The Crown of Thorns Starfish (CoTS) invoke horror and wonder. They also spur important science.
Horror and wonder
CoTS (Acanthaster) are indigenous to the Great Barrier Reef. They eat coral. In “normal” periods there are relatively few adult CoTS to be found and their presence does not threaten the Reef ecosystem. But every decade or so they over-populate and cause major coral loss.
To appreciate the horror of CoTS from the perspective of a coral polyp, imagine a predator of humans with similar characteristics. Some CoTS grow quite large – up to a metre wide. They have light sensors (primitive eyes) in each of their 7 to 23 arms, an acute sense of smell, and defend themselves with poisonous spines.
CoTS move between reefs and coral feasts at a pace that is quite surprising for a sea star, typically at night. To see how, view this video by Jon Allen, Dione Deaker, Emily Richardson, Antonio Agüera Garcia and Maria Byrne. The first part is at normal speed. To skip normal and see the ‘kangaroo hop’ at 8x speed, drag the time bar to 3:02 for the last 17 seconds:
CoTS feed by extruding their stomach to envelop their prey (i.e. living coral) and digest it externally. When they pull their stomach back in, all that is left is a dead white coral skeleton.
CoTS are prodigious breeders.
Like an alien in a science fiction movie, CoTS have the ability to regenerate a lost organ (i.e. if they lose an arm they grow a new one). At the larval stage they can clone themselves, and may be able to use their symbiotic bacteria as a food source. (see below).
They are unusually hardy; larval CoTS do well in low nutrient conditions, and adults can live up to 9 months without eating.
To appreciate the wonder of CoTS, think of these same attributes from the perspective of scientific enquiry. CoTS are biological superstars, with attributes more remarkable than science fiction. Scientists have already learnt a lot about them, but there is still much more to know, especially about how to cost-effectively limit the coral loss resulting from their over-population.
Recent science publications on CoTS
Uthicke et al (2015): Dr Sven Uthicke (AIMS) and others have developed a technique to confirm the presence and concentration of CoTS by eDNA. During an outbreak in 2014 Dr Uthicke and his colleagues used eDNA to survey larval CoTS along a section of the GBR from Osterlund Reef (near Cooktown, south of Lizard Island) to a reef area about 200 km south of Cairns. Most of the 48 water samples collected contained CoTS larvae, including those taken 100 km south of the outbreak zone, indicating a continuous cloud of CoTS larvae over the entire 320km survey section. The larvae numbers in the outbreak zone (>1010) were about 4 orders of magnitude higher than adults (~106 ) in the same area, implying that attempts to halt outbreaks by removing adults may be futile.
Wolfe et al (2017): The water of the GBR is naturally low in nutrients (oligotrophic), especially in the northern sector where Lizard Island is located. It can become enriched (eutrophic) by phytoplankton (tiny photosynthetic plant-like organisms). Phytoplankton “bloom” (i.e. multiply) when exposed to run-off events from the rivers and streams that run through farming land onshore from the central and southern sectors of the Reef. Kennedy Wolfe and his colleagues found larvae survive and metamorphose into juvenile starfish in both low and rich nutrient conditions. Their paper concludes that CoTS population outbreaks are probably not caused by nutrient blooms associated with agricultural run-off. [But see also Allen et al (2019) below]
Carrier et al (2018) reports the results of a study on the microbiome of CoTS larvae. It found they associate with a bacteria community that is distinct from environmental microbiota, and that this community experiences a community-level shift in response to different feeding that is maintained over development. It also reports an indication that the larval microbiome may be partly phototrophic (i.e. able to produce its own food from sunlight). If so, this bacteria could be a symbiotic energy source for CoTS larvae and part of the reason why they are so successful in ogliotrophic (low nutrient) seas.
Allen at al (2019): Scientists have known for some time that CoTS are among the most prodigious breeders. Babcock et al (2016) found that a single large female can produce over 100 million eggs in a breeding season. That’s already scary and remarkable. This 2019 research paper adds to the horror and wonder. Jon Allen and his colleagues found larval CoTS are able to clone themselves. In other words, a single larval CoTS can split itself, resulting in TWO larval CoTS, each of which has the capacity to become a fully fertile adult. This research also found that larval cloning of CoTS occurs more frequently in rich nutrient conditions. See our posts on Life stages of CoTS and Larval cloning of CoTS for further detail.
Significant parts of the research reported in this post (Zara-Louise Cowan’s work on life stages of CoTS; Sven Uthicke’s pioneering work on CoTS eDNA; Maria Byrne’s contributions to the Wolfe & Carrier research; and Jon Allen’s and Maria Byrne’s contributions on CoTS cloning) were supported by grants from The Ian Potter Foundation, arranged through LIRRF and the Australian Museum’s Lizard Island Research Station.