Marine Invertebrates of Bermuda Green Sea Urchin (Lytechinus variegatus) By Amy Norris Dr. James B. Wood - Editor Taxonomy Habitat Ecology Recent Research Commercial Importance Bermuda Laws Personal Interest References Links

Phylum: Echinodermata
  Subphylum: Echinozoa
    Class: Echinoidea
      Subclass: Euechinoidea
        Superorder: Echinacea
          Order: Temnopleuroida
            Family: Toxopneustidae

 Class Echinoidea includes sea urchins, sand dollars, and heart urchins. The word itself come from the Greek word for ‘spiny’ and describes the general appearance of this class. As an echinoderm, sea urchins are related to sea stars, brittle sea stars, and sea cucumbers. Defining characteristics of echinoids include the presence of Aristotle’s lantern, a specialized mechanism used for feeding, and a rigid test formed by joined ossicles (Pechenik 2000). Sea urchins are small, normally globular shaped organisms surrounded by spines with radial pentamorous symmetry. Lytechinus variegatus, known commonly as the green or variegated sea urchin, is a regular urchin that inhabits the warm waters of the Western Atlantic Ocean. It has relatively short spines and can reach a diameter of about 110mm (4in) (Hendler et al. 1995). This species has a multitude of globiferous pedicellariae that are easily seen and appear as white or pink structures. L. variegatus also has a water vascular system and madreporite. It does not have a centralized brain but instead has a nervous system made up of radial nerves that extend into the spines, pedicellariae, and tube feet (Ruppert et al. 2004). This species also has tube feet and spines that aid in various activities such as locomotion and protection.
 

Habitat
 

Lytechinus variegatus is most commonly found in calm, clear waters such as seagrass beds but is also found on rock or sand. Studies have shown that L. variegatus is intolerant of water that is too turbid with suspended sediment (Moore et al. 1963). It inhabits the waters from North Carolina and Bermuda southward to the Caribbean and Brazil (Hendler et al. 1995). This species is usually not found in waters deeper than 50m (164 ft).

Ecology
 

General Characteristics
  The skeletal structure of the sea urchin is a rigid test or theca that is made up of plates encircling the mouth in the center of the oral side, encompassing the urchin’s inner organs (Nichols 1962). As the sea urchin grows, the entire test enlarges equally (Pechenik 2000). Pores in the five ambulacral plates allow the tube feet access to the external environment (Nichols 1962). The tube feet are one of the main components involved in locomotion and are discussed later. Spines, which can regenerate, cover the entire surface of the sea urchin test and play a role in protection as well as aiding in movement. All regular sea urchins have a structure known as Aristotle’s lantern which is adapted specifically for feeding. At the beginning of the intestine, a structure known as the siphon breaks off and re-enters the system further along. The function of the siphon is not certain although the common theory states that it is involved in removing water from food particles, aiding the digestive process. Like other echinoids, sea urchins, including Lytechinusvariegatus, also have pedicellariae, specialized pincer like structures that have been divided into four main types. First there are the tridactyles (three fingered), whose stem is supported by a calcite rod leaving the head and neck flexible. Second are the ophiocephalous (snake headed) pedicellariae, which are the most common. Next are the trifoliate (three leaved) pedicellariae that, as their name suggests, have broad leaf like blades and a highly flexible neck. Finally, there are the gemmiform or glandular pedicellariae which also have a calcite rod for support but have toothed blades to pierce and poison glands to paralyze prey. Tridactyle and ophiocephalous pedicellariae are proposed to play a role in cleaning off particles from the test and catching moving particles (Campbell 1983).  Sea urchins also have a water vascular system that transports water throughout the urchin, aiding in tube feet expansion (Pechenik 2000). The madreporite is located on the aboral surface and is thought to aid in water transport to the water vascular system but there is no conclusive evidence supporting that theory. A sea urchin’s nervous system, known as the ectoneural system, includes a ring around the esophagus as well as five nerves branching out to the edges of the test and into the spines.

