Marine Invertebrates of Bermuda Long-spined sea urchin (Diadema antillarum) By K. Clay Farland Dr. James B. Wood - Editor Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Taxononmy


Phylum: Echinodermata
  Subphylum: Echinozoa
    Class: Echinoidea
      Subclass: Euechinoidea
        Superorder: Diadematacea
          Order: Diadematoida
            Family: Diadematidae


Sea urchins are spiny, globe-shaped animals that are related to starfish. Classified in the Phylum, Echinodermata, the long-spined sea urchin, Diadema antillarum, categorized as a "regular" sea urchin, has obvious radial symmetry, a central mouth on the underside, and a central anus on the upper surface (Sterrer 1986). They are usually black in color, but can sometimes be white (Sterrer 1986). Diadema, in adult form, can have a diameter of 50 cm including spines that can cause painful puncture wounds (Sterrer 1986). Their movement is slow, using either spines, which can be three times as long as the diameter of the test, or tube feet for motion (Sterrer 1986). They are found in shallow water up to 400 m (Sterrer 1986). Collecting is best done by SCUBA on sediment bottoms (Sterrer 1986).

Habitat


Diadema antillarum are found on the surfaces of hard and soft substrates to several hundred meters depth (Sterrer 1986). These echinoderms are important herbivorous urchins that prey on macroalgae. They are often found in coral reef ecosystems where increased macroalgal presence can be attributed to immense productivity of reef ecosystems.

The long-spined sea urchin is widely distributed in both the eastern and western tropical Atlantic Ocean (Ogden 1973). Its habitat ranges throughout the West Indian region from Florida to Surinam, and was abundant until about 20 years ago (Aronson and Precht 2000) in shallow subtidal waters protected from strong wave action (Ogden 1973).

Ecology


General Introduction and Characteristics

The class: Echinoidea, under which Diadema antillarum are classified, includes sea urchins and sand dollars. Some shared physical characteristics of this class include: (1) shape ranges from spherical to disc shape, (2) skeletal plates join to form a rigid test, (3) independently moveable spines, approximately three times as long as the diameter of the test, (4) five radial areas with plates perforated for tube feet (Sterrer 1986). Life expectancy is approximately 5 years for most species of class Echinoidea (Sterrer 1986).

Watch out! Be careful when handling these creatures. Their extremely sharp spines can crumble after penetration into the skin, inflicting painful wounds (Sterrer 1986). They will not try to hurt you, unless you grab them; this is a defensive mechanism of Diadema that protects them from predation. Predators include gastropods, crabs, starfish (within their own phylum!), and fish including surgeonfish, porcupine fish, and wrasses.

Sterrer (1986), in his guide to the systematics of marine organisms of Bermuda, describes this species as "common" in Bermuda. However, this is not the case anymore. A global mass-mortality of Diadema antillarum, occurring over a two-year period from 1983-1984, caused by a yet to be identified pathogen, significantly reduced the Bermudan population.

Timing of Activity Patterns

It is well known that these urchins are more active at night, and many hide in crevices during the day (Ogden 1973). However, bright light may be tolerated (Ogden 1973). In his report, Ogden (1973) observed that the activity cycle of Diadema antillarum begins from 1500 to 1800 hours and continue to peak at 2400 to 0300 hours; daytime inactivity is fully established at 0600. At the time of his report, John C. Ogden (1973) reported that the animals were often seen in fair number exposed during the day in shallow water in Bermuda.

Judging by the presence of food in the esophagus and foregut, Ogden (1973) observed that feeding activity is greatest in the afternoon and evening, although feeding may occur at any hour. In contrast, the specimens kept in their laboratories were mostly inactive during the day and fed most often at night. Ogden (1973) suggests that activity could be controlled by more than just light level, as movement can vary greatly from location to location. He suggests further research.

Movement

From his research, Ogden (1973) observed at least two different types of movement patterns in Diadema antillarum. Urchins that have a crevice in which they hide by day tend to move less than 1 m away from the crevice at night and return to it at dawn. Those without crevices in which to hide move around without homing tendency.

