Feeding Guilds in Reef Echinoderms

Echinoderms have a number of larval forms, but some classes that are distinctly different as adults have similar larvae. Almost all of them have a digestive tract and are planktotrophic, with the exception of crinoid larvae, which apparently do not feed. Asteriods are almost always gonochoric and broadcast spawn eggs and sperm. A single female can produce >2x10⁶ eggs per season. The initial larval stage is called a bipinnaria (below, left) which is a bilaterally symmetrical larva with ciliated bands around the mouth and on the larval arms. Phytoplankton and other planktonic organisms are captured by the cilia and transported to the moiuth. An advanced asteroid larva with multiple arms is called a brachiolaria as shown below (right). The brachiolaria has an adhesive disc or sucker (yellow portion) that is used for attachment. The attachment phase precedes metamorphosis to the adult. In a few asteroids the bipinnaria is the settling stage and no brachiolaria is formed.

Ophiuroids and echinoids can be considered together since they both produce a distinctive and similar larval form. The majority of brittlestars are gonochoric (but hermaphrodite species are not unusual). Most are broadcast spawners like asteroids, but some brooders occur. The larval form, called an ophiopluteus, has a distinctive V-shape with 6 larval arms. Unlike asteroids, there is no settlement stage prior to metamorphosis. The larvae of a six-armed group (genus Ophiactis) are capable of clonal development by casting off an arm and then regenerating the rest of the larva from it.

All echinoids are gonochoric, and most of them are broadcast spawners. Their larvae arise from small eggs, develop into an echinopluteus, and spend a few weeks feeding and growing in the water column before metamorphosing. However, approximately a third of all echinoids with known mode of development have larvae that do not feed, and instead develop directly from large, well-provisioned eggs, eliminate the pluteus larva, and metamorphose after only a few days in the water column.

Echinopluteus larva

http://faculty.washington.edu/ziglerk/Helio.JPG

Holothurians go through an initial larval stage called an auricularia, which is similar to the asteroid bipinnaria. However, it then forms a barrel-shaped doliolaria larva which closely resembles the larva with the same name, typical of crinoids. As mentioned above, the crinoid doliolaria does not feed.

Holothurian auricularia larva

Source unknown

Fish

In common with many marine animals, fishes that live on coral reefs have a two-part life history: a relatively sedentary adult phase on the reef, and a potentially very mobile pelagic larval phase in open water. Adult reef fishes take little or no care of their young: most larvae end up off the reef into open water where they are left to fend for themselves for anywhere from two to 20 weeks before they must find a coral reef to settle on. Little is known of the biology of these tiny (typically, 1-20 mm long) fishes during this pelagic period in open water. Because most dispersal takes place during the pelagic larval phase, it is important to both researchers and managers to know what the larvae are doing, where they are doing it, and how far they move during the pelagic phase. Fish larvae are provisioned with a yolk sac, the size of which varies considerably, and determines whether the larva will be lecithotrophic and have a short larval life, or if they adopt a planktivorous habit, as many do, and have a longer life in the plankton.

Feeding Guilds in Reef Echinoderms

There are five living classes of echinoderms. All are represented on reefs and contribute to its structure by means of adding their calcitic remains to the sediments. This section will focus on each group and will examine representative cases of echinoderm interaction on the reef. It is assumed that you are familiar with the basic elements of echinoderm biology, including pentamerous symmetry, the water vascular system, (including the podia, madreporite, ambulacrum), and other general anatomical features..

