Marine Biology I
Colloquial Meeting of Marine Biology I Headed by: dr. A. Goldschmid Author: Pierre Madl Salzburg, 22nd Nov. 1998 (revised in Dec. 2002) |
1. Introduction:
Asteroids (sea stars or pin cushion stars) are mostly detritus feeders, but some are omnivores or even predators. Both
species of Culcita (in particular C.coriacea, C.novaeguineae, C.schmideliana) and Acanthaster planci
are corallivores. Although the former are feeding on coral polyps, their impact on scleractineans (hard or stony corals) is
moderate to neglectable as their dietery uptake only in parts consits of coral polyps. This is not valid for the latter -
A.planci is a voracious predator upon sclaractinean corals. A.planci occurs over a wide range, through the
tropical waters of the Pacific and Indian oceans, however, it is unknown to waters of the tropical atlantic (Caribean sea).
This short article should help to understand the nature of that controversy and help to comprehend the complex interactions involved in the occurrence of this phenomenon. | |
2. Identification, anatomy, and reproduction of Acanthaster planci:
This sinister looking and rather flattened sea star with a large central disk and 7 to 23 arms (usually 14 to 18) displays on the
aboral side elongate spines (ca. 5cm) which make it immediately distinctive (see fig. 1). The body "appears" to be quite stiff, but
can bend and twist into all sorts of shapes to fit the contours of the coral substrate it feeds on. The skin is a sensitive membrane,
used for extracting dissolved oxygen from the seawater. A meshwork of small magnesium calcite-ossicles, from which project
numerous movable and immovable spines, are embedded in the tissue of the body wall. CoTs should be handled carefully, since
the long, sharp spines are mildly venomous and can inflict painful, slowly healing wounds - see end of report
(8. Toxicity of A.planci). PHYLUM Echinodermata, (Gk.: echino, spike + derma, skin) spiny skinned animals CLASS Asteroidea, (Gk.: aster, star) sea stars or starfish ORDER Valvatida, (Gk.: valvata, flap) valve-like FAMILY Acanthasteridae (Gk. acantha, thorn + aster, star) thorned starfish |
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Reproduction: Like most starfish, A.planci is separate in sexes. It breeds once each year in midsummer, when eggs and sperm are released into the water (fig.3). Group spawning appears to increase the normally rather low chance of fertilization. And during an outbreak and synchronized spawning a fertilization rate of 95% can be reached (when male and female are less than 2m apart, and 25% when 6m apart). As in all asteroids, the number of eggs spawned increases with body size. One large female is capable of producing up to 60E6 eggs per season, so when A.planci do occur in high numbers, their reproductive potential is enormous. Their populations can quickly increase by as much as six orders of a magnitude over several breeding seasons. | |
Larval stages: Eggs and sperms are shed into the seawater; since they can’t survive very long independently,
spawning is synchronized by rapid change of temperature or light along with hormones secreted to stimulate others to spawn
as well. After fertilization, the zygote divides synchronously into a ciliated blastula, hatches from the egg membrane
and forms an infold (blastopore) which later becomes the coelom of the gastrula. |
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Within the first 24 hours, a series of radical changes occur. After several contractions, lateral swellings appear which form
the first pair of tube feet. This phase is followed by some more gradual internal changes (formation of cardiac portion, closure
of anus (which will reopen after metamorphosis is completed), formation of the nervous system, oral and aboral coelomic
extensions separated by mesenteries, and finally the secretion of spicules originating from the mesenchyme - which elaborates
into fenestrated plates and terminates by the formation of adult ossicles). This last stage initiates the stars independent existence. |
3. Distribution of
A.planci:
The large scale distributions if asteroids are affected by water-current pattern: |
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4. A.planci’s Food Resources and Feeding Patterns:
A.planci has a large potential for food input per unit biomass, which facilitates rapid growth; a young species can
transform from the juvenile to the adult feeding biology within a 1 month transition period. It will reach a total diameter of
about 25cm after just 2 years. PHYLUM Cnidaria (Gk. knide, nettle) CLASS Anthozoa (Gk. anthos, flower + zoon, animal) SUBCLASS Hexacorallia (Gk. hexa, six) 6-parted symmetry ORDER Scleractinia (Gk, skleros, hard) Stony corals with a heavy external calcareous skeleton arranged in a hexamerous cycle. FAMILY: almost any family is affected by A.planci predation: the most common species are found among the: Acroporidae (see fig. 5), Agariciidae, Faviidae, Poritidae, etc. |
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A brief look at the Anatomy of Scleractinian Corals: |
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How can this species create such
damage?