Reproduction and Development

In order for their eggs to be fertilized, male and female Diadema must release their eggs and sperm into the water at the same time (Simon 1973). To make sure this happens, the male or female sea urchin sends a chemical substance into the water, and the smell of this substance makes every sea urchin in the vicinity release its eggs or sperm (Simon 1973). This mass release increases the chance that the eggs will be fertilized (Simon 1973).

Shortly after fertilization, the blastula is free-swimming (Sterrer 1986). The gastrula develops into a planktonic larval stage, known as echinopluteus, with a laterally compressed shape, bearing 4-6 pairs of arms supported by calcareous spicules (Sterrer 1986). After a few weeks, the echinopluteus sinks to the substrate where it rapidly metamorphoses into a juvenile urchin. The metamorphoses can occur within one hour (Sterrer 1986).

Population Dynamics and Structure

Predators of Diadema antillarum include at least 15 fish species, especially several species of grunts (Pomadasyidae) and queen triggerfish (Balistes vetula), as well as several gastropods (Ogden 1973). At the time of Ogden's (1973) research, D. antillarum populations were abundant. He hypothesizes that a reason for the abundance could be overfishing of the fish that prey on D. antillarum, many of which are highly favored as food by humans. When Ogden (1973) performed his study, he recorded high densities of D. antillarum on the patch reefs of St. Croix. He recorded densities in excess of 20 individuals per meter squared in Christiansted Harbor alone.

The urchins were unequally distributed with respect to size in a patch reef used for Ogden's (1973) study. He observed the smaller urchins in the middle of the patch reef and the larger ones toward the edge. He suggests that this may be due to an aggressive dominance hierarchy based on size, and that the larger urchins select 'favored' areas at the edge of the reef.

Food

Although starved individuals may show predatory behavior, Diadema antillarum are generally herbivorous, consuming mainly algae or detritus and sediments (Ogden). A total of 14 genera of red, green, brown, and blue-green algae plus diatoms of red and blue-green algae were found in the guts of 6 D. antillarum from different locations along a transect taken along the barrier reef in St. Croix (Ogden 1973). Species of algae were not identified.

These urchins often feed in Thalassia (turtle grass) beds, feeding on various algae and plants (Ogden 1973). It is uncertain whether or not urchins can efficiently digest the cellulose in Thalassia (Ogden 1973). However, Thalassia beds support a rich flora of epiphytic algae and diatoms (Ogden 1973).

General Ecology

Sea urchin grazing is important to the structure and diversity of benthic communities. In areas where its predators have been eliminated, Diadema antillarum may increase in numbers and come to occupy a position near or at the top of the food web. In order to support high concentrations of D. antillarum, the substrates on which they live must have fairly high productivity in order to support the nutritional needs of the urchins. In his study on urchin populations in St. Croix, Ogden (1973) found that the biodiversity, based on the diversity of algal species, was greater in the patch reef with predation by urchins. In contrast, the patch reef lacking a Diadema population was dominated by several species, mainly Padina sancta-crucis with Dictyota spp. and Turbinaria sp. of secondary importance. Padina is a fast-growing algae that may outcompete or shade out other species, ultimately reducing biodiversity.

(This is elaborated below in subcategory: Interesting Research.)

Recent Research


Annual and lunar reproductive rhythms

In Bermuda, Diadema antillarum spawning occurs from early summer to early winter, peaking in early summer and late fall (Iliffe and Pearse 1982). A well-defined lunar rhythm stimulates the creation of gametes, a process that is closely synchronized among individual urchins, and the animals in Bermuda spawn around the same time as the new moon stage in the lunar cycle (Iliffe and Pearse 1982). Iliffe and Pearse (1982) suggest that lunar synchrony, coupled with a spawning pheromone, may serve to maximize fertilization success. In their study, specimens of D. antillarum were collected from the north side of the Causeway in Castle Harbour, Bermuda. It is interesting to note that they picked this location, in addition to its easy accessibility, because it was the only inshore area where they found a large population of these sea urchins.