The Asteriodea

There are about 1500 species of starfish, a few of which are well represented on reefs. Most are scavengers and carnivores that feed on a variety of other invertebrates, especially mollusks. They have acquired a bad reputation for invading and wreaking havoc on oyster beds. But the most notorious for the coral reef biologist is Acanthaster planci, the crown of thorns starfish. Rather than the more usual pentamerous symmetry,

BLUE LINCKIA

Above: A. planci with aboral

spines, 16 arms and large central disc. Note white coral skeleton at upper right & lower left where tissue has been consumed. Left: detail of arm with podia with sucker-like ends.

www.richard-seaman.com/Wallpaper/Nature/Underwater/Invertebrates

www.geo.lsa.umich.edu/~kacey/ugrad/coral3.html

this star has a large central disk and 7 to 23 arms (usually 14 to 18) and an adult may grow anywhere from 25 to 35 cm in diameter with the largest recorded individual being 80 cm. Its arms may be lost to predators or shed when the organism is stressed, but can be re-grown within 5 to 6 months. The podia are elongated with sucker-like ends as in other predatory asteroids. The arms are supported by a complex endoskeleton of aboral ossicles that have projections forming protective spines. Each spine is a single crystal of magnesium calcite 4 to 5 cm (~2”) long. The spines are covered with a toxic epidermis and can easily penetrate human skin. link for symptoms.

This organism is a specialized coral predator found only in the Indo-Pacific. Here it emerges at night, everts its stomach on a coral colony and digests years of growth in a single feeding. Usually this organism occurs in low densities and is not a cause for alarm. However, in the late 1960’s and early 70’s a population outbreak of plague proportions occurred on many reefs throughout Acanthaster’s range.

ACANTHASTER PLAGUE

map showing reefs where Acanthaster outbreaks have occurred

http://www.ogp.noaa.gov/mpe/paleo/coral/sor/images/Map6L.gif

The starfish were moving in large numbers during the day, instead of being of minor impact, they were now major predators on the reef with as many as 15 individuals per m². link for control of A. planci outbreaks. In Guam, for example, over 90% of the coral population was killed within just a few years. The reasons for the outbreak are a matter of opinion, but boiled down to their essence, the two camps are a) that it is a natural, cyclical phenomonon or b) that it is connected to anthropogenic activity. Sediments about 7,000 years old contain aggregations of starfish spines, suggesting that outbreaks have occurred in the past as a natural phenomonon. On the anthropogenic side, pollution and predator removal have been the main themes. One predator is Charonia tritonis, the giant triton, shown consuming an Acanthaster specimen in the section on carnivorous gastropods. Charonia is a favorite of shell collectors, and some supported the hypothesis that it's collection allowed the population explosion of the A. planci. But Charonia has always been uncommmon, takes a week to eat a single Acanthaster, and actually prefers other prey! Whatever the reason, populations explosions continue sporadically, possibly corresponding to plankton blooms or nutrient input after rains. Recovery is evident in some locations from the initial outbreaks, while damage on others was so severe that they have yet to recover.For more information on recent outbreaks, and Acanthaster biology in general, see http://www.sbg.ac.at/ipk/avstudio/pierofun/planci/planci.htm. Some corals, especially branching pocilloporids (see coral family Pocilloporidae), gain some protection from "coral crabs" (Family Trapeziidae) that live within the branches of the colony. These small crabs defend their home colony by biting the tube feet of Acanthaster, often driving the predator away. There aee also shrimp living in pairs (the harlequin shrimp, Hymenocera picta) that prey on A. planci and other asteroids

TWO CRAB

HARLEQUIN SHRIMP

The Ophiuroidea

There are about 2000 species of ophiuroids, most of which are brittlestars. Like asteroids, most brittlestars have five arms and a central disc, but the arms are slender and extend from the disc more abruptly. Podia are reduced and are internalized as gills. There is no ambulacral groove. Instead the arms are covered with calcified shields on the oral, aboral and lateral sides of the arms; shields also cover the oral and aboral sides of the central disc. These calcified elements eventually contribute to reef sediment, which makes their geological role relatively minor, but ophiuroids are significant contributors to the complex biology of reef organisms. While the shields in most species are used as body armor, they sometimes have additional uses. Examination of the arm shields in the brittlestar Ophiocoma wendtii showed the presence of a regular array of spherical microstructures that look like lenses. Experiments showed that these microstructures, which are absent in closely related but light-indifferent species of brittlestars, were indeed sophisticated optical elements that have the optimal design for focusing light.

