Fossil Starfish Story: The Deep Sea-Antarctic-Global Warming in Reverse Connection!

(from Littoraria on Flickr)

Believe it or not, when you get under the ice there are more than a few decent places to find fossils in Antarctica (see above).

Work by Dan Blake at the University of Illinois and colleagues have recovered a great many invertebrate fossils from Seymour Island, a small island in the Antarctic Peninsula region.

Seymour Island deposits a great many invertebrates, including mollusks, crinoids, and sea urchins. But there was an incredible diversity of fossil asterozoans, including 6 genera and species of asteroids and one genus/species of ophiuroids.

Specifically, we see how paleontology intersects with the deep-sea, the Antarctic and Climate Change.

To illustrate how this all ties together..let me introduce you to one animal in particular:

The genus Zoroaster (family Zoroasteridae). A genus of starfish with about 20 species that occurs primarily in the deep-sea (~300-5000 meters) throughout the world in the Pacific, Atlantic, and Indian ocean basins. Mah 2007 recently developed a phylogeny and monographed the group. Zoroasterids are interesting because they have only a single marginal plate series, which is a character shared by many Paleozoic forms, but that is only one of the many reasons they are among the most interesting of starfish to study..

Zoroaster sp. from the North Pacific:
and now, here is one from the Eocene La Meseta formation of Seymour Island in Antarctica!! Fragments apparently were not uncommon in the area.

(courtesy of Dan Blake, UIUC)

Another quick comparison of a living Zoroaster sp.
and an Eocene (i.e., fossil) Zoroaster from Seymour Island.

Morphologically, they are VERY close.

Some interesting points:

1. Living Zoroaster live in deep-water habitats. vs. fossil Zoroaster which lived in shallow-water sediments (i.e., the ancient environment can be inferred by geologists to be shallow-water based on distribution of the fossils, and the sedimentary features such as size, composition, sorting and a number of other physical characteristics including the form of the sediment distribution and formation ).

In theory, you could have literally gone wading on Seymour Island during the Eocene and picked up one of these starfish (that today live in 1000 m depths!) in knee-deep water!!!

(from Littoraria on Flickr)

2. Eocene Zoroaster lived in pre-glacial settings. The Eocene Antarctic was much warmer than it was today, resembling the Pacific Northwest in many ways. Temperate rorests extended to the poles. A substantial climate change event took place during the Eocene/Oligocene, which saw the formation of the Antarctic Counter Current and the glaciation of Antarctica.

3. Interestingly, to my knowledge, Zoroaster (and indeed no zoroasterids) have ever been recorded from the Southern Ocean today. (although they are found in adjacent sub-Antarctic waters outside the ACC). But of course, absence of presence is not presence of absence..

Lots of Questions:

• Did Zoroaster go extinct? or shift into deeper water?
  
• Was this part of the recent radiation of Zoroaster in the deep-sea?
• What about other fossil starfish or invertebrate taxa at Seymour Island?
• Why did Zoroaster "lose its foothold" in the Antarctic? Climate? Food? Both?

So we see some interesting recent precedents. Starfish give us insights into major and relevant questions simply by being there.....and then not being there.

Deep-Sea Evolution-Consider the vertical instead of the horizontal! Discovering different species along a slope!

Image courtesy of Hawaiian Undersea Research Laboratory, Honolulu, HI

All of this talk of deep-sea submersibles goin' down to the bottom of the ocean and so forth has everyone focused on the abyss and the deep-DEEP-sea!

Now, echinoderms are a BIG component of life found below 1000 m and are among the deepest animals found in the abyss. Brisingid starfishes for example are found as deep as 6000+ meters!

Probably one of the most fascinating questions that interest biologist-regardless of where they work is to ask "How did that species evolve?" 

One such question is the subject of today's blog! This features results from a paper published by my colleague Kerry Howell and her colleagues at Southampton Oceanographic Center in the journal Marine Biology

Howell studied a deep-sea starfish called Zoroaster fulgens which occurs all throughout the Atlantic Ocean, typically between 220 and 4810 meters!  

 

Although you can find it both in European AND North American waters, Howell's study site was

in the Porcupine Seabight located in the northeastern Atlantic.  

From Howell et al., Fig. 2

This species occurs widely and yet, the different external features of different individuals varied slightly between several individuals.

Usually, when we see different features on different individuals we think about whether or not those variations might actually represent a different, possibly new species. 

Or alternatively, this is what's known as natural variation. 

Human beings for example are widespread and can have blond, black, or brown hair, blue or brown eyes. And yet are all considered the same species. Maybe having different body shape is how starfish express natural varaition?

In Zoroaster fulgens, Howell observed that there were 3 different "variant" types...

From Howell et al, 2004 Fig. 1

This top one for example was referred to as the "robust" form with much more solid, shorter arms.

This middle one was referred to as the "slender" form and it had more tapering, more elongate arms..

And finally, this bottom one was referred to as the "long-armed" form due to its very tapering and elongate arms.

As it turns out, each of these different forms, each occurs in a different depth range in the deep-sea. The long-armed one occurs deepest at the 3300-4020 meter depth range whereas the other two were present in shallower (but still deep) waters...

Howell then sampled DNA (2 different genes) from each of the different forms and found that indeed-there were three distinct lineages that corresponded with each distinct morphotype!