Herbivory and Algal Dynamics: The importance for coral reefs

Aronson and Precht (2000) have observed a decline over the past two decades in coral cover and a substantial increase in the cover of flesh and filamentous macroalgae on coral reefs in the western Atlantic and Caribbean region. Many scientists attribute this shift in coral and algal populations to coral mortality and reduced herbivory, as the primary factors. Herbivory was reduced by the mass mortality of the echinoid D. antillarum in 1983-1984 (Lessios 1988). Lessios (1988) discovered urchins dying near the Atlantic outlet of the Panama Canal in January 1983 (Knowlton 2001). The mortality spread at first slowly, but then with greater speed, reaching Bermuda in September 1983 (Knowlton 2001). Diadema populations were reduced to at least 7% and often to less than 2% of their sizes prior to the mass-mortality within a few days of the onset of symptoms (Knowlton 2001). According to Knowlton (2001), we still do not know what caused the mass mortality. She explains that the pattern of spread, largely following currents, and specificity (no other urchin species was affected) make a pathogen a near certainty.

In their study on herbivory and algal dynamics, Aronson and Precht (2000) conducted surveys on the coral reef at Discovery Bay, Jamaica, in February of each year during the period from 1993-1999, with the exception of 1997. Using Line Point Technique, counts were made of the following categories: (1) fleshy and filamentous macroalgae, (2) hard corals, (3) calcareous green algae, and (4) a category combining crustose coralline algae, algal microturfs, and bare space. Counts were also made of D. antillarum. The species was rare from 1993-1996, but after 1996, the population density of D. antillarum at their study site remarkably increased. Their results showed high macroalgal cover through 1998 with a sudden drop from about 60% to 10% in 1999. Counts for the category combining crustose coralline algae, algal microturfs, and bare space showed the opposite pattern with low cover through 1998 and a remarkable increase from about 15% to 60% in 1999. Coral cover was low throughout the study, but gradually rose from 1995-1999. The counts for the calcareous green algae did not show a trend. To summarize their results, benthic cover changed as the abundance of the population of D. antillarum increased. This reversal of trends in herbivory and macroalgal cover confirms the importance of herbivory (Aronson and Precht 2000).

The increased abundance of macroalgae negatively affects coral growth and recruitment, having long-term effects on the physical structure of the reef (Ostrander et al. 2000). Macroalgae can rapidly colonize a substrate as long as herbivores such as D. antillarum, nutrient limitation, and physical factors do not inhibit them (Ostrander et al. 2000). Corals cannot successfully colonize a substrate dominated by algae (Ostrander et al. 2000). Therefore, it is not surprising that the removal of a major herbivore would obviously lead to a significant change in the interaction between corals and algae (Ostrander et al. 2000). In their study on the recovery of D. antillarum, Edmunds and Carpenter (2001) document a sustained recovery on a Jamaican reef, not only for the urchins but for the corals as well. They monitored shallow reef at five sites along the Jamaican coastline now characterized by a sea urchin-grazing zone. In comparison to the seaward algal zone, they found macroalgae to be rare in the urchin zone, where they measured the density of D. antillarum to be ten times higher and the density of juvenile corals to be eleven times higher. The densities found by Edmunds and Carpenter (2001) are close to those recorded in the late 1970s and early 1980s. They suggest that the widespread recovery of D. antillarum populations alone may result in reefs dominated again by corals.

Some interesting questions raised by Knowlton (2001) at the end of her report include:

  1. Will recruitment success spread rapidly across the Caribbean, and will it follow the paths of currents (as the apparent pathogen did)?

  2. Can recruitment be facilitated locally by algal removal or removal of potential predators on juvenile urchins?

  3. Will this apparent recovery persist at all, much less spread?

With the importance of biodiversity in mind, especially because of recent coral vulnerability and mortality, we can only hope that this apparent recovery of the Diadema population will continue and spread.

