Research

Juvenile Crown-of-Thorns Starfish

By June 30, 2020 No Comments

Background on Coral Predator

Crown-of-thorns starfish are a natural coral predator within the Great Barrier Reef, but they do occur between the Red Sea and the Eastern Pacific (Not reported in the Caribbean). These animals have a unique mechanism of feeding, where their stomach is pushed out through their mouth and over the live coral surface. Once the coral tissue is digested, their stomach is pulled back through the mouth. These animals leave a feeding scar which appears bright white from where the coral tissue has been digested. CoTS are voracious feeders and can kill entire colonies of corals, even big ones. They tend to favour preying on fast growing hard corals like the branching, plate or table forms. Other corals such as boulders, brain and softs are not as liked as much. This may be due to the amount of coral tissue available, coral defences or possibly they just don’t like the taste.

These animals have the usual starfish body plan with numerous arms (>20) that radiate from the central disc. CoTS like several other starfish species have the ability to regenerate appendages lost through predation, stress or disease. The body and arms are covered in sharp venomous spines, and they can grow up to almost 80cm in diameter. Cots grow fast and reach sexual maturity about 15cm in diameter, but a large CoTS can potentially produce millions of eggs. Generally separate sexed, with male and females releasing their reproductive material through broadcast spawning. CoTS have high fertilisation success even if males and females are separated by distance. Recent discoveries have shown that CoTS are capable of being hermaphrodites in having both male and female organs and also have the capacity to self-fertilise. Which means one CoTS has the potential to release 100,000’s of fertilised eggs.

Crown of Thorns Starfish Outbreak
COTS Diver

Population outbreaks

Crown-of-thorns starfish have some interesting biological features, but what they are probably most famous for is their capability to go from a very low density (<1 starfish/hectare) on a reef to extremely high density (>1000 starfish/hectare). When these outbreaks occur on coral reefs, they can have devastating effects, where a high live coral cover reef can be reduced to a very low coral cover reef in a matter of months. When these outbreaks were first witnessed in the 1960’s they were often associated with plague or biblical references.

Although we are uncertain what factors initiate a primary outbreak on a coral reef, the suspected causes are reduced predation pressure, oceanographic features (related to ENSO cycle) and food availability for CoTS larvae. We believe a primary outbreak occurs on coral reefs in the section between Cairns and Lizard Island and is referred to as the initiation box. Then secondary outbreaks spread to neighbouring reefs and generally start heading south. Primary outbreaks occur roughly every 15years, although secondary outbreaks can last for a decade. On the Great Barrier Reef there have been four recognised outbreaks which initiated; 1962, 1979, 1993 and 2009.

Tourism perspective

From a tourism point of view, high numbers of CoTS are devastating to hard coral populations in a small area such as Reef Tourism site. Tourism operators inside the World Heritage Great Barrier Reef, utilise several methods to increase coral resilience at their sites. Ongoing CoTS control programmes are one initiative operators use to preserve and protect live coral cover. These programmes usually use SCUBA divers to inject CoTS with a corrosive substance. Traditionally these methods were multi-shot and used swimming pool acid (sodium bisulphate), but have recently evolved to single shot methods and using organic substances such as ox bile salts or vinegar. These advances along with the Pest Management strategy have seen CoTS control at an individual reef level become increasingly efficient in recent years. For more information, visit www.gbrmpa.gov.au and www.ampto.org

Finding juvenile COTS
Juvenile cot on underside coral
Juvenile cot feeding scar

Our Research

Reef Magic Cruises pontoon site on the North-western corner of Moore Reef has had long term involvement with the Great Barrier Reef Marine Park Authority Eye on the Reef Monitoring Programme. In 2009 these surveys have revealed increasing CoTS abundance at this site.

A multi-shot injection programme was conducted between 2010 and 2012 which removed 650 Adults and 2,060 juvenile CoTS from this site. Individuals in between the size range of 5 and 150mm class are classed as juvenile. This programme identified:

  • Key areas or habitats where juvenile CoTS are found
  • That the proportion of juvenile to adult CoTS was increasing

In 2014 our juvenile Crown-of-thorn starfish research/control programme originated to focus only on locating and collecting juvenile CoTS. This is quite difficult because juvenile CoTS move around less in the daytime compared to adults, always quite hidden and mainly active at night when they feed. We use belt transects to locate Juvenile CoTS, this is where two of our trained staff, lay out a 60m tape across the coral reef, and each member systematically works 3m either side of the tape.

The programme was designed to provide quantitative information on:

  • Juvenile CoTS densities (see eDNA Research)
  • Coral preference by juvenile CoTS
  • Size structure of juvenile CoTS populations over time to infer age and growth of juveniles.
Small juvenile COT
Interpretation

Key Findings so far

  • From 2014 too present, 11,049 juvenile CoTS between 5 and 150mm in size have been collected.
  • Small Cots less than 7cm like to feed on small diameter hard branching coral species.
  • Possible two age classes progressing each year

Relevance

Sampling designs that focus on juvenile CoTS abundance combined with other early detection methods could be incorporated into an early warning system to predict future outbreaks in the initiation region of the Great Barrier Reef. This long term juvenile research programme provides information on juvenile CoTS ecology that may be useful to innovative programmes involved with suppression of future outbreaks.