Abstract

Numerous diatom taxa, including six endemic genera, have been reported to live on the skin of cetaceans, none of which appear to be host specific. Presence of such epizoic diatoms becomes obvious if macroscopic patches of different coloring develop.

Reports of diatom infestations of cetaceans most commonly involve Mysticetes harvested during whaling activities. Diatoms are apparently not uncommon in skin films of, Blue (Balenoptera musculus), Fin (B. physalus), Sei (B. borealis), Humpback (Megaptera novaengliae), and Minke (B. acutorostrata) whales. The infestations are reported to be most prevalent in whales with blubber thickness' indicating they have been in cold water climates for some time. Whales described as "lean," presumably recent arrivals from warmer waters, were noted to be free from diatom films. Nemoto et. al., (1980) report diatom skin films to be more common in colder waters and less common in temperate waters. Thus diatom films have been used to indicate movement patterns of the large cetaceans. Diatom skin films are reportedly responsible for the description of Blue whales as "sulfur bottom" whales.

While less common, descriptions of skin film diatoms have been found for the following odontocetes- Dall's Porpoise (Phocoenoides dalli), Sperm Whale (Physeter catodon), Bottlenose Whale (Hyperoodon rostratus), Killer Whale (Orcinus orca), Franciscana Dolphin (Pontoporia blainvillei), Harbor Porpoise (Phocoena phocoena), Pacific white-sided Dolphin (Lagenorhynchus obliquidens), Cuvier's Beaked Whale (Ziphus cavirostis), Strap Toothed Whale (Mesoplodon layardii), and Baird's Beaked Whale (Berardius bairdii). "Springer Dolphins" (Lagenorhynchus cruciger and Cephalorhynchus commersoni) are also suspected of being infested. Apparently, diatom skin films on Atlantic Bottlenose dolphins have not been reported in the literature, although there are unconfirmed anecdotal reports of such films from Tursiops in Hawaii; the diatom taxon involved was not described, however.

Two geriatric (>30 year) Atlantic Bottlenose dolphins, Tursiops truncatus, one male and one female, housed at the same open ocean enclosure complex in San Diego Bay, but not in-contact, developed "coffee-colored" skin films in January of 1997. Skin film samples were collected with a 4mm bone curette into sea water and 1% neutral buffered formalin. Light microscopy (LM) revealed numerous diatom cells. Subsequently a diatom skin film has been noted on a juvenile, incontact, male, however, the surface film is much smaller and much more difficult to observe grossly. No clinical signs have been attributed to the skin films.

The skin films were essentially monospecific (a single valve of the sessile Fragilaria investiens (W. Smith) Cleve-Euler was observed in addition). LM and Scanning Electron Microscopy (SEM) of material treated with hydrogen peroxide was performed to elucidate the valve structure of the diatom involved. This allowed it to be identified as Epipellis oiketis Holmes, the single representative of this monoraphid genus known so far, and reported previously only from Killer Whale and Dall's Porpoise at latitudes north of Monterey Bay (Holmes, 1985). Although agreeing completely with this species in ultrastructural architecture, the valves collected from Bottlenose dolphin revealed a significantly denser ornamentation pattern of striae, areolae and marginal alveolae, which could possibly relate to intra specific variation.

It is interesting to note both original animals noted with infestations (which persist at time of abstract preparation) are geriatric and housed at the same location. No other animals in our population, spread throughout San Diego Bay, have developed skin films. It is also interesting that water temperatures in San Diego Bay are consistently the coldest in January and February.

References

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