Protozoa and Sponges

The Protozoa are acellular (having a body which is not composed of cells) organisms and constitute the simplest animal group. They comprise about 50,000 species and are found in almost every habitat where moisture is present, and also as parasites in most animals. Protozoa are divided into 4 subphyla: Sarcomastigophora (amebae and flagellates), Ciliophora (ciliates), Sporozoa, and Cnidospora. Because of their resistant spores, some protozoa can resist extremes of temperatures and humidity. It's not surprising that they are among the most numerous animals in the world. Some protozoa share similar features with sponges, which suggests that these two groups may be related.

Amebae and Flagellates

Amebae have a constantly changing body shape and more by producing pseudopodia (false feet). The cytoplasm of the ameboid cell is extruded at one point to form the pseudopodium as the animal moves forward. The common Amoeba has a naked cell surface, but a variety of shell forms exist; the genus Difflugia, for example, constructs as a case from minute grains of sand, whereas other amebae secrete intricate shells of calcium carbonate and silica. Some amebae are parasitic, such as Entamoeba coli which lives in the human large intestine. It does not transmit disease, but scavenges bacteria and food detritus (organic debris). Such an association is termed commensalism. But the related Entamoeba histolytica is harmful and causes amebic disentery.

In contrast to amebae, flagellates move using a long hairlike structure called flagellum, which beats like a whip to provide propulsion. Most flagellates have a fixed body shape (usually oval) and almost reproduce asexually, but there is considerable diversity between the species. Most flagellates feed on small particles and organic materials dissolved in the surrounding water. But some, such as the green Euglena commonly found in ponds, are able to produce their own food by photosynthesis. May flagellates live in association with other animals. Trichonympha, for example, lives in the intestine of termites. The termite relies on the protozoon to digest the wood that it eats, a mutually beneficial relationship known as symbiosis. But some flagellates are blood parasites, such as Trypanosoma, and cause disease (in this case sleeping sickness).

Vorticella, a bell-shaped body protozoan that attaches to the substrate by means of a stalk.
Photo by Larysa Johnston

The most complex and diverse species of protozoa owe their names to the cilia (short hairlike fibers similar in structure to flagella) which grow in orderly rows on the body and beat rhythmically to propel the animal. In many ciliates, the cilia occur only on parts of the body, whereas in others they form plates or "membranelles." Still others have cilia that are fused into stiff cirri and used as legs for crawling. Ciliates reproduce asexually by binary fission, or sexually by conjugation. They differ from other ptotozoa in that they have two nuclei, a macro- and micronucleus (other protozoa have only one).

Sporozoa and Cnidospora
These organisms, the spore-formers, have no distinct locomotory adaptations because they are all parasitic. They live in all animals and are often transmitted by insect vectors. Their name comes from the production of spores, or cysts, during the infective stage of their life. The life cycle of this group is complicates, reproduction alternate from sexual to asexual.


Sponges are exclusively aquatic animals, which are fixed on the substrate and live by drawing in water and filtering microscopic-size food particles from it. They comprise the phylum Porifera (pore-bearers) and represent the simplest level of multicellular whose origins date back to the Precambrian more than 600 million years ago. Most of the 11,000 species are found in shallow waters, although some live in deep water, but as many as twice that number are thought to exist. Sponges are currently divided among four distinct classes, 25 orders, 128 families and 680 genera. They range in length from one-quarter of an inch to more than 3 feet. Functionally, sponges share many features with unicellular protozoa, particularly with respect to nutrition, cellular organization, gas exchange, reproduction, and response to environmental stimuli. Instead of organs or tissues, sponges possess amoeboid cells that move freely through the three-dimensional sponge matrix, termed the mesohyl. Sponges are filter feeders that pump large volumes of water through a unique and highly vascularized canal system, leaving the expelled water essentially sterile. Nutrients are acquired by phagocytosis of bacteria that are removed from the water column.