The aboral side of A.planci is not made of a thick and rigid test that characterizes other asteroids; it is rather elastic and pliable which allows it to crawl out and wrap itself around the pointed tips of branching corals. It is maybe this plasticity of the body which gives juvenile A.planci its early ability to attack coral polyps and to undertake adult feeding bahavior early in life. | |
Under normal conditions, it is not uncommon to spot a few starfish and to see where it has fed on a coral over night. CoTs are
extraoral grazers; they feeds by turning their stomach outward (an area equivalent to that of its oral disk - see also fig.9), press
it against the coral and digest it. Once the stomach is pushed out through the mouth it secretes digestive enzymes on to the
polyps which break down the coral's living tissue into a sort of "polyp soup". After several hours, specialized cilia convey this
solution to the caeca where it is absorbs through the stomach wall (this process is called extracellular digestion). In fact this
oral part of a CoT starfish (dotted with shorter and blunter spines - fig.8), is known to some predating fish; they flip them over
on their backs before starting to chew on the soft parts of the echinoderm (see predation). |
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5. Effects on Coral-Reef Communities:
Spines in sediments from fossil reefs (dating back some 3500 years) proof that they have been an important part of reef life
ever since. Like hurricanes, starfish outbreaks play a role in maintaining high species diversity on the reef; but recent mass
outbreaks as seen on the Great Barrier Reef seem to be more devastating than in the past.
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Figure 10 depicts the periodical fluctuations of coral regeneration and outbreaks of A.planci. Outbreaks itself, last for
about 4-5 years (may differ from reef to reef) and end suddenly once food becomes scare or the starfish die of diseases. The
time frame between outbreaks used to be 15 years, enough to allow fast growing coral species to recover to pre-outbreak levels.
CoTs found today along the GBR, still belong to the massive occurrence observed in 1979.
The recent events of Crown of
Thorns Starfish outbreak since 1996
6. Selected Predators feeding on A.planci:
As long as the new generation of CoTs are still in the planktonic phase, any filter feeding organism is capable to (passively) prey
on eggs and larvae (refer to Life Cycle). These are mainly Porifera (sponges), Bivalvia (shells
like giant clams, pearl shells), and Tunicata (sea squirts). Unfortunately, in many tropical countries, filter feeders like
Tridacna have been extensively collected and wasted for the curio trade. | |
PHYLUM Annelida (L. annel, ringed) ringed animal CLASS Polychaeta (Gk. polys, many + chaite, hair) many haired animal ORDER Amphinomida (Gk. amphi, around + ?) fireworms FAMILY Amphinomidae (Gk. amphi, around + ?) This small predator and scavenger is commonly known as bristle or fire worms. Its flattened dorso-ventrally body with dense and protective bundles of setae, give this species a characteristic appearance. It approaches the juvenile CoTs laterally by nibbling its way from the tip of an arm towards the disk. |
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PHYLUM Arthropoda (Gk. arthr-, joint + pod, foot) SUBPHYLUM Crustacea (L. crusta, crust) hard surfaced animal CLASS Malacostraca (Gk. malac-, soft + ostracon, shell) soft shelled animal ORDER Decapoda (Gk. deca, ten) ten-footed animal INFRAORDER Caridea (Gk. cary-, hearty?) FAMILY Gnathophyllidae (Gk.gnath, jaw + phyll-, leaf)
The pink, white and blue coloring of the harlequin shrimp (Hymenocera picta) makes it very noticeable as it moves around
slowly in male-female pairs. With its vivid colors, it has a deterring effect on attacking fish. The shrimp attacks the less protected
tissues of a juvenile sea star by turning the animal over. As it also feeds on other starfish (Fromia, Nardoa, Linckia, etc.)
it is not known to what an extent these shrimps affects CoT population (fig.13). |
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PHYLUM Arthropoda (Gk. arthr-, joint + pod, foot) SUBPHYLUM Crustacea (L. crusta, crust) hard surfaced animals CLASS Malacostraca (Gk. malac-, soft + ostracon, shell) soft shelled animal ORDER Decapoda (Gk. deca, ten) ten-footed animal SUBORDER Pleocyemata (Gk. ?) INFRAORDER Brachyura (Gk. brachy, short + ura, tail) short-tailed animal FAMILY Trapeziidae (Gk. ?) Trapezia are usually associated with corals or other invertebrates. Both species shown here help prevent corallivores from eating the live coral tissue (fig.14) They do not feed on the starfish themselves, but rather fiercly attack the slowly approaching invader by pinching the highly sensitive podia which prompts the starfish to turn back (fig.15). |
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PHYLUM Mollusca (L. mollos, soft) soft bodied animals CLASS Gastropoda (Gk. gaster, belly + pod, foot) as snail, limpets, and slugs SUBCLASS Prosobranchia (Gk. prosos, forward + branchios, gills) slow creepers ORDER Mesogastropoda (Gk. meso, middle + gaster, stomach) middle footed SUPERFAMILY Cymatiacea (Gk. cymatos, ?) FAMILY Ranellidae (Gk. ?)
