DESCRIPTION OF THE PHYLUM CNIDARIA

EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>RADIATA>CNIDARIA

PHYLUM CNIDARIA LINKS

Cnidaria (ni-DA-re-a) is the Latinized form of a Greek word (κνιδοσ) that means sting. The reference is to the stinging cells called cnidae or nematocysts.

INTRODUCTION TO THE CNIDARIA

The Cnidaria is a natural group of diploblastic organisms with a mostly acellular mesogloea that is derived from the ectoderm. They contain specialized cells, the cnidocytes, which produce a variety of adhesive and stinging structures collectively called cnidae, most of which are the stinging structures called nematocysts. The phylum includes hydroids, jellyfish, and corals. The calcareous corals are responsible for the occurrences of tropical reefs and whole tropical islands. These frequently form the basis of very productive tropical and subtropical marine ecosystems. Hinde (2001) says that the Cnidaria have a fossil history that goes back to the late Pre-Cambrian and are members of the Ediacaran fauna. Corals and coral-like animals appear in the fossil record of the Ordovician Period and form reef systems like the modern corals.

The apparent simplicity of the cnidarians and their radial symmetry suggested a basal position in the animal kingdom together with the Ctenophora. However, a recent review of metazoan relationships by Collins et al. (2005) suggests that the Cnidaria are sisters to the Bilateria and more recently derived than the Ctenophora (see also Cook 2002; Martindale et al. 2002; and Aleshin and Petrov 2002). Traditionally, the phylum is separated into two large groups: Anthozoa (soft and stony corals) and Medusozoa (jellyfish and hydrozoans, animals that have a medusoid stage). Collins (2002), using SSU rDNA, Marques and Collins (2004), using 87 morphological characters, and Kayal et al. (2013), using mitogenomics, present a very complex relationship within the phylum. All seem to agree on the more derived position of the Hydrozoa (e.g. Schuchert 1996; Collins et al. 2005; and Cartwright and Nawrocki 2010). Topologies suggested by the analyses of rRNA phylogenies (Figure 1A) and mitochondrial coding genes (Figure 1B) and not entirely compatible with Kayal et al. (2013, Figure 1C). The main difference between Figures 1A and 1B is the monophyly of the Anthozoa and the complexity of the Medusozoa (which is monophyletic in both figures). Figure 1D is a summary cladogram showing the relationships of the major taxa according to Marques and Collins (2004) and Kayal et al. (2013).

ANTHOZOA

Anthozoans (hard and soft corals plus the anemones) appear to be basal in the Cnidaria and made of two nested groups: Hexacorallia and Octocorallia. The anthozoans are marked by having no medusoid stage; thus, the polyp is the sexual stage which produces the dispersive planula larva. Mature polyps are large with thick, cellular mesogloea . The coelenteron is partitioned by mesentaries which have glands, filaments and nematocysts. The oral disc is surrounded by tentacles and the stomodaeum extends from the mouth down into the enteron. They have one or two ciliated grooves that direct water into the coelenteron. They have both epidermal and gastrodermal nematocysts, but no operculum. Often, the animals are colonial and have about 6,000 extant species.

Corals (Figure 2) and anemones are among the most important marine organisms. The hard corals form the basis for whole marine systems in coastal tropical and subtropical seas. The biodiversity of a coral reef can be astounding and rival that of a tropical rain forest, far greater than any other marine biome. The extraction of the calcium carbonate that produces the coral skeleton is facilitated by the photosynthesis of symbionts called zooxanthellae, symbiotic dinoglagellates, which make the coral polyps brightly colored.

Charles Darwin (1842) was the first to describe the formation of a fringing tropical reef relative to changes in sea level based on observations that he made while on the voyage of the H. M. S. Beagle. The karst limestone region on the northern part of Puerto Rico is from a fringing reef made high and dry by tectonic uplift. One collapsed cave provides the circular basin within which the Arecibo radio telescope was built. Fossil coral reefs form major deposits of limestone in other areas of the world as well.

Typically, there are two types of anthozoans. The Hexacorallia [sea anemones (Figure 3) and stony corals] are animals that secrete calcium carbonate to the outside of the animal, which may be solitary or colonial. They have six internal mesentaries and six or multiples of six tentacles. The Octocorallia, [sea fans, organ-pipe corals, soft corals (Figure 4), sea pens, and sea pansies] have eight internal mesentaries, polyps with eight tentacles, and calcareous mineral deposits are internal.

MEDUSOZOANS

The medusozoans have a motile jellyfish or medusa as a sexual stage in most taxa. The phylogeny presented here suggests that the polyp is the most primitive form of the cnidarians, and medusae were added to the life history in this clade.