Howell et al. Fig. 3 from 16S data

She observed that the deeper long-armed form was most closely related to the one with slender arms relative to the shallower "robust" form.

Thus, even though the "robust" form and the "slender" form occur together at the same depth they were the most genetically different from one another. Two separate lineages but living side by side.

In contrast, the "long-armed" form was separated from the other two by a vertical gap of 1100 m.  

This suggested that all 3 of these species were reproductively isolated from one another. 

The "robust" form is pretty clearly separated from the other two and given the amount of genetic distance relative to the other two forms on the tree, it is very likely what is called a "cryptic species"-one that has effectively "hidden" in plain sight but has been determined to be different as supported by independent (usually genetic) evidence. 

So, in Context...

Many times we often look to examples of speciation that involve models that show how different populations have become isolated from one another (eventually becoming species) in a horizontal plane.  For example, like this..

From the Understanding Evolution website

Its unclear exactly how these different lineages of Z. fulgens may have separated. What kinds of natural barriers may have led to their isolation??  Other deep-sea invertebrates, such as crustaceans and mollusks also show speciation along deep-sea slopes.

Have the gametes and/or larvae been transported along these great distances?  Any number of oceanographic barriers or settings in the deep-sea (at different scales) could have played a role.

For example, currents that isolate settlement, or the topography (ie shape of the ocean bottom) are all possible ways that different populations/lineages of Z. fulgens might have been isolated thus leading to different species. 

But is evolution across a vertical gradient really that unusual?  Check out this blog I wrote a few years ago about the evolution of Zoroaster in and around the Antarctic..

Here is a pic of some Eocene fossil Zoroaster sp. (that looked like Z. fulgens) which, when alive, likely lived in shallow-water and looked surprisingly like the living ones..

and today.. ALL of the species in the genus Zoroaster (including Z. fulgens) live in the deep-sea.  Clearly.. a puzzle is at work and hopefully one day it will get worked out!

Image courtesy of Hawaiian Undersea Research Laboratory, Honolulu, HI

Who Eats Who? Figuring out Feeding in Deep-Sea Starfish

thanks to MBARI!

People are always fascinated by how animals eat and, I think, the weirder the better. Starfish have one of the most distinctive feeding modes of all animals and I think, that's why people get fixated on how they consume their prey.

The feeding posts on the Echinoblog are among the top ten highest hit and the research literature is full of comprehensive studies detailing the feeding ecology and behavior of a great many starfish species. 

As one might expect, the great majority of feeding studies were initially those of shallow-water species which were easy to observe. Feeding is important to understanding marine ecology in many systems. So, its not just some casual trivia that comes in handy at cocktail parties.

But what about deep-sea species? Understanding the role of predators in these often inaccessible systems is important.  In cases, such as with deep-sea coral, its important to understand what role these animals play, especially given how little we know about the individual animal's importance.

How does one figure out the role..and indeed, the importance of these species in such far away and forboding habitats??

Today's Echinoblog post features work by Ms. Katie Gale at Memorial University who resides in the lab of Annie Mercier (and including Jean-Francois Hamel).

Data from this paper was recently published in the new issue of Deep-Sea Research studying the trophic biology of various deep-sea asteroids from the coast of eastern Canada. (here).

1. The Starfish

Fig. 1 from Gale et al. 2013

Ms. Gale's research focused on seven deep-sea starfish species from the North Atlantic:
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 the old, old days, figuring out the feeding ecology of deep-sea species was difficult. Specimens were collected via net-and usually brought up badly damaged. The animals were mostly dead and had often emptied all of their gut and stomach contents. Rarely did you have an opportunity to see the animal interacting with any possible prey items. Any interactions you might have spied could have been caused by the trawl net scooping up any and all of the bottom fauna...

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

This is Hippasteria phrygiana, a widely occurring cold-water/deep-sea coral (and cnidarian) predator, but Gale et al. observed several other species from the deeps, about 500-1100 meter depths,  of the North Atlantic.

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

nodosa 

Image from Natioal Museum of Northern Ireland via EOL

and the deep-sea coral Flabellum alabastrum 

Image from Fisheries & Oceans Canada via marinespecies.org

used their tentacles (which all have stinging cells) as a defense against the oncoming hunger dogs!  And this was effective against the more timid Ceramaster but not against Hippasteria. Flabellum was fed upon by Hippasteria VERY quickly (in 18 minutes)..

Indirect Evidence. This is where some newer techniques shows us some cool ecological stuff!!

Stable Isotopes!!
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

You've seen Hippasteria, but here's Ceramaster granularis

and Mediaster bairdi

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.

One of the subject animals, Zoroaster fulgens has been one of the more mysterious deep-sea starfish species in my experience is an infaunal predator, that is, a species which eats animals living in bottom mud and sediments.  A related zoroasterid called Doraster is shown here with a snail in its mouth with snail food in the red circle

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..

Fr. Arcodiv.org

• 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

I've recently discussed some recent observations of a seemingly innocuous species, Porania pulvillus as a predator rather than a passive ciliary feeder. Understanding deep-sea ecosystems is an exciting endeavor, who knows what we'll find!  Simple things like feeding are intriguing and interesting-but poorly known. What will the important impacts of these species be down the line?

But even BASIC knowledge such as this is a complex and time-intensive process. It starts with work like this...