Sea urchins, echinoids

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Taxonomy

Sea urchins belong to phylum Echinodermata (echinoderm = "spiny skin").

Phylum Echinodermata is represented by 20 extinct and 6 extant classes:

• Class Crinoidea - sea lilies and feather stars;
• Class Asteroidea - sea stars;
• Class Ophiuroidea - brittle stars;
• Class Echinoidea - sea urchins and sand dollars;
• Class Holothuroidea - sea cucumbers, radial symmetry is not that obvious;
• Class Concentricycloidea - sea daisies

Echinoderms possess the following common characteristics:

• marine, benthic, slowly moving organisms;
• possess an endoskeleton composed of calcium-plate ossicles;
• the plates of the endoskeleton have perforations through which tube feet are extended;
• thin epidermis completely covers the endoskeleton including spines;
• have no head or brain, five-part body plan;
• have a unique water vascular system that controls the extension and contraction of flexible tube feet;
• possess several organs, which functions are not yet clear;
• larvae are bilaterally symmetrical, while the adults exhibit secondary radial pentamerous symmetry

Class Echinoidea (echinoids - sea urchins and sand dollars) possess the following common characteristics:

• lack distinct arms;
• five rows of tube feet protrude from plates of skeleton;
• possess distinct openings for mouth and anus;
• endoskeletons are made up of fused calcareous plates;
• all echinoids except for Heliocidaris, which brood young, exhibit indirect development through planktotrophic echinoplutei, larvae that have cilia on long arms, unlike those of other classes

Taxonomic lineage

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Deuterostomia - Echinodermata - Eleutherozoa - Echinozoa - Echinoidea - Euechinoidea - Echinacea - Echinoida - Strongylocentrotidae - Strongylocentrotus

Species

• Strongylocentrotus droebachiensis (green sea urchin) - northeast and northwest coasts of North America and in Europe;
• Strongylocentrotus franciscanus (red sea urchin) - North American Pacific coast from Alaska to Baja, harvested commercially for roe;
• Strongylocentrotus intermedius - northern regions in the Pacific coastal waters of Choshi, Chiba, the Sea of Japan around Toyama, the Korean peninsula, northeastern China, Sakhalin, and Vladivostok;
• Strongylocentrotus nudus - harvested commercially in the Pacific Ocean from Ibaragi to Hidaka, Hokkaido, and in the Sea of Japan from Toyama to Soya, Hokkaido;
• Strongylocentrotus pallidus (pale sea urchin) - widespread epibenthic species in high-Arctic waters;
• Strongylocentrotus polyacanthus - along the coasts of Kamchatka peninsula;
• Strongylocentrotus purpuratus (purple sea urchin) - North American Pacific coast from Alaska to Baja.

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Brief facts

• Ecology

  Sea urchins are herbivorous and as such are major kelp grazers in shallow seas worldwide. They are preyed upon by many predators. In some ecosystems, they are a keystone species that define conditions of living of majority of other members of the ecosystem and their harmonious co-existence. Explosions of sea urchin populations such as occurred in the early 1970s along the Atlantic coast of Nova Scotia result in conversion of macroalgal beds (kelp forests) into coralline barrens. Decimations of sea urchin populations such as happened when disease struck the long-spined sea urchin Diadema antillarum in 1980s in Caribbean basin result in algal overgrowth and severe depression of coral reefs ecosystems.

  In nature the sea urchin feeds upon various algae (green, red and brown). If starved, sea urchins can consume almost anything available such as rotten carcasses, fruits, leaves, etc.

  Along Pacific coast of United States, Strongylocentropus purpuratus (purple sea urchin) populations are controlled by large spiny lobsters Panulirus interruptus, sea star Pycnopodia helianthoides, sheephead fish Semicossyphus pulcher, large predatory sea snail Fusitriton oregonensis, and by sea otters (Enhydra lutris, which tend to decimate urchin population .

• Importance for humans

  The red sea urchin, S. franciscanus, a conspicuous member of subtidal communities in the North Pacific, is sought for its gonads, which are considered as delicacy.

  The purple sea urchin, S. purpuratus, is an important model organism in biology.

  Common regular sea urchins often used in invertebrate zoology courses to exemplify echinoid anatomy. Example of regular sea urchins are urchins from genera Strongylocentrotus, Arbacia, Lytechinus, and Echinus.

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External morphology

Oral pole of sea urchin

Image demonstrates elements of external morphology: peristome, pentameric distribution of tube feet, and spines.