MEDUSOZOAN LIFE HISTORY: POLYP —> MEDUSA—>PLANULA—> POLYP

SCYPHOZOA
The scyphozoans and cubozoans are sister groups in which the medusa is the dominant life stage. The polyp is either highly reduced, absent, or unknown. The orientation of the medusa is mouth down rather than mouth up as in the polyps. Contractions of the bell of the medusa provide locomotion.
Scyphozoans and Cubozoans have medusae that are free living with gastrodermal gonads (8), four septa partioning the coelenteron, and with cellular mesogloea. The bell of the Scyphozaons is relatively large, generally with many tentacles, and without a velum. Scyphozoans have several feeding strategies represented by the following taxa.
- Aurelia, the Moon Jelly (Figure 5), is a planktivore. has small marginal tentacles and four large oral arms surrounding the mouth. Plankton are caught in mucus on the under side of the bell and transported to the margin where the oral arms remove the food particles and carry them to the mouth.
- Cyanea, the Lion’s Mane Jelly (Figure 6), is an animal that can have a bell 2m across and long flowing tentacles that can reach more than 30m. They live in cold waters of the northern Pacific and Atlantic where they feed on fish and other jellyfish.
- Cassiopeia, the Upside-Down Jelly or Mangrove Jelly (Figure 7), generally lies upside-down on the substrate where it tends its internal garden of zooxanthellae, which give it a greenish color. While there, the bell margins pulsate creating a current across the oral surface where plankton and other particles are subdued by nematocysts and caught in a gelatinous coating. The captured particles are carried to the mouth or to other secondary mouths that occur on the oral arms. These are animals occur in warm, shallow waters of the West Indies, the Pacific, and the Indian Oceans.

CUBOZOA + STAUROZOA
The cubozoans, considered to be a separate class of jellyfish in most modern systems, are small and delicate medusae. They have a bell that has a velum and four flattened sides like a box with tentacles emerging only from each of the four the marginal corners of the bell. The small Cubozoan jellies are very toxic. Chironex, the Sea Wasp (Figure 8), is one of the most venomous animals in the oceans. They are relatively common on the Great Barrier Reef where they can grow up to 30cm across and have tentacles up to 2m long. The sting of the Sea Wasp is deadly and each year claims the lives of at least two swimmers, usually within minutes of being stung.
Staurozoans live in nearshore habitats in cold water regions (e.g. Haliclystus in Antarctica, see Figure 9), and like the cubozoans, they have a square symmetry. The staurozoans are the stalked jellyfish in that they are attached with the appearance of hydroids. Interpretations vary as to the nature of the stauropolyp; either it can be an inverted attached medusa or a typical polyp. Kayal et al. (2013) treat them as medusans that have lost the medusoid stage. They were recognized as a new class by Marquez and Collins (2005), which has been confirmed by more recent molecular phylogenies (e.g. Kayal et al. 2013). Though attached as adults, these cnidarians have a planktonic larva that settles down to an appropriate substrate as a creeping planula, which forms the stauropolyp (Miranda et al. 2010).

HYDROZOA
Hydrozoans are the sister group to the Scyphozoans. Their life histories typically include both polyps and medusae, though the polyps tend to dominate. The polyp of a hydrozoan is not complex in that it has a simple mouth and no internal septa. Many taxa have colonial polyps in which individual animals are connected, and often individual polyps have different functions. The hydrozoan medusa is small with a simple mouth and a bell with a velum. Various life history strategies can be seen in the Hydrozoa as represented by the following taxa.
- Hydra (Figure 10), one of the most common cnidarians in the Biology Lab, is a relatively simple, elongate polyp without a periderm, a medusa, or a planula. Gonads develop on mature polyps. Zygotes develop directly into polyps without an intervening planula. However, most of the individuals are produced by asexual reproduction.
- Physalia, the Portuguese Man-of-War (Figure 11), is a colonial animal in which the individual zoids develop into one of five different types. One polyp, the pneumatophore, becomes a gas-filled float. The stinging tentacles are provided by gastrozoids (with a mouth) and dactylozoids (without a mouth). The gonozoids give rise to medusae which may remain attached for some time before being released and complete the life cycle. Other taxa produce sex cells directly and produce planulae. Protective shelf-like structures, called bracts, are formed by a fifth kind of zoid. The colony operates as a single, floating organism that feeds on plankton and small fish. During certain times of the year, on-shoer winds can drive Physalia to beaches in the Gulf of Mexico and warm Atlantic by the thousands with painful results by bathers. I can attest to the pain from personal experience.
- Obelia, (Figure 12) is most obvious as an attached colonial organism that resembles a bryozoan or a branching alga. The colony is formed of gasterozoids and gonozoids, all of which are connected and have a common coelenteron. The colony is contained within a leathery periderm, which is open at the mouth end of the gasterozoids and the pore end of the gonozoids. The gasterozoids feed on plankton caught by nematocysts on the tentacles which carry food to the mouth. Gonozoids produce medusae by budding. Then, the gonad-bearing medusae, release gametes, which produce zygotes that develop into planulae.
- Narcomedusae (Figure 13) have lost the polyp stage all together. They disperse themselves as small simple medusae which produce larvae that are parasitic on other cnidarians. Very likely, the Myxozoa, parasites of fish, are sisters of this group.