 Movement
  Sea urchin locomotion varies greatly from other organisms such as fish and mammals. Most roving echinoderms move by means of their tube feet. Regular sea urchin tube feet are composed of highly developed suckers which are supported by a series of calcareous ossicles in the form of a ring (Nichols 1962). This ring, along with the rosette, forms the frame of the sucker area of the tube feet. The rosette is what keeps the tube feet shape during adhesion and release with a surface. It is actually the combination of these tube feet and the sea urchin spines that perform locomotion. The sea urchins’ spines are hollow tubes and most are highly developed (Nichols 1962). Spines, especially those with ball and socket joints, are able to rotate and push against a surface, aiding the activity of the tube feet (Lawrence 1987). Lytechinus variegatus spines have a ball, known as a tubercle, and socket joint which allows for a greater control over movement. L. variegatus is known to use its spines only while on a sandy surface and has been recorded at a rate of 82 mm per minute average (Parker 1936). The spines can also be used for a variety of tasks including digging protection, bearing poison glands, and creating currents.

 Reproduction
  Echinoids exhibit an annual reproductive cycle that can extend to over several months (Harvey 1956). Tennent (1910), looking at specimens along the Carolina coast, found that Lytechinus variegatus’ breeding season was between the end of May through July. Moore et al. (1963) discovered that L.variegatus spawned throughout the summer in Miami, Florida, but in Bermuda the spawning period was brief and exhibited a connection to the lunar cycle. There is inconclusive evidence that L. variegatus reproductive organs are ripe at the full moon and empty soon after (Tennent 1910). Sea urchins reproduce by releasing the unfertilized eggs and sperm into the water column, where the eggs are fertilized and develop into free swimming larvae known as plutei (Pechenik 2000; Harvey 1956). The larval form of L. variegatus, as with most echinoderms, is very different from the adult form and must undergo a complex metamorphosis to achieve adult form (Harvey 1956).

 Feeding
  Lytechinus variegatus feeds mainly on Thalassia sp., a type of seagrass, although is known to be omnivorous in captivity (Moore 1963). Many different structures aid in the feeding of a sea urchin. The spines and tube feet are both involved in the capture of drifting particles in the water column. However, L.variegatus has relatively short spines and uses tube feet primarily for feeding (Lawrence 1987). The tube feet act as an anchor for sea urchins while they graze on algae using Aristotle’s Lantern. The lantern functions as a rasping device to scrape at the turf algae growing on rock or other substrate. Aristotle’s lantern is provided with strong joints and teeth that are semi erect and constantly growing (Markel 1979; Nichols 1962). In L. variegatus, the lantern, made up of five calcareous pyramid plates, is in constant feeding motion, so seawater is taken up even when there is no food (Klinger 1984). Experiments on feeding with L. variegatus reveal that the green sea urchin feed more rapidly upon food that is in a large block shape versus food that is flat and blade like (Klinger 1982). It was hypothesized that the thinner food items took more time to eat since there was more repositioning involved. The manipulation of a piece of food was found to be important in determining the feeding rate of L. variegatus. Food particles travel through the esophagus, running through the center of the lantern, and straight into a long intestine that encircles the urchin internally, doubles back, continues halfway around and then rises to the aboral side to form the anus (Nichols 1962 ;Pechenik 2000).
 

Recent Research
 

Lytechinus variegatus has a covering mechanism that, according to popular theory, is due to photosensitivity. However, most past research on L. variegatus has involved embryology, with studies observing their ecological behavior being more recent (Cheng unpublished, 2000). L. variegatus eggs are used to study the roles of various chemicals and proteins throughout fertilization (Bachman and McClay 1995) as well as isolate chemical ions (Salmon and Segal 1980). Although not as recent, Sharp and Gray (1962) reported that L. variegatus displayed a covering reaction, loading various materials from the surrounding area onto its aboral surface and keeping the objects stable through the use of tube feet. Millott’s (1956) previous experiments had revealed that in L. variegatus, the covering behavior is connected with change in light and that the covering was a response to block strong light and UV radiation. Other experiments have been done on the physical and chemical properties of L. variegatus. Veis et al. (2002), examined the pattern of chemical distribution in the five teeth located in Aristotle’s lantern.

Commercial Importance

Sea urchins are primarily used by humans for food, although only certain species are edible (Harvey 1956; Pechenik 2000). The only part of a sea urchin that is eaten are the gonads, and they are usually eaten raw. Urchins are served throughout Italy, France, the West Indies, and Japan although are not usually found in England or America unless in specialty stores (Harvey 1956). L. variegatus is not one of the sea urchins commonly used in Bermuda for gourmet dishes. Sea urchin shells are used as cups, lamps, and ground up for inks in various cultures. The sea urchin skeleton is also valued in art or as jewelry and can be found in various tourist shops across Bermuda.