O. wentdii, a typical ohiuoroid except that their shields are composed of multiple calcitic microlenses (above right) that serve as light-focusing devices

www.bell-labs.com/news/2001/august/22/1.html

The lenses focus light about 5 microns below their surface where nerve bundles running through the skeleton underneath the lenses are placed, exactly in the correct position to pick up the light signals. Acting together, 10,000 calcite crystals in the five arm shields, form a kind of primitive compound eye that covers much of the organism's body, and researchers think this may account for this species’ ability to detect and escape from predators.

From the perspective of Bell Labs, where the research was done, the calcite microlenses expertly compensate for birefringence and spherical aberration, defects in lenses that distort light. The research suggests ways to mimic the brittlestar design where they may prove useful as components of fiberoptic networks, and in chip design.

Reference: www.nature.com/nsu/010823/010823-11.html

Reef brittlestars occur in several different feeding guilds. These range from carnivores and scavengers to deposit, suspension, or filter feeding. Some are epizoic and may function in a number of feeding modes on reefs. These include snake stars and basket stars. Unlike typical brittlestars, the arms of these ophiuroids are less calcified and are capable of twisting arm movements. They are often found in sponges, or with their arms tightly wrapped around gorgonian or other branched corals.

The serpent star Astrobrachion constrictum on a black coral branch
(K. Grange, NIWA, New Zealand)

Depending on the species, these may extend their arms at night to capture plankton with their sticky podia, or they may feed on the mucus secretions of their hosts. Plankton feeding is accomplished by “basket stars” which have multiply divided arms that form a fine plankton net. The arms unfold at night and unlike other ophiuroids the prey is caught directly by the arm-net without the use of the podia.

The Caribbean basketstar Astrophyton muricatum coiled around a gorgonian host during the day (left) extends finely divided arms at night to feed on plankton

Composite image composed from

www.dwave.net/~donauw/Keys/ June2002/page_01.htm

and www.utrd.com/ basketstar01.htm

The Echinoidea

Echiniods include about 950 species of sea urchins and their dorso-ventrally flattened relatives, the heart urchins, sea biscuits and sand dollars. The latter group burrow in sediments and will not be considered here. The sea urchins are composed of an external shell or test that has five ambulacral areas with pores for the podia, interspersed with and fused to five additional plates that lack such pores. The test is covered with spines that are longer at the equator and aboral surface than around the oral region. Many species have long primary spines interspersed with shorter secondary ones. Both of these are mounted on a ball-and-socket joint and can be moved for both locomotion on the aboral surface, and for defense, among other purposes, elsewhere. While the test and spines ultimately contribute to the sediments as do other echinoderms, sea urchins have two specific and important roles on coral reefs while they are alive. The first is their food and feeding habits, while the second

The oral half of a sea urchin test is shown exposing the Aristotle’s lantern apparatus and some of its musculature. A completely dissected lantern is shown in the upper left corner.

Composite image from http://virtual.yosemite.cc.ca.us/randerson

and www.zoology.uwa.edu.au/staff/rblack/06112002.html

(see below) is their activity as bioeroders. Sea urchins graze the substrate with a jaw composed primarily of five separate teeth, an apparatus is called Aristotle’s lantern. The lantern is actually more complicated than just the five teeth and has 50 skeletal elements. Specialized muscles protrude and retract the lantern through the mouth, while other muscles control its opening and closing. There are about 60 muscles that are involved with the movement of Aristotle’s lantern, allowing sea urchins to pull, scrape and tear a wide variety of foods. While sea urchins can be generally regarded as omnivores, they are particularly important as herbivores on reefs, playing a prominent role in the control of macroalgal and turf algae populations. Some indication of their importance was noted when the sharp-spined urchin Diadema antillarum was discovered creating “halos” around patch reefs. The halos were areas where Diadema emerged from patch reefs at night and grazed heavily on algae and seagrass. The intense grazing pressure close to their home reef created bare sand rings around each patch. It was clear that this animal could influence the abundance of plant material within its range of nocturnal movement.