LITERATURE CITED

Aleshin, V. V. and N. B. Petrov. 2002. Molecular evidence of regression in evolution of metazoa. Zh. Obshch. Biol. 63(3):195-208.

Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia.

Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257.

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago.

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Cartwright, P. and A. M. Nawrocki. 2010. Character evolution in Hydrozoa (phylum Cnidaria). Integrativeand Comparative Biology. 50(3): 456-472.

Collins, A. G. 2002. Phylogeny of the Medusozoa and the evolution of cnidarian life cycles. Journal of Evolutionary Biology. 15: 418-432.

Collins, A. G., P. Cartwright, C. S. McFadden, and B. Schierwater. 2005. Phylogenetic context and basal metazoan model systems. Integrative and Comparative Biology. 45:585-594.

Daly, M. M. R. Brugler, P. Cartwright, A. G. Collins, M. N. Dawson, D. G. Fautin, S. C. France, C. S. McFadden, D. M. Opresko, E. Rodriguez, S. L. Romano, and J. L. Stake. 2007. The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa. 1668: 127-182.

Darwin, C. 1842. The Structure and Distribution of Coral Reefs. Smith, Elder, and Co. London.

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis.

Hinde, R. T. 2001. The Cnidaria and Ctenophora. In: Anderson, D.T., ed. Invertebrate zoology. Oxford University Press. Oxford, UK. pp. 29-57.

Kayal, E., B. Roure, H. Philippe, A. G. Collins, D. V. Lavrov. 2013. Cnidarian phylogenetic relationships as revealed by mitogenomics. BMC Evolutionary Biology. 13(5): doi:10.1186/1471-2148-13-5

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York.

Marques, A. C. and A. G. Collins. 2004. Cladistic analysis of Medusozoa and cnidarian evolution. Invertebrate Biology. 123(1): 23-42.

Martindale, M. Q., J. R. Finnerty, and J. Q. Henry. 2002. The Radiata and the evolutionary origins of the bilaterian body plan. Molecular Phylogenetics and Evolution. 24:358-365.

Miranda, L., A. G. Collins, A. C. Marques. 2010. Molecules clarify cnidarian life cycle – The “Hydrozoan” Microhydrula limopsicola is an early life stage of the Staurozoan Haliclystus antarcticus. PLoS ONE. 5(4): e10182. doi:10.1371/journal.pone.0010182

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX.

Schuchert, P. 1993. Trichoplax adhaerens (Phylum Placozoa) has cells that react with antibodies against the neuropeptide RFamide. Acta Zool. 74: 115–117.

Steele, R. E., C. N. David, and U. Technau. 2011. A genomic view of 500 million years of cnidarian evolution.Trends in Genetics. 27(1): doi:10.1016/j.tig.2010.10.002

Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York.

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

By Jack R. Holt and Carlos A. Iudica. Last revised: 12/22/2015


FIGURE 10. Hydra, a common hydrozoan that has lost the medusoid phase. Image from the Systematic Image Archive	FIGURE 11. Physalia, the Portuguese Man-of-War, is a colonial hydroid with many different individuals forming a functional floating feeding platform. Image by NOAA, in the Public Domain	FIGURE 12. Obelia, a hydroid that alternates between a medusoid (left) and colonial polyp (right). Images: medusa the Systematics Image Archive; polyps by TIMS, the Wikimedia Commons	FIGURE 13. A narcomedusa, a hydroid that has no polyp stage but has a parasitic planula larva. Image by NOAA, in the Public Domain


FIGURE 1A. Cnidarian topology based on rRNA phylogeny.	FIGURE 1B. Cnidarian topology based on mitochondrial coding gene phylogeny.


FIGURE 2. Living corals from the Great Barrier Reef. Image from http://www.ucmp.berkeley.edu/cnidaria/hydrozoa.html	FIGURE 3. The Giant Green Anenome, a large anthozoan with many concentric rows of tentacles around the oral disk. Image by NOAA, in the Public Domain	FIGURE 4. Iciligorgia schrammi, a soft coral. It has flexible branches that are covered with polyps. Image by NOAA, in the Public Domain


FIGURE 5. Aurelia, the Moon Jellyfish. Image by Dante Aligheri (sic), Wikimedia Commons	FIGURE 6. Cyanea, the Lion’s Mane Jellyfish. This is the largest known scyphozoan. The bell can be up to 2m in diameter. Image by Ole Kils, Wikimedia Commons	FIGURE 7. Cassiopeia, the Upside-Down or Mangrove Jellyfish. Image by Chris Hind, Wikimedia Commons


FIGURE 8. Chironex, the Sea Wasp. Image from http://www.aims.gov.au/pages/research/project-net/dma/pages/seawasp-01.html	FIGURE 9. Two Haliclystus polyps in the Antarctic. Image from Miranda et al. (2010)

DESCRIPTION OF THE PHYLUM CNIDARIA

Document Footer