Sponges have developed intimate contact with diverse microorganisms such as viruses, bacteria, archaea, fungi, protozoa, and single-celled algae and the nature of the sponge-microbe interaction is manifold. In addition to a transient seawater population serving as a food source, sponges harbor large amounts of bacteria in their tissues that can amount to 40% of their biomass, including Acidobacteria, Actinobacteria, Chloroflexi, Cyanobacteria, Gemmatimonadetes, Nitrospira, Planctomycetes, Proteobacteria, (Alpha, Delta, Gamma subclasses), Spirochaetes, Kocuria, Mycobacterium, Nocardia, Rhodococcus, and Actinomycetes. Genomic analysis showed that sponges can be viewed as reservoirs that are highly concentrated in yet uncultured, elusive marine microorganisms, while other bacteria are nearly exclusively found in sponges (Poribacteria).5,6,7

Sea sponge
Sea sponge

Sponges react to external light or mechanical signals with contractile or metabolic reactions and are devoid of any nervous or muscular system. Furthermore, elements of a photoreception/phototransduction system exist in those animals. Recently, a cryptochrome-based photoreceptor system has been discovered in the demosponge (Amphimedon queenslandica). The assumption that in sponges the siliceous skeleton acts as a substitution for the lack of a nervous system and allows light signals to be transmitted through its glass fiber network is supported by the findings that the first spicules are efficient light waveguides and the second sponges have the enzymatic machinery for the generation of light.3

The simplest sponges are tubular with an external layer of epithelial, or lining, cells. The internal surface is covered with flagellated collar cells (choanocytes) which maintain a water current through the sponge. Food particles are extracted fron this current which is also used for gas exchange and waste removal. Water is drawn in through small pore cells in the walls of the sponge, and ejected from the large mouth (osculum). More advanced sponges have complex sytems of canals and chambers, through which water is channeled. Sponges reproduce sexually and asexually. Most are hermaphroditic, and produce eggs and sperm at different times. In addition some sponges produce gemmules (buds or internal groups of cells) which survive when the parent disintegrates in winter. In spring, the gemmule develops into an adult sponge.

A great diversity of symbiotic organisms often thrive inside or on the body of a sponge, from microscopic prokaryotes to macroscopic organisms such as shrimps, polychaetes, hydrozoans and fishes. Shrimps are often parasites adapted to consumption of sponge tissues. An example of such relationship is West Atlantic tropical shrimp living in fire sponges of the genus Tedania.8

Regarding their physiological roles, sponge lectins exhibit a broad range of bioactivities, including modulation of inflammatory response, antimicrobial and cytotoxic activities, as well as anticancer and neuromodulatory activity.4 Steroids isolated from Haliclona simulans were found to be active against Trypanosoma brucei brucei and Mycobacterium marinum.9


  1. Mike Janson and Joyce Pope (consultant editors). The Animal World
  2. U.S. Fish and Wildlife Service Multimedia database
  3. Public Health Image Library (PHIL)
  4. Transcriptome profiling of the demosponge Amphimedon queenslandica reveals genome-wide events that accompany major life cycle transitions Cecilia Conaco, Pierre Neveu et al.
  5. Porifera Lectins: Diversity, Physiological Roles and Biotechnological Potential Johan Gardères, Marie-Lise Bourguet-Kondracki, Bojan Hamer, Renato Batel, Heinz C. Schröder, Werner E. G. Müller
  6. Molecular Evidence for a Uniform Microbial Community in Sponges from Different Oceans. Ute Hentschel, Jörn Hopke, Matthias Horn, Anja B. Friedrich, Michael Wagner, Jörg Hacker, Bradley S. Moore
  7. Single-cell genomics reveals complex carbohydrate degradation patterns in poribacterial symbionts of marine sponges. Janine Kamke, Alexander Sczyrba, Natalia Ivanova, Patrick Schwientek, Christian Rinke, Kostas Mavromatis, Tanja Woyke, Ute Hentschel
  8. These Squatters Are Not Innocent: The Evidence of Parasitism in Sponge-Inhabiting Shrimps. Zdeněk Ďuriš, Ivona Horká, Petr Jan Juračka, Adam Petrusek, Floyd Sandford
  9. Isolation and Identification of Antitrypanosomal and Antimycobacterial Active Steroids from the Sponge Haliclona simulans. Christina Viegelmann, Jennifer Parker, Thengtheng Ooi, Carol Clements, Gráinne Abbott, Louise Young, Jonathan Kennedy, Alan D. W. Dobson, RuAngelie Edrada-Ebel
  10. Global Diversity of Sponges (Porifera). Rob W. M. Van Soest, Nicole Boury-Esnault, Jean Vacelet, Martin Dohrmann, Dirk Erpenbeck,,Nicole J. De Voogd, Nadiezhda Santodomingo, Bart Vanhoorne, Michelle Kelly, John N. A. Hooper



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