Patch reef "halos" caused by Diadema grazing pressure

Reference: Ogden, John C., Richard A. Brown, and Norman Salesky. 1973. Grazing by the echinoid Diadema antillarum Philippi: formation of halos around West Indian patch reefs. Science 182, no. 4113: 715-17.

Diadema antillarum, the Caribbean long-spined sea urchin

http://typhoon.wcp.muohio.edu/tropicalvideos/coralecology2003/diadema_antillarum_deep_b.jpg

This message was delivered more clearly when the ubiquitous Diadema began suffering mass mortality in 1983. The as yet undiagnosed pathogen killed most all Diadema beginning in Panama, and over the course of a few years spread to Florida and Bermuda, wiping out an average of 98% of the species throughout its range. Other species of Diadema found in the Indo-Pacific were unaffected. Dramatic increases in benthic turf algal cover followed the urchin mortality, increasing from approximately 31% in Jamaica before the die-off, to nearly 50% within 2 weeks afterwards. A maximum coverage of 72% was observed 4 months after the event, then declined somewhat. Drastic changes in community structure were on Jamaican reefs in the years immediately following the urchin die-off. A survey of well-studied sites along the North coast showed a consistent decrease from more than 50% coral cover in the late 1970’s to less than 5% in 1993. This major ecological phase shift from a coral dominated system to an algal dominated system was certainly not only due to the Diadema population decline, but it is clear that Diadema’s loss was a major force in the decline or coral reefs in the Caribbean region. There is evidence that populations of this sea urchin are just now beginning to recover. However, the recovery of coral reef structure is a more complicated problem that we will discuss later.

Reading assignment: Mass Mortality in Diadema antillarum (Echinodermata:Echinoidea): A Large-Scale Natural Experiment

Sea urchins as agents of bioerosion

Sea urchins erode calcareous surfaces in two ways. First, as grazers, they their use their Aristotle’s lantern, to scrape reef surfaces as they graze. This is not considered a significant source of bioerosion except in cases where large populations may occur (e.g., 20 or more per m²). Second, sea urchins possess many tough spines that can scrape surfaces and result in abrasion as the urchins move within their burrows. Members of the pan-tropical genus Echinometra are particularly well known for their rock-boring abilities.

Echinometra viridis in rock gallery

www.riebesell.net/aquaria/ livestock.cfm?fid=214

The Holothuriodea

There are about 1200 species of holothurians (sea cucumbers), ranging in size from a few millimeters to 2 meters. They lack arms and ambulacra, but they generally have three rows of podia on the ventral surface and two rows on the dorsal surface. The body wall is leathery on reef species that are in the open during the day and most commonly observed.

¬Ossicles are isolated as individual calcitic elements buried within the dermis, rather than the more obvious endoskeletons of other echinoderms. These ossicles contribute to reef sediment upon the death of the animal, but that is the least of their contribution to the biology of the reef system. Holothurians typically have 10-30 oral tentacles (sometimes highly branched) around the mouth. These are extended from the body only when feeding. Their food consists of detritus and bacteria

Holothurian in feeding mode www.reefseekers.com/ PIXPAGES/Yap_Palau

picked up by the sticky tentacles (indirect deposit feeding) along with sediment. Some species prefer feeding on hard substrate (as above) while others prefer reef sediments. Sea cucumbers consume and grind sediment and organic material into finer particles, turning over the top layers of sediment in lagoons, reefs. In absence of fishing pressure, sea cucumbers may occur on Indo-Pacific reef flats at densities in excess of 35 per square meter, where individuals process an immense amount of sediment each day. For example, a common Bermudian species which is about 20 cm in length, can process 160 grams of ocean debris in 24 hours and in an area of 4.4 km populations of this species have been estimated to ingest 500-1,000 metric tons of sand annually.