Peristome

Peristome is a region around mouth of sea urchin that is devoid of spines and is not underlain by skeleton.

The triangular white tips of the fife teeth of Aristotle's lantern can be seen protruding from the mouth.

Tube feet (podia)

The 10 meridional rows of tube feet, or podia, extend between the oral and aboral poles. The 10 rows of tube feet are arranged into five ambulacra consisting of two rows of tube feet each. The ambulacra are separated by zones without tube feet, known as interambulacra. The tube feet are mostly used for locomotion and respiration. Some urchins have specialized tube feet that can be suckerless. The sucker is reinforced with tiny flat ossicles.

Spines

The spines of a sea urchin is an example of natural molecular engineering. Each tough, fracture-resistant spear is a crystal of calcite, which was rendered flexible by incorporation of protein molecules throughout its structure.

The spines are part of the endoskeleton and are covered with epidermis.

Spines that are articulate with tuberceles (sockets) may be used in locomotion, defense, protection against sediment abrasion, excavating, feeding, and burrowing.

Spines can move in a wide range of directions by contraction of muscles. Locking mechanism implemented with an inner ring of collagen fibers allow fixing spines so rigidly that they cannot be moved without breaking.

Walking Diadema antillarum

Strongylocentrotus franciscanus

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Internal morphology

High resolution posters of this image for a classroom or office are available at GeoChemBio shop

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Life cycle

S. purpuratus requires temperature 15 °C or lower for its development.

Life Cycle Stages

• Unfertilized egg The eggs complete meiosis in the ovaries and can be fertilized immediately after spawning. Millions of gametes are produced by one sea urchin annually.
• Fertilized egg MeSH
• Embryo The eggs and embryos of echinoderms have remarkable abilities to reorganize after experimental disturbances and form normal larvae.
  • Cleavage MeSH Dividing egg, approx. 7 h after fertilization.
  • Morula MeSH
  • Blastula MeSH Approx. 20 h after fertilization.
  • Gastrula MeSH Approx. 40 h after fertilization.
• Larval The larva of sea urchin is called pluteus. The larval period lasts for ~1 month. The maximum life span of the pluteus that was not induced to metamorphose is probably about 4 months and depends on nutrition (underfed larvae live longer). However, in about 2 months larva loses its ability to metamorphose.
  • Early pluteus Pluteus is bilaterally symmetrical. In laboratory, the larvae are fed on microscopic flagellated algae such as Dunaliella, Rhodemonas, and Pyranimonas. Echinoderm larvae have remarkable ability to regenerate and , in some taxa, to propagate asexually.
  • Mature pluteus Mature larva is about 1.5 mm in size. The external features of the developing larvae gradually increase in complexity: new spicules and their associated arms develop, and in some species, dense ciliary bands called epaulets form. Three pedicellarieae appear: 2 on the right side and one at the posterior end of the larva.
  • Urchin embryogenesis Development of the embryonic urchin inside the growing larva. Few, if any, of the larval organs give rise to comparable organs in the adults. The developing urchin, while it is in the larva, is called rudiment. Rudiment constitutes a ventral half of the future urchin and is represents primarily by ventral skeleton and water vascular system. The larva provides nutrients and protects the rudiment.
• Metamorphosis This stage is initiated by a chemical cue of bacterial origin and usually starts with the settling of the larva on its left side on to an appropriate substrate covered with bacterial biofilm. Within about an hour, all that remains of the larva is a lump of tissue on top of the young urchin. Over the next 24 h the spines of the urchin greatly elongate. The digestive system and other organs develop in 4 or 5 days after settling and urchin begins feeding.
• Juvenile Less than 25 mm in diameter, growing sea urchin: the number of spines and tube feet increases, gonads start developing at about 2 months of age.
• Adult Age of sexual reproductive maturity is attained at about 2 years of age in wild, however, well-fed laboratory Lytechinus pictus can be induced to spawn at 4.5 month of age when urchin reaches about 1 cm in size. Average life span of sea urchin is about 7-10 years in laboratory and about 2-3 years in the wild.

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References

• Ziegler A, Ogurreck M, Steinke T, Beckmann F, Prohaska S, Ziegler A. Opportunities and challenges for digital morphology. Biol Direct. 2010 Jul 6;5:45.

  

  Various traditional and digital morphological visualization techniques, shown in an exemplary fashion using cidaroid sea urchins (Echinoidea: Cidaroida).

