Anthozoa Sea anemones and most corals Polyp is dominant and only life form –NO MEDUSA –Presents evolutionary question: Were the original cnidarians all medusae and pelagic, with the medusa being lost in the anthozoans and most hydrozoans? OR is the polyp and a benthic existence the primitive state with the pelagic medusa evolving later?
Anthozoa Reproduction –Asexual Longitudinal or transverse fission Pedal laceration –Parts of pedal disk detach and differentiate into new animal –Sexual Gametes produced by polyp Planula larvae produce another polyp
Anthozoa Defense and territoriality –Acrorhagi: special stinging cells used to defend territory against neighboring anemones –Also have capture tentacles –Sweeper tentacles in corals and some anemones
Anthozoa Feeding –Carnivorous Use nematocysts on tentacles –Anatomy differs from hydrozoan polyps Food enters mouth and then goes through tubular pharynx before entering the gastrovascular cavity Ciliated grooves called siphonoglyphs extend down the pharynx from the mouth Gastrovascular cavity is partitioned by mesenteries –Infoldings of gastrodermis –Increases surface area for secreting digestive enzymes and for nutrient absorption
Anthozoa Muscles and movement –Longitudinal and circular muscles –Sea water in gastrovascular cavity acts as hydrostatic skeleton –Some anemones can “crawl” slowly –Others are carried around by other invertebrates –A few can swim –Most are sessile
Sea Anemones Symbioses –Zooxanthellae “Anemone fish” –Percula clown fish in aquarium –These fish have substituted the mushroom corals for sea anemones Anemones are too dangerous in a reef tank They will sting other corals and kill many of the hard and soft corals and polyps
Corals and Coral Reefs Types of corals –Subclass Hexacorallia Tentacles in multiples of 6 around mouth Solitary forms are the sea anemones No special protective covering Colonial species are the true or “stony” corals Found in warm, clear water Largest reefs are found in tropical areas of the Indo-Pacific One of the largest reefs is the Great Barrier Reef off the northeast coast of Australia –2000 km long –145 km wide
Sublcass Hexacorallia (=Zoantharia) True anemones: Order Actiniaria Mushroom corals: Order Corallimorpharia Zoanthis polyps: Order Zoanthidea True corals: Order Scleractinia
Hexacorallia Corals are carnivorous but usually occur where plankton is not abundant –Coral polyps have zooxanthellae Endosymbiotic dinoflagellates Require high amounts of light for photosynthesis Therefore silty deposits from land can suffocate a reef Zooxanthellae are very sensitive to heat so global warming is another danger for reefs as sea temperatures rise Zooxanthellae provide photosynthate to coral polyp and use the metabolic wastes from the animal for its own growth
Hexacorallia Formation of coral skeleton –Rate of reef growth is higher in light than in dark Therefore: implies zooxanthellae play a role –Mechanism? Have an effect on availability of bicarbonate ion which is essential for calcification process Contribute some critical component of the organic matrix that allows for calcium carbonate deposits to form Localized removal of dissolved phosphates (nutrient for algae) which can inhibit formation of calcium carbonate Generate extra oxygen through photosynthesis which speeds up rate of coral metabolism
Hexacorallia Subclass Octocorallia (Alcyonaria) –8 tentacles –Tentacles are pinnate –Some have no support other than the thick mesoglea These are the soft corals –Others have proteinaceous or calcareous internal skeletons These are the gorgonians and pipe corals
Miscellany Corals have medical uses –Tiny pieces of coral are used for some bone grafts especially face and jaw Tiny pores are rapidly infiltrated by capillaries and bone forming osteoblasts –Used to make artificial eyes move! Blood vessels and muscles invade coral fragment and then patient can use their own muscles to move the fake eyeball
Emerging Patterns in Evolution Skeleton (Hydrostatic Skeleton) Contractile tissues/locomotion Nervous system Gut Gastrulation Radial Symmetry Two embryonic tissues
3 Germ Layers Ectoderm forms epidermis and central nervous system Endoderm forms the lining of gut and associated organs –It is argued that the early embryo may have more than just these two Mesoderm forms all the stuff in between gut tube and skin, such as, muscle, bone, connective tissue, blood, etc.
The Cnidaria are good candidates as the crossroad of metazoan evolution. Sponges are the living proof of how a protozoan colony can become a metazoan, without reaching a level of organisation involving a recognisable body form, real tissues and a nervous system. The Cnidaria are the first animals with tissue layers, muscles, and sense organs. –They lie at the base of the tree because they are diploblastic, have radial symmetry, and do not have a real brain.
Cnidaria and Metazoan Evolution They are present in the fossil record since the Precambrian, when animals similar to modern forms were absent. The strange creatures of the Precambrian, besides cnidarians, apparently did not pose the foundations of future metazoan organisations, whereas these are evident in the Cnidaria. Boero, Bouillon, and Piraino. 2005. The role of Cnidaria in evolution and ecology. It. J. Zool. 72:65-71
Cnidaria and Metazoan Evolution From an ecological point of view, the cnidarians probably play roles that are much more important than usually perceived. –Both Cnidaria and Ctenophora feed on the eggs and larvae of most benthic, planktonic and nektonic organisms Might be keystone species Maintain high biodiversity, by feeding on potentially highly competitive, resource monopolizing species. During periodic outbreaks of their populations –serious effects on fisheries yields and other problems
Phylogenomic Analyses Firmly Place Myxozoa in Cnidaria All Myxozoa possess polar capsules that are similar to nematocysts of Cnidaria in ultrastructure and ontogeny and are used for host attachment Presence of a minicollagen gene –Minicollagens are cnidarian-specific constituents of nematocyst walls. Phylogenetic analyses placed Myxozoa as sister group of Medusozoa within Cnidaria Maximilian P. Nesnidal, Martin Helmkampf, Iris Bruchhaus, Mansour El-Matbouli, B. Hausdorf. Agent of Whirling Disease Meets Orphan Worm: Phylogenomic Analyses Firmly Place Myxozoa in Cnidaria. PLOS One Published: January 30, 2013 DOI: 10.1371/journal.pone.0054576
Phylogenetic tree based on a dataset including 128 genes
Myxozoa Myxozoa ("slime animals") are so unusual that they were long believed to be protozoa - entirely the wrong kingdom - until molecular analysis proved a direct relationship with the jellyfish, corals and other cnidaria. It's an understandable mistake, considering these "jellies" can consist of as little as a single cell and live as parasites in other animals; jellies who evolved into germs.
Myxozoa Most slime animals pass through two different hosts in their life cycle. Actinospores Myxospores
Myxozoa –Actinospores Work the same way but attack larger hosts; Most species attack fish, but a few are known to favor amphibians or aquatic reptiles, and at least one invades shrews. In larger hosts, the plasmodium can become visible to the naked eye, a concentration of thick, clear slime growing and feeding in the host's tissues. Some attack the muscles, some attack the heart, some even dwell within the spinal column, causing severe skeletal deformities and neurological damage. Thousands of spores are released once the host dies, with some species killing more than 90% of infected fish.
The newly-discovered parasite which creates mysterious holes in the Greenland halibut was discovered by Greenlandic fishermen, and researchers have yet to figure out how prevalent this parasite is. (April 7, 2012)
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