Commercial Importance


Medical Research

Sea urchin larvae are used in embryology and molecular biology research, in addition to biomedical research.

Seafood

In some countries, the gonads of some species of sea urchins are eaten (Sterrer 1986). However, they are not eaten in Bermuda. Diadema antillarum have indirect commercial importance in Bermuda, due to their importance in the marine food web.

Bermuda Laws


None.

Personal Interest


My interest in Diadema antillarum originates in my interests in climate change and marine ecology. Because of its importance for corals as an algae consumer, the role of D. antillarum in the health of some of the worlds most important regions of biodiversity captured my interest.

Many other animals and plants rely on corals for shelter and food. It is the reliance of these species on corals, and in some cases, the reliance of the corals on some of these species, that results in the vast biodiversity found on reefs. Recent damages to coral reefs are most often attributed to human impacts, including overexploitation, overfishing, increased sedimentation, and nutrient overloading. From my research on the mass-mortality of Diadema antillarum, it is possible and likely that some of these coral reefs were suffocated by the algal population explosion following the urchin wipeout.

During the 1980s, nearly all of the world's coral reefs experienced various amounts of bleaching and mortality. Before the 1980s, most coral bleaching events were induced by non-temperature related events such as storms and aerial exposure due to extreme low tides. When bleaching events occurred together with periods of elevated water temperatures before the 1980s, they were geographically isolated and within certain reef zones. Quite the reverse is the case with coral bleaching events recorded during the 1980s; these occurred over large geographic regions (Pockley 1999). The mass-mortality of Diadema antillarum occurred over a two-year period from 1983-1984 (Lessios 1988). The lack of geographic isolation of the 1980s combined with a rapid unprecedented decline of the D. antillarum population, with populations reduced to at least 7% and often less than 2% of their former numbers over the course of two years (Knowlton 2001), led me to believe that this species may have been an important factor in the rapid decline of coral populations in the 1980s.

References

Aronson, Richard B., and William F. Precht. "Herbivory and algal dynamics on the coral reef at Discovery Bay, Jamaica." Limnol. Oceanogr. 45.1 (2000): 251-255.

Edmunds, Peter J. and Robert C. Carpenter. "Recovery of Diadema antillarum reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef." Proc. Natl. Acad. Sci. 98.9 (2001): 5067-5071.

Iliffe, Thomas M. and John S. Pearse. "Annual and lunar reproductive rhythms of the sea urchin, Diadema antillarum (Philippi) in Bermuda." International Journal of Invertebrate Reproduction 5 (1982): 139-148.

Knowlton, Nancy. "Sea urchin recovery from mass mortality: New hope for Caribbean coral reefs?" Proc. Natl. Acad. Sci. 98.9 (2001): 4822-4824.

Lessios, H. A. "Mass Mortality of Diadema antillarum in the Caribbean: What Have We Learned?" Ann. Rev. Ecol. Syst. 19 (1988): 371-393.

Ogden, John C., ed. Studies on the Activity and Food of the Echinoid Diadema antillarum Philippi on a West Indian Patch Reef. U. S. Virgin Islands: West Indies Laboratory, 1973.

Ostrander, Gary K., Kelley Meyer Armstrong, Edward T. Knobbe, Donald Gerace, and Erik P. Scully. "Rapid transition in the structure of a coral reef community: The effects of coral bleaching and physical disturbance." Proc. Natl. Acad. Sci. 97.10 (2000): 5297-5302.

Pockley, Peter. "Global warming 'could kill most coral reefs by 2100'." Nature 400 (1999): 98.

Simon, Seymour. From Shore to Ocean Floor: How Life Survives in the Sea. New York: Franklin Watts, Inc., 1973.

Sterrer, Wolfgang., ed. Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marine Organisms. New York: John Wiley and Sons, 1986.

Links

University of California Cooperative Extension: Sea Grant Extension Program Publication
Sea Urchin Embryology
Mass Mortality in Diadema antillarum
Color images of living echinoderm larvae