Bermuda Laws

            None

Personal Interest
 

My personal interest in Lytechinus variegatus is mainly due to my choice of research project for an invertebrate zoology course. I had observed sea urchins along the benthos of Ferry Reach in Bermuda and was interested in why they had various materials covering their aboral side. I decided to do my project on the preference of materials that L. variegatus chose as well as what made them cover in the first place. The favored theory is that the sea urchin is protecting itself from high intensity sunlight by blocking the UV radiation with objects found in the surrounding environment.

References

Bachman, E., McClay, D. 1995. Characterization of moesin in the sea urchin Lytechinus
 variegatus: redistribution to the plasma membrane following fertilization is
 inhibited by cytochalasin B. J. of Cell Sci. 108:161.

Cheng, Y. 2000. Lytechinusvariegatus covering behaviour: is this light intensity   dependent and is there a preference for covering materials? Unpublished. Bermuda Biological Station for Research.

Campbell, A.C. 1983. Form and function of pedicellariae. In: Jangoux, M. and Lawrence,
J.M., editors. Echinoderm Studies. Netherlands: A.A. Balkema p 139-167.

Harvey, E.B. 1956. The American Arbacia and Other Sea Urchins. New Jersey: Princeton  University Press Princeton; 298p.

Hendler, G., Kier, P.M., Miller, J.E., Pawson, D.L. 1995. Sea Stars, Sea Urchins, and  Allies: Echinoderms of Florida and the Caribbean. Washington: Smithsonian  Institution Press; 390p.

Klinger, T.S. 1982. Feeding rates of Lytechinus variegatus Lamarck (Echinodermata:  Echinoidea) on differing physiognomies of an artificial food of uniform  composition. In: Lawrence, J.M., editor. Echinoderms: Proceedings of the   International Conference, Tampa Bay. Netherlands: A.A. Balkema p 29-32.

Klinger, T.S. 1984. Feeding of a marine generalist grazer:Lytechinus variegatus    (Lamarck) (Echinodermata: Echinoidea). Unpublished PhD thesis, University of  South Florida, Tampa.

Lawrence, J. 1987. A Functional Biology of Echinoderms. Maryland: The Johns Hopkins   University Press; 340p.

Markel, K. 1979. The lantern of Aristotle. In: Jangoux, M., editor. Proceedings of the   European Colloquium on Echinoderms Brussels in Echinoderms: Present and   Past. Netherlands: A.A. Balkema p 91.

Millott, N. 1956. The covering reaction of sea urchins. 1. A preliminary account of  covering in the tropical echinoid Lytechinus variegatus (Lamarck), and its relation   to light. J. Exp. Biol. 33:508.

Moore, H.B., Jutare, T., Bauer, J.C., Jones, J.A. 1963. The biology of Lytechinus  variegatus. Bull. Marine Sci. Gulf Carib. 13:23-53.

Moore, H.B. 1966. Ecology of echinoids. In: Boolootian, R.A. editor. Physiology of  Echinodermata. New York: John Wiley and Sons; pp. 157-218.

Nichols, D. 1962. Echinoderms. London: Hutchinson & CO LTD; 192p.

Parker, G.H. 1936. Direction and means of locomotion in the regular sea urchin  Lytechinus. Mem. Musee Roy. d’Hist. Nat. Belgique. Ser. 2. 3:197.

Pechenik, J.A. 2000. Biology of the Invertebrates, Fourth Edition. U.S.A.: McGraw-Hill  Companies, Inc.; 578p.

Ruppert, E.E., Fox, R.S., Barnes, R.D. 2004. Invertebrate Zoology: A Functional  Evolutionary Approach 7th Edition. California: Brooks/Cole; 963p.

Sharp, D.T., Gray, I.E. 1962. Studies on factors affectin the local distribution of two sea  urchins, Arbacia punctulata and Lytechinus variegatus. Ecology 43:309.

Tennent, D.H. 1910. Variation in echinoid plutei. J. Exp. Zool. 9:657

Vies, A., Barss, J., Dahl, T., Rahima, M., Stock, S. 2002. Mineral-related proteins of sea
 urchin teeth: Lytechinusvariegatus. Microscopy Research and Technique.
 59:5:342.
 
 

Links

Smithsonian Marine Station at Fort Pierce Field Guide to the Indian River Lagoon, Florida

Urchins II

Wikipedia The Free Encyclopedia