This giant triton Charonia
tritonis (fig.16) has been and among many other tropical nations still is heavily collected for the ornamental
shell trade, leading some people to suggest that its increasing rarity is leading to CoT outbreaks, there is as yet no
conclusive evidence for this. |
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PHYLUM Chordata backboned animals SUBPHYLUM Vertebrata (Gk. vertebros, spine) CLASS Actinopterygii (Gk. actinos, ray + ptergy, fin) Bony fish ORDER Perciformes (Gk. ?) FAMILY Lethrinidae - Emperors SUBFAMILY Lethrininae - This species usually occurs in schools in sand or coral rubble areas. It approaches echinoderms by picking them up at one arm and turning them over to feed on the oral part of the starfish. Molluscs and crustaceans are also included in its dietary habits Unfortunately it is a favorite among anglers as it is a good food fish. |
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PHYLUM Chordata backboned animals SUBPHYLUM Vertebrata (Gk. vertebros, spine) CLASS Actinoptergyii (Gk. actinos, ray + ptergy, fin) Bony fish ORDER Perciformes (Gk. ?) FAMILY Lutjanidae - Snappers SUBFAMILY Lutjaninae - Species feed on fishes, crabs, stomatopods, other benthic crustaceans, cephalopods, and occasionally on echinoderms. It is often encountered in the vicinity of lagoons, and over adjacent sand flats. Can occur in schools or as solitary individuals. Again it is appreciated as a food fish and commercially important but in certain regions of the Indian Ocean, large individuals are known to cause ciguatera poisoning. |
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PHYLUM Chordata backboned animals SUBPHYLUM Vertebrata (Gk. vertebros, spine) CLASS Actinoptergyii (Gk. actinos, ray + ptergy, fin) Bony fish ORDER Perciformes (Gk. ?) SUBORDER Labroidei (L. labbra, lip) lipfish FAMILY Labridae - Wrasses SUBFAMILY Cheilininae - Most labrids are benthic feeders; they prey mainly on invertebrates with hard parts such as shelled mollusks, sea urchins, sea stars, and crustaceans. Besides that, this species is one of the few predators of other toxic animals such as sea hares, and boxfishes. They crush the animals with their pharyngeal teeth. The wrasses are diurnal; they are among the first fishes to retire to an inactive state on the bottom with the approach of darkness and among the last to resume activity the following morning. It has been a popular species for spear fishermen in the past, and was taken in large quantities by commercial fisherman on the Great Barrier Reef. Nowadays, larger species (can reach up to 2m in length) are frequently fed by divers (dive tourism, even though it is quite wary and solitary) which trains it away from its natural food-sources. |
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PHYLUM Chordata backboned animals SUBPHYLUM Vertebrata (Gk. vertebros, spine) CLASS Actinoptergyii (Gk. actinos, ray + ptergy, fin) Bony fish ORDER Tetraodontiformes (Gk. tetra, four + dental, tooth ) FAMILY Balistidae - Triggerfish The orange-green striped Triggerfish Balistapus undulatus reaches 30cm in length and the Titan Triggerfish Balistoides viridescens that can be up to 75cm long are known to feed on CoT. Pseudobalistes flavimarginatus with likewise sharp teeth has been observed to attack (and take bites out of) crown-of-thorns with apparent immunity. But they also feed on a variety of other benthic organisms such as algae (often coralline red algae), fishes, mollusks, tunicates, brittle stars, urchins, polychaetes, sponges, and hydrozoans. Females of B.viridescens guarding their nest are reported to have attacked divers. |
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PHYLUM Chordata backboned animals SUBPHYLUM Vertebrata (Gk. vertebros, spine) CLASS Actinopterygii (Gk. actonis, ray + ptegy, fin) Bony fish ORDER Tetraodontiformes (Gk. tetra, four + dental, tooth ) FAMILY Tetraodontidae (Gk. tetra, four + dental, tooth) - Pufferfish A.planci is also preyed upon by a variety of pufferfish, which are sometimes taken for food (although poisonous if prepared not adequately - Arothron hispidus contains TTX, a toxic tetrodotoxin. Puffers feed on calcareous or coralline algae, detritus, mollusks, tunicates, sponges, corals, zoanthid anemones, crabs, tube worms, sea urchins, brittle stars, hermit crabs, hydroids, and of course echinoderms. With its powerful teeth, used for chewing corals, it make easy work on starfish spines and all the rest. The trigger fish preys on adult starfish by seizing it by an arm and turn it over before biting into the soft parts. Where a puffer has eaten a starfish there is often a neat ring of discarded spines. |