• Ziegler A, Faber C, Mueller S, Bartolomaeus T. Systematic comparison and reconstruction of sea urchin (Echinoidea) internal anatomy: a novel approach using magnetic resonance imaging. BMC Biol. 2008 Jul 23;6:33.

  

  Overview chart showing analyzed specimens of 'regular' sea urchins and corresponding 3D reconstructions of selected internal organs.
  (A) Information on species name, geographic distribution, and systematics.
  (B) Photograph of scanned specimen, aboral view.
  (C)-(E) 3D models of reconstructed selected internal organs, stepwise turned by 90°;
  (C) aboral view (interambulacrum 5 facing upwards);
  (D) lateral view (interambulacrum 5 at back);
  (E) oral view (interambulacrum 5 facing downwards).

• Ziegler A, Mooi R, Rolet G, De Ridder C. Origin and evolutionary plasticity of the gastric caecum in sea urchins (Echinodermata: Echinoidea). BMC Evol Biol. 2010 Oct 18;10:313.

  

  Digestive tract anatomy of selected "regular" sea urchin taxa (Parechinidae - Toxopneustidae).
  Parechinidae (A-D),
  Echinidae (E, F),
  Echinometridae (G, H),
  Strongylocentrotidae (I, J),
  and Toxopneustidae (K, L).
  (A) from [27, Fig. 9] - reproduced in modified form with kind permission from Mr. Thierry Powis de Tenbossche. (C, F, K) from [22, Figs. 1, 4, 7, Pl. II] - reproduced in modified form with kind permission from L'Institut Océanographique, Fondation Albert Ier, Prince de Monaco. (H) from [23, Fig. 9] - reproduced in modified form with kind permission from The Royal Society of New Zealand. AB = aboral view, OR = oral view. d = dilation, e = esophagus, f = festoon. Not to scale.

• Holland ND, Ghiselin MT. Magnetic resonance imaging (MRI) has failed to distinguish between smaller gut regions and larger haemal sinuses in sea urchins (Echinodermata: Echinoidea). BMC Biol. 2009 Jul 13;7:39.
• Ziegler A, Faber C, Bartolomaeus T. Comparative morphology of the axial complex and interdependence of internal organ systems in sea urchins (Echinodermata: Echinoidea). Front Zool. 2009 Jun 9;6:10.
• Mooi R, David B, Wray GA. Arrays in rays: terminal addition in echinoderms and its correlation with gene expression. Evol Dev. 2005 Nov-Dec;7(6):542-55.
• Littlewood DT, Smith AB. A combined morphological and molecular phylogeny for sea urchins (Echinoidea: Echinodermata). Philos Trans R Soc Lond B Biol Sci. 1995 Jan 30;347(1320):213-34.
• Cameron RA, Hinegardner RT. Initiation of metamorphosis in laboratory cultured sea urchins. Biol Bull. 1974 Jun;146(3):335-42.
• Hinegardner RT. Morphology and Genetics of Sea Urchin Development. Amer. Zool. (1975) 15(3): 679-689.
• Hinegardner RT. Growth and development of the laboratory cultured sea urchin. Biol Bull. 1969 Dec;137(3):465-75.
• Vickery MS and McClintock JB. Comparative Morphology of Tube Feet Among the Asteroidea: Phylogenetic Implications. Amer. Zool. (2000) 40(3): 355-364.
• Mead D, Moseley L. Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms. AMER. ZOOL.. 38:965-974 (1998) (.pdf)
• Lacalli TC. The Structure and Arrangement of Echinoid Tubercles. Invertebrate Biology Vol. 119, No. 2 (Spring, 2000), pp. 234-241.
• Smith AB. Larval Budding, Metamorphosis, and the Evolution of Life-History Patterns in Echinoderms. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences Vol. 289, No. 1033 (May 7, 1980), pp. 1-54.
• FARMAFARMAIAN A and PHILLIPS JH. DIGESTION, STORAGE, AND TRANSLOCATION OF NUTRIENTS IN THE PURPLE SEA URCHIN (STRONGYLOCENTROTUS PURPURATUS). Biol Bull 123: 105-120. (August 1962)
• FENNER DH. THE RESPIRATORY ADAPTATIONS OF THE PODIA AND AMPULLAE OF ECHINOIDS (ECHINODERMATA). Biol Bull 145: 323-339. (October 1973).

Websites and other references

• Invertebrate Anatomy OnLine: Strongylocentrotus drobachiensis

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