While they are clearly visible to observers, sea cucumbers seem to suffer relatively low predation. Some species deter predators by distasteful chemicals in their tissues, while others use small needle-like spines (spicules) hidden within the skin. But to really appreciate holothurian defense mechanisms you have to appreciate their plumbing system

One of the more unusual defences is to spray the would-be attacker with sticky white threads (tubules of Cuvier) emitted from the anus.The tubules swell once expelled are extremely sticky. They are said to be able to immobilize a lobster. In addition, there are toxic saponins carried by the tubules. Some compounds isolated to date exhibit antimicrobial activity or act as anti-inflammatory agents and anticoagulants. Fishermen in the Pacific islands use the toxins, some of which act as respiratory inhibitors, to entice fish and octopus from crevices so that they may be more easily speared.

Tubules of Cuvier after discharge from anus

www.bims.buu.ac.th/.../aquarium/ sea_cucumber.html

Furthermore, the sticky tubules are placed over bleeding wounds as a bandage, apparently without ill effects, but eating the tubules would be a mistake. Eating the body wall of holothurians is considered haut cuisine in the far east, where, when suitably prepared, they are one of many “Pu foods”- those considered to possess aphrodisiac qualities. There are many problems with the demand for certain high value species of holothurians: the ease with which such shallow-water forms can be collected or poached from reserves, and their top-heavy age structures (larger, more desirable and sexually mature individuals are collected preferentially) have all contributed to over-exploitation and collapse of populations in many regions.

Sea Cucumbers also act as a mobile home for many other creatures; such as shrimp, crabs, snails, and worms, and even a fish that resides in their cloaca. These pearlfishes (are from the aptly named genus Carapus) may leave their host at night to go out and feed. Upon their return they may find themselves locked out of their homes by a less than enthusiastic host. However patience

pays off, as eventually the anus must open up, as holothurians have the unusual characteristic of respiring via the cloaca to ventilate its respiratory trees (see diagram above).

The Crinoidea

Crinoids are the most ancient group of living echinoderms and were abundant on Paleozoic (especially Silurian) reefs in the central United States, among other places. I’m willing to bet you didn’t know that the crinoid is the “official state fossil” of Iowa, but it is.

More than 6,000 fossil species have been described, and most of them were stalked “sea lilies” as depicted in the Silurian reef diorama on the left. Others, however were stalkless forms as shown below.

Stalked crinoids depicted on a Silurian reef

www.mpm.edu/research/geology/

third/tp6-pmb.html

Stalkless crinoid fossils

http://whyfiles.org/100oil/cret.html

Of the approximately 600 species of living (extant) crinoids, 85% are stalkless “feather stars”; the remaining stalked forms are found exclusively as deep sea animals with most species living at depths >200m. As with many groups, the tropical Indo-West Pacific is the richest region, with single reefs supporting as many as 50 species, almost as many as recorded for any individual fossil assemblage. Here, abundance and diversity reach 115 specimens and 12 species per m 2, respectively. In contrast, there are only a few readily observable reef species in the Caribbean region.

Most of a crinoid's body, in fact usually at least 80% or so, is made of a calcareous endoskeleton consisting of individual plates (ossicles) each of which is formed from a single high-magnesium calcite crystal.

The ossicles held together by collagenous ligaments and muscles. This skeleton explains both why crinoids make good fossils, and because only 20% of a crinoid is digestible, few organisms subsist on them.The salient anatomical features of a feather star are shown here. The mouth (and anus projected on an anal cone) are in the center of five upward-facing arms (rays). The rays are attached to a heavily calcified cup or calyx. The upside of the arm contains a longitudinal ambulacral groove that extends into the lateral branches or pinnules. The feather star perches on the substrate by its cirri, but the animal can crawl or swim using its arms.