Worm+Shrimp Versus Crown of Thorns! aka the Importance of the Little Guy!

(Photo by Klaus Jost, Animal Diversity at U. Michigan)

This week, I am a featured Guest Blogger for the great folks at the Ocean Portal Smithsonian NMNH Website! Click here or on the pic above!

I write about one of the world's most well-known sea star species, the Crown of Thorns aka Acanthaster planci! So, here on the Echinoblog-I thought I would give you a little something extra about this dazzling and spiny beast!!

An epic skirmish between this huge monstrous toxic, spine-covered monster VS. a crazy colored shrimp and a pugnacious polychaete worm!!

First-a little introduction.

I've mentioned the Crown of Thorns Starfish (or Sea Star) (aka the COTS) before. They are voracious coral predators whose appetite has become an ecological problem due to huge population booms (click here to see earlier post).

They occur on coral reefs throughout the tropical Indian and Pacific Oceans and occur from the west coast of Mexico and central America, north up to Japan, south to New Caledonia and westward to the east coast of Africa.
They are abundant but are also ecologically quite important. Because, not only are they big predators of coral but they are themselves FOOD (and prey!) for other invertebrates in the tropical reef ecosystem!

There are actually a surprising number of animals that feed on the Crown of Thorns Starfish (including giant snails and fish), but today we deal with two tiny but very important predators and the potential impact they may display!

Data for the below comes from work by Peter W. Glynn on populations of Crown of Thorns in Panama (Pacific side) from the Bull. Mar. Sci (1984) and a paper from the Proc. of the 4th International Coral Reef Symposium in Manila in 1981 Vol. 2: 607-6712.

1. The Shrimp! Meet: Hymenocera picta.

H. picta is known commonly as the Harlequin or Painted Shrimp owing to their very eye-pleasing designs and colors..

But this hides their TRUE murderous nature! This species and several of its relatives are predators on starfish and other echinoderms..Here is a video of one in action feeding on a hapless ophidiasterid... As you can see, they are pretty vicious little crustaceans!

It was estimated by Glynn that the attack frequency on average varied between 3.2% to 7.7% relative to the number of sea stars present. (this could be as low as 0 but as high as 20%!)

(Fig, 2 from Glynn, 1981)

2. The Worm! Meet Pherecardia striata- an amphinomid polychaete worm....with a vicious streak!

Pherecardia striata is an opportunistic omnivore/predator, feeding mostly on algae and detritus-but is also heavily into grabbing and eating crustaceans, mollusks and echinoderms (including other species of starfish) as they become available..

Worms such as these can be quite abundant. Although Glynn did not sample the whole population, sampling recovered many individuals. Based on Glynn's sample, there were from 90 to 380 individuals per square meter of reef!!

(photo by Gustav Paulay, image from CalPhotos)

Similarly, Glynn estimated that the mean attack frequency was about 6.5 % (ranging from 0 up to 25%! of the population studied).

(Fig. 3 from Glynn, 1981)

3. The Action! What happens is this:

FIRST-the shrimp (H. picta) attacks the starfish-tears apart the sea star's surface or dismembering one of the arms. Basically creating some prominent tear in the body wall..

THEN-the worm picks up on the opening and DIVES INTO THE wound/open body wall!!

Pherecardia then proceeds to attack the starfish from the INSIDE!! They enter into the body cavity and begin devouring its various guts! In other words- the pyloric caeca, gonads, water vascular system and so forth..

Sometimes, there can be anywhere from 10 to 18 worms INSIDE a single Crown of Thorns!

4. The Impact!
The relative frequency of COTS that are attacked by both shrimp and worm was pretty low (about 0.6%) but very effective. Based on their observations of wounded Crown of Thorns taken to aquaria, individuals attacked by both shrimp and worms died within a week..

Glynn actually found that the correlation between COTS abundance (i.e., the number of individuals) and shrimp attacks showed an INVERSE relationship (i.e., number of starfish goes down when shrimp attacks are up)!

So, shrimp attacks were MOST frequent when Acanthaster was LEAST abundant.

(there is alternatively the possibility that there was an explanation for the relationship owing to statistical bias of having too few sea stars)

(from the Guam Biodiversity Website)

Glynn points out that BOTH Hymenocera and the worm Pherecardia are VERY abundant on the coral reefs in which they live.

So, even if the attacks frequency was not very high-these predators would be very effective and potentially affecting COTS populations (at least in localized regions..)

Glynn goes so far as to imply that populations of East Pacific (i.e., Panama) Acanthaster planci might well be kept "in check" by the constant attacks (and feeding) on the COTS created by being constantly under attack by these tiny but abundant little predators working in tandem!

and if by chance you can only understand tropical Pacific ecology using Star Wars analogs, then you are in luck! Think of it this way...

Millenium Falcon = the Harlequin shrimp Hymenocera picta

X-wings = the worm Pherecardia striata

Death Star= Crown of Thorns Starfish Acanthaster planci

and watch this....

That's the basic message. :-)