This week-a neat paper from Miles Lamare and team (who are mostly from the University of Otago in Dunedin, New Zealand) about using electronic tagging to study individual movement in the temperate-water New Zealand asteriid starfish Coscinasterias muricata in a 2009 issue of the Journal of Experimental Marine Biology and Ecology (click here).
As much as the idea of tagging echinoderms has come up (and this is actually one of the most common professional inquiries I receive) its not been done very much.
Largely, because its very difficult to stick a pin, tag, flag or any kind of inorganic device into an echinoderm without any number of deleterious effects, including:
-seriously affecting its behavior
-causing the arm/spine/limb to be cast off and regenerated and/or variations thereof
.......and so on.
But technology has progressed and the authors introduce the usage of this handy little electronic tag (the DST-milli electronic tag) made by Star-Oddi. The tag is programmed with electronics that record water temperature, and depth every 5 minutes.
The authors attach these tags to the animal with a metal wire to which the tracker tag is attached so that the animal looks like so...
1. Where they went! and how fast! The Data
Individuals vary-but on average they seem to be capable of 15.6 meters/day.
This still pales in comparison to the west coast sunflower star (Pycnopodia helianthoides) which is practically a starfish lamborgini at the rate of 3 meters (possibly 2 from other sources) a minute as outlined here-I'll let you do the math)
And that's MUCH faster compared to The Crown-of-Thorns (Acanthaster planci) which can go 2.3 to 4.3 m/day versus the much smaller and slower asterinids (i.e., bat stars) Patiria pectinifera (1.5 to 3.7 meters/day) and Patiriella regularis (5.7 meters per day).
Why is speed important? Read on below...
We see from the data above that different individuals of this species can travel quite a bit and over a pretty broad vertical distance (four to 14 meters). So, what's going on?
2. The Salinity Story
It turns out that C. muricata displays "vertical migration" which is to say that it TRAVELS back and forth from deeper to shallower regions in order to forage for its favorite food, the blue mussel, Mytilus galloprovincialis, which live in the intertidal.
But based on the tracking info, two of the individuals showed some variation. One occupied deeper water below the mussels and then migrated into shallow water for short periods of less than a day and at intervals of about five days. Another occupied shallow depths for the first week before moving on to deeper depths.
Another important behavior they noticed was how physical factors, such as salinity affected the behavior of the observed individuals. As a species, Coscinasterias muricata is VERY intolerant of low salinity and experiments suggest that individuals WILL die in a relatively short period if salinity is not maintained.
So, if there's freshwater runoff or a storm-it will retreat from shallow water.
Figure 6 below charts the position of each of THREE individuals relative to salinity, temperature, height above the "mussel line" and the amount of rainfall (on the bottom) versus the day during a two week period.
NOTE in the top "blue" box and the bottom box. The depth inhabited by the animals DECREASES when rainfall is highest (i.e., the dilution of salinity is lowest).
Salinity also gets higher as you get deeper...
So, as a physical influence, the salt water layer influences how C. muricata forages for food! An influence that is apparently very significant to the animal's biology..
The authors note something of particular interest... that the animals begin their descent BEFORE the deepening/formation of the low-salinity water layer!! This suggests that there are other physical cues (e.g., changes in wave action) to the animal, so that so that it KNOWS when to move!
This is kind of neat, because these are the KINDS of cues that are not obvious to us. Asteroids have a very different world of perception.
The speed of the animal..observed as between 1.78 and 23 meters/hour is faster than the rate at which the low salinity layer deepens during the weather!!
In other words- the starfish can outrace the "low salt" layer as it approaches!
I think most people interpret movement as a function of something observable-like predation. But in this case-maybe not so much?
Undoubtedly, the authors probably have more (or could have more) than this-but the paper's intent was mainly to see what they could learn using these new tags. Foraging behavior for an animal like this is important to marine ecologists. You can't exactly follow a starfish like a panther or a racoon.
And if our recognition of Pisaster (same family as Coscinasterias by the way..) as a keystone species is considered important to ecology-then understanding things like foraging patterns is also potentially significant to what's going on in marine ecosystems.
And just on a pure curiosity level wouldn't YOU like to know the secret lives of starfish?