Sessile Protists

Contributed by
Igor V. Dovgal, Senior Research Scientist,

Office address: Department of Invertebrate Fauna and Systematics,
I.I. Schmalhausen Institute of Zoology of the Ukrainian Academy of Sciences
B.Khmelnitsky str.15, 01601- Kiev Ukraine
Phone:(38-044) 235-7053
Fax: (38-044) 234-1569

Illustrated Guide IISessile protists are principally unicellular organisms that are attached to substrate by special adhesive organelles (such as stalks, etc). These organisms are usually have attached adult stage (trophont) and free-swimming larval stage (swarmer). Two main morphotypes of sessile protists are distinguished (Dovgal, 2000): stalked and flattened on substrate. These protists are inhabited on unanimate substrates (stones, etc) and as commensals or ectoparasites on various water animals and plants.

The attached mode of life is rather common among protists. There are sessile species even among foraminifers and radiolarians. There are also many attached species among autotrophic flagellates. The majority of heliozoans and representatives of three orders of flagellates (Choanophlagellata, Bicosoecidae, and Chrysomonadida) are sessile. Attached mode of life is quite characteristic for ciliates. Several high taxa (class Suctorea, subclasses Peritrichia and Chonotrichia, order Pilisuctorida and family Folliculinidae) are include hundreds sessile species. Evidences for an important role of these protists in the fouling communities are not uncommon in the literature (Burkovsky, 1984, Fenchel, 1987). The density of sessile predating suctorian ciliates can comprise 140 million individuals per square meter in rivers (Dovgal, 1990). Under this abundance their are a principal controllers of planktonic ciliates numbers. It should be remarked that the abundance of filter-feeding heterotrophic protists (such as bicosoecids, choanoflagellates and peritrichous ciliates) may be greater (Fenchel, 1987). After I.V. Burkovsky (1984) data the density of sessile ciliates only may comprise up 102-107 individuals per square meter.

Some additional evidence on sessile protists can be found in following review works:

Additional references on sessile protists (In English):

Harmful Algal Blooms

(Contributed by Don Anderson, Woods Hole Oceanographic Institution, MA, USA)

Among the thousands of species of microscopic algae at the base of the marine food chain are a few dozen that produce toxins. These species make their presence known in many ways, ranging from massive "red tides" that discolor the water, to dilute, inconspicuous concentrations of cells noticed only because of the harm caused by their highly potent toxins. Impacts include mass mortalities of wild and farmed fish and shellfish, human illness and death, alterations of marine trophic structure, and death of marine mammals, seabirds, and other animals.

"Blooms" of these algae are commonly called red tides, since, in some cases, the tiny plants increase in abundance until they dominate the planktonic community and change the color of the water with their pigments. The term is misleading, however, since non-toxic species can bloom and harmlessly discolor the water, or can cause ecosystem damages as severe as those linked to toxic organisms. Adverse effects can also occur when toxic algal cell concentrations are low and the water is not discolored. Given the confusion surrounding the meaning of "red tide", the scientific community now prefers the term "harmful algal bloom" or HAB. This new descriptor includes algae that cause problems because of their toxicity, as well as non-toxic algae that cause problems in other ways. It also applies to macroalgae (seaweeds) which can cause major ecological impacts as well.

Impacts from Toxic Algae
HAB phenomena take a variety of forms, with multiple impacts. One major category of impact occurs when toxic phytoplankton are filtered from the water as food by shellfish which then accumulate the algal toxins to levels which can be lethal to humans or other consumers. The poisoning syndromes have been given the names paralytic, diarrhetic, neurotoxic, amnesic, and azaspiracid shellfish poisoning (PSP, DSP, NSP, ASP, and AZP). Except for ASP and AZP, all are caused by biotoxins synthesized by a class of marine algae called dinoflagellates. The ASP toxin, domoic acid, is produced by diatoms that until recently were thought to be free of toxins. The toxin responsible for the recently discovered poisoning syndrome called AZP has not yet been linked to a particular algal species or group. A fifth human illness, ciguatera fish poisoning (CFP) is caused by ciguatoxins produced by dinoflagellates that attach to surfaces in many coral reef communities. Ciguatoxins are transferred through the food chain from herbivorous reef fishes to larger carnivorous, commercially valuable finfish. The final human illness linked to toxic algae is called Possible Estuary-Associated Syndrome (PEAS). This vague term reflects the poor state of knowledge of the human health effects of the dinoflagellate Pfiesteria piscicida and related organisms that have been linked to symptoms such as deficiencies in learning and memory, skin lesions, and acute respiratory and eye irritation – all after exposure to estuarine waters where Pfiesteria-like organisms have been present.

Another type of HAB impact occurs when marine fauna are killed by algal species that release toxins and other compounds into the water, or that kill without toxins by physically damaging gills or by creating low oxygen conditions as bloom biomass decays. Fish and shrimp mortalities from these types of HABs at aquaculture sites have increased considerably in recent years. HABs also cause mortalities of wild fish, seabirds, whales, dolphins, and other marine animals. To understand the breadth of these ecosystem impacts, think of the transfer of toxins through the food web as analogous to the flow of carbon. Just as phytoplankton are the source of fixed carbon that moves through the food web, they can also be the source of toxins which cause adverse effects either through toxin transmitted directly from the algae to the affected organism or indirectly through food web transfer.

A prominent example of direct toxin transfer was the death of 19 whales in Massachusetts in 1987 due to saxitoxin that had accumulated in mackerel that the whales consumed. Similar events occurred in Monterey Bay, California in 1998 and again in 2000 when hundreds of sea lions died from domoic acid (the ASP toxin) vectored to them via anchovies. In both cases, the food fish accumulated algal toxins through the food web and passed those toxins to the marine mammals.

Another significant concern relates to sublethal, chronic impacts from HABs that can affect the structure and function of entire ecosystems. Adult fish can be killed by the millions in a single outbreak, with obvious long- and short-term ecosystem impacts. Likewise, larval or juvenile stages of fish or other commercially important fisheries species can experience mortalities from algal toxins. Impacts of this type are far more difficult to detect than the acute poisonings of humans or higher predators, since exposures and mortalities are subtle and often unnoticed. Impacts might not be apparent until years after a toxic outbreak, such as when a year class of commercial fish reaches harvesting age but is in low abundance. Chronic toxin exposure may therefore have long-term consequences that are critical with respect to the sustainability or recovery of natural populations at higher trophic levels. Many believe that ecosystem-level effects from toxic algae are more pervasive than we realize, affecting multiple trophic levels, depending on the ecosystem and the toxin involved.

Major journals appropriate to research on HABs:

Harmful Algae
Journal of Phycology
Journal of Plankton Research
Limnology and Oceanography
Marine Ecology Progress Series

Harmful Algal Blooms (HAB) images

Announcements of meetings, proposal requests, etc.

Links to major sites: