticola
forming 'diatom film' on whale skin. J. lap. Bot. 29: 271-277.
OMURA, H. 1950. Diatom infection on blue and fin whales in the Antarctic whaling area V (The Ross Seaarea). Sci. Rep. Whales Res. Inst., Tokyo 4: 14-26.
UsACHEV, P.I. 1940. The overgrowing of whales with diatoms (in Russian). Zoo[. Zhur. 19: 306-312.

EXPLANATIONS OF PLATESPLATE I
Each scale bar indicates 1 µm.
Fig. 1. A colony of Plumosigma on the skin of a sperm whale, source 2, SEM.
Fig. 2. Co-occurrence of Plumosigma with Stauroneis sp. and Bennettella ceticola on the skin of sperm whales,
source 1, SEM.
Fig. 3. Exterior of P. hustedti, source 1, LM.
Fig. 4. Interior of P. hustedti, source 1, LM.
Fig. 5. Frustule of P. hustedti, source 1, SEM.
Fig. 6. Frustule of P. rimosum, source 1, SEM.
Fig. 7. Three valves of P. hustedti without girdle bands, source 1, TEM.
Fig. 8. Another valve of P. hustedti without girdle bands, source 1, TEM.

PLATE II
Each scale bar indicates 1 µm.
Fig. 1. Valve of P. rimosum, source 1, TEM.
Fig. 2. Central area of P. rimosum, source 1, TEM.
Fig. 3. Double valves of P. hustedti, source 1, TEM.
Fig. 4. Exterior of P. hustedti with valve mantle, source 1, SEM.
Fig. 5. Interior of P. hustedti, source 1, SEM.
Fig. 6. End of interior valve of P. hustedti, source 1, SEM.
Fig. 7. Detached interior valve mantle of P. hustedti, source 1, SEM.

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