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7. Controlling A.planci outbreaks: Labor-intensive though it is, there have even been projects that involved injecting the offending starfish with a variety of poisons, such as formaldehyde (40% HCHO-gas in water, or formalin; i.e. 1 part of formaldehyde and 19 parts H2O yields a 5% formalin solution), copper sulfate (CuSO4), sodium hypochlorite (NaClO), ammonia (NH3), ammonium hydroxide (NH4OH), compressed air, and acetic acid (CH3COOH). Currently, the most environmental tolerable control method of this kind is diluted sodium-bisulfate (NaHSO4·H2O). At the peak of the last A.planci outbreak, up to 15000 individuals have been caught in one single day. But poisoning or removal by hand are likely localized measures, tending to be used only in popular tourist areas and providing cosmetic, short-term solutions, which prevent further reef damage only until the next outbreak. The implementation of biological controls (such as diseases or predators) may be successful but it should be borne in mind that not all biological control programs undertaken by man have been successful. Many of them have caused far greater problems in the environment than the original problems they were meant to overcome. With or without such action, most of the stars disappear, and if the time between outbreaks is long enough, sea-surface temperatures do not exceed the thermal tolerance threshold, then new corals will settle, will thrive and replace those lost. CoT occurs throughout the Indo–Pacific and shows a classical boom–bust population dynamics with low background densities and intermittent outbreaks (see fig.10). Three waves of population outbreaks have affected Australia's GBR since the 1960s. The waves of outbreaks appear to start at the northern section of the GBR and progress southward through the central GBR, causing devastating effects on the coral fauna and the associated reef-ecosystem (see fig.4). Extensive surveys by the GBR Marine Park authorities have shown that protection from fishing affects the frequency of outbreaks - that is, the relative frequency of outbreaks on reefs that were open to fishing was 3.75 times higher than that on no-take reefs in the mid-shelf region of the GBR, where most outbreaks occur, and seven times greater on open reefs if all reefs were included. However, the explosive appearance of CoT-outbreaks is coupled to a likewise rapid and massive die-off. The reasons behind are probably not so much related to the scarcity of food resources but rather linked to a probable viral infestation with lethal effects on the dense CoT-population. Echinoderms are deuterostomes and thus are rather primitive compared to vertebrates (they lack notochords and do not possess an adaptive immune system). Recent evidence suggests that these sea stars are subject to massive viral infection and show a similar seasonal die-off characteristics as observed in the sea slug Elysia chlorotica. During mass mortality, these slugs produce large quantities of nuclear and cytoplasmic retroviral particles, apparently from endogenous retrovirus (ERV). The biological significance of this virus induction has not been well evaluated, but it is likely to be involved in the phased mass mortality of these slugs. Similar mass mortalities are observed in maricultural practices, like that in 1994, when the French oyster aquaculture industry crashed, as well as in 1999, when a similar crash was seen in Japan. Other major crashes in commercial shellfish populations have also been experienced, such as in the shrimp aquaculture in China. |
References: Coral Reef Animals of the Indo-Pacific by T.M.Gosliner, D.W.Behrens, G.C.Williams; Sea Challengers 1996 - USA Echinoderm Studier 1-4 by M.Jangoux, J.M.Lawrence; A.A.Balkema Publ. 1989 – NL Fishes of the GBR and Coral Sea by Randall J.E., Allen G.R., Steene R.C.; University of Hawaii Press; Honolulu 1990 - USA Great Barrier Reef; Mead & Becket; Reader's Digest Services Pty Ltd.; Sydney 1984 - AUS Greenpeace Book of Coral Reefs, by S.Wells, N.Hanna; Sterling Publ. 1992 - USA Guidelines for Managing Risks in Recreational Water issued by the Australian Government - NHMRC. 2008 - AUS Impacts of Climate Change on Australian Marine Life: Part C; edited by Alistair J. Hobday, Thomas A. Okey, Elvira S. Poloczanska, Thomas J. Kunz, Anthony J. Richardson; CSIRO Marine and Atmospheric Research 2006 - AUS Living Invertebrates by R.&M.Buchsbaum, V.&J.Pearse; Boxwell Publ. 1986 - USA No-take reserves protect coral reefs from predatory starfish by H.Sweatman in Current Biology , Vol.18/14: 598-599 Sea Stingers and other venomous and poisonous Marine Invertebrates of WA by L.Marsh WA-Museum 1986 - AUS Starfish Wars - Coral Death and Crown of Thorn by R.Raymond, McMillan Co. 1986 - AUS Pesci e Coralli del Mar Rosso di A.Mojetta, A.Ghisotti, Mondadori, Milano 1996 - ITA Red Sea Invertebrates by P.Vine; Immel Publ. 1986 - UK The Blue Planet by Fothergill A., Allen D., Allen P.; ABC/BBC 2002 - UK/AUS Viruses and the Evolution of Life by L.P.P.Villarreal; ASM Press 2005 - USA Zoology Intl ed. By R.L.Dorit, W.F.Walker, R.D.Barnes; Saunders Publ. 1991 - USA |