|thanks to MBARI!|
|Fig. 1 from Gale et al. 2013|
A. Ceramaster granularis
B. Ctenodiscus crispatus-aka the mud star (learn more about it here)
C. Hippasteria phrygiana (learn more about it here)
D. Leptychaster arcticus
E. Mediaster bairdi
F. Novodinia americana-a brisingid
G. Zoroaster fulgens (more about Zoroaster here)
but also had some feeding notes on this weird guy.. Tremaster mirabilis! (about which very little is known)
Most of these species occur in a primary range of about 500 to 1500 meters, but some can get relatively shallow. Most are difficult to study and live on the deep-dark sea bottoms...
2. Figuring out feeding
There's generally TWO ways to study feeding:
- Directly: i.e., you watch a species consuming its prey (or whatever food) and voila! You have a direct feeding observation.
- Indirectly: You have something which provides inference about what the animal has already consumed. Look at the gut contents or something similar...
In contrast, I think Ms. Gale et al's paper has acquired some great information using some modern techniques and good ol' fashioned detective work!
Direct Observation So, the most obvious and direct way to observe feeding is by watching it! These days, submersible robots aka ROV's (Remotely Operated Vehicles) are one of the main platforms for these types of observations. I've done some similar work in the Pacific (here)
|Fig 9A from Gale et al. 2013|
Several species were observed as predators for the first time, whereas others were confirmed. For example, Novodinia is a brisingid with a documented suspension-feeding mode and we saw more of that in Gale et al. 2013.
Gale et al. also reported feeding for Tremaster for the first time! Feeding on coral...
Laboratory Feeding Experiments
As a complement to the direct observations, Gale also performed several laboratory feeding experiments and was able to observe several direct feeding moments!
Predictably, Hippasteria fed on various cnidarians, including sea pens and other deep-sea cnidarians.
Whereas Ceramaster fed on sponges..
But not all the prey allow the predators to just...eat them. Some, like the sea anemone Hormathia
|Image from Natioal Museum of Northern Ireland via EOL|
|Image from Fisheries & Oceans Canada via marinespecies.org|
Here is a video that explains the basics of stable isotopes but basically what it comes down to is this: elements like nitrogen (N) and carbon (C) undergo changes as they pass through different ecological levels in the environment.
In doing so, they become kind of like a "fingerprint" for a particular kind of ecological role. So, for example, species with a stable isotope N (Nitrogen) value of about 16, but w/ Carbon value of about -14 (Hippasteria, Ceramaster and Mediaster) are higher within the overall trophic relationship among these asteroid species.
|Fig. 6 from Gale et al. 2013|
|Image by K. Gale|
All the other species, including Novodinia americana, Leptychaster arcticus, Ctenodiscus crispatus and Zoroaster fulgens display lower values which would be consistent with their previously thought of feeding modes as suspension feeder (the brisingid) and deposit feeders/detritivores (mud stars, including Leptychaster and Ctenodiscus) and Zoroaster.
Gut Contents & Prey Items!
One other indirect way of looking at food items? Gut contents. What were they eating?
Ms. Gale did a LOT of work looking through the guts of many starfishes.. Much of how they fed is based on detective work. For example, many animals such as deep sea gorgonians and such, after being digested leave only skeletal bits called sclerites.
Fortunately, these can be used to identify the animals with some accuracy. Curiously Hippasteira also had some crustaceans in its gut..
|Figure 3 from Gale et al.|
Gale et al reiterate the importance of the feeding ecology of many of these species...
- Hippasteria is a widely occuring asteroid which likely affects coral populations
- Ctenodiscus-the mud stars occur in LARGE numbers, up to ~6000 individuals per hectare and influence the sediment as they move around through it feeding on mud..
- Suspension feeding asteroids such as Novodinia capture food from the water column that would ordinarily not be made available to bottom feeders
|not N. americana. Image by NOAA National Ocean service|
But even BASIC knowledge such as this is a complex and time-intensive process. It starts with work like this...