• Anthropic causes 

• Bio-erosion , Cyclones , Acanthasters , Coral bleaching

Acanthaster

Chantal Conand, ECOMAR (univ. Réunion) and Loïc Charpy, IRD

Acanthaster planci is a spiny star-fish which is often called the "crown of thorns". In New Caledonia, it is known as the "step-mother’s pin-cushion".

This is one of the best-known creatures inhabiting the reef systems. Its reputation is not based on either its beauty or its commercial value, however, but on the fact that acanthaster tend to form large clusters which are fatal to the coral, because these animals feed on the polyps.

During the last 20 years or so, much  debate has focused on the biology and the ecology of acanthaster, and there has even been some controversy as to what their effects on the tourist industry might be.

The following points are central to all these debates:

• What has caused the recent blooms of acanthaster?

• Might they be affected by human activities?

Morphology

• Diameter : 18 to 50 cm in New Caledonia

• Most of these animals have between 11 and 12 arms, sometimes 16 or 17

• The arms of 53 % of the individuals undergo a process of regeneration

• Weight: 200 g to 3 Kg.

Reproduction

• The members of the two sexes always remain apart: since the reproductive substances are ejected into the sea, fertilisation is most likely to occur if a large number of individuals have congregated. It has been observed that the males and females adopt a special position during the process of fertilisation

• The reproductive periods: these have been determined on the basis of the data obtained in various countries at diverse latitudes on the mean monthly gonado-somatic ratios (GSR), the volume of the gonads and the animals’ stage of sexual maturity.

The life-cycle

Some questions still remain to be answered about the acanthaster growth rates:

• Do the growth rates vary depending on the animals’ diet and / or on their age?

• Is it possible to determine the animals’ age from their size?

Nutrition et Mobility

Acanthaster are to be found throughout the Indo-Pacific region, but they are difficult to quantify because their numbers vary considerably in time and space.

Acanthaster contain sapotoxins  which protect them from predators. However, there exist 12 species of fish and coral which feed on acanthaster when they are still at the stage of eggs and larvae.

Definitions

The term “bloom” or “population explosion " began to be used in the framework of observations carried out on the Great Australian Barrier Reef. High rates of infestation by acanthaster were previously reported in the Ryukyu Islands (Japan), where 220 000 acanthaster were collected in 1957. During the last half-century, many other acanthaster blooms have occurred on the Great Barrier Reef (from 1962 to 1974), in Micronesia, in Ponapé, Truk, Palau and Guam, in Okinawa, Tahiti, Fidji , Hawai, in the western Samoa and in New Caledonia (where more than 1000 acanthaster were collected on the islet of Tabou).

However, the distinction between "normal" populations and blooms tends to be a rather subjective one. Some authors have attempted to quantify their findings in terms of either the numbers per   observation period or the numbers per reef surface area or length.

The counts carried out per reef surface area in New Caledonia resulted in a continuum of values ranging between  1/hectare and more than 300/hectare.

It is worth quoting the following definition, which was proposed by a scientist called Potts: 

Primary and secondary infestation

The distinction needs to be made between primary infestations, where individuals begin to appear quite suddenly, and secondary infestations, which result from the scattering of the larvae or adults originating from a primary infestation. Secondary infestations of acanthaster have been occurring, some of them quite recently, at the Great Barrier Reef. 

Damage to the coral

Just as is it difficult to define a bloom, it is also difficult to assess the resulting damage:

• Sometimes the damage is considerable: 90 % of all the coral on the 38 -Km long coast of Guam were destroyed by blooms,  as were 80 % of all the coral on Green Island (Australia) down to depths of 40 m. The acanthaster have luckily caused much lower death rates in Hawai.

• The damage varies from one point to another: the coral in shallow places with turbulent waters has been less severely damaged by acanthaster than the coral in other places.

• The extent of the damage also depends on the coral species: those such as Porites and Pocillopora, which form the most massive blocks, are less highly exposed to acanthaster; in some places where the coral has been scraped bare, there can still be some tiny surviving patches measuring only a few square centimetres,  which may suffice for a process of recolonisation to be able to start up. 

Damage to other species

• Due to the disappearance of the coral, other species in the ecosystem are also liable to suffer from these infestations. In Japan, for example, the number of species present on live coral substrates is 62, whereas there are only  43 species living on dead coral substrates and 22 on coral debris.

• The trophic category of herbivores are on the increase, whereas the corallivores are tending to disappear.

The possible causes

The question as to how these infestations originated has given rise to several theories, which are often assumed to be contradictory: 

• Anthropic factors disturbing the environment

• Natural fluctuations in the size of the populations

Fishing for tritons, Charonia tritonis, gastropods which prey on both juvenile and adult acanthaster, may have favoured the proliferation of the latter species. Marine pollution, which contributes to the weakening and destruction of the coral, may be another decisive factor. 

Some authors have claimed that acanthaster blooms are natural events, based on evidence tending to show that these events already occurrred in the past. References to previous blooms can be found in the history and folklore of some Pacific islands. In addition, the remnants of acanthasters skeletons have been found in both recent and fossilised sediments on the Great Barrier Reef.

Theories about blooms 

Two main hypotheses have been put forward: the one is that adult populations with normal densities gather in the wake of cyclones which have destroyed coral reefs; the other is that an increase in the larval survival rates may lead to a sharp increase in the recruitment rates.

In the framework of the first hypothesis, the increase in the density of the populations occurs suddenly, three years after a reproductive season associated with heavy rainfall. Several places located in the same region are usually involved.

In the framework of the second hypothesis, heavy rainfall on the high-rising islands may lead to the lagoon waters being enriched with nutrient salts, and thus result in the development of the phytoplankton on which the survival of the acanthasters larvae mainly depends.

These two theories are not so much contradictory as complementary: the primary blooms may have various causes, and the contribution of the diverse factors liable to account for them can vary from one place to another.