Tuesday, January 31, 2012

Sea Cucumber Evisceration! Defense! Regeneration! Why? GROSS!

(From the Echinoblog Art Department!)
So, you've probably heard of this.

EVISCERATION! Explodoing Guts! One of the things that sea cucumbers (class Holothuroidea) are probably best known for when you learn about them at aquaria and in introductory marine biology classes!

Sea cucumbers are, of course, the "worm-like" echinoderms. They usually eat fine mud or organic goo and are composed of 3 main parts!
First-a mouth (surrounded by tentacles)
Second-some guts!
Third-a second hole for spewing out what doesn't get eaten!
You wouldn't think that something so seemingly basic-would be capable of so many WEIRD ass things. But as we'll see they are.

Information from today's post comes from 2 primary sources: 1 from a review by Jose Arraras and Michael Greenberg and a second paper by Maria Byrne from 1985 on evisceration in the west coast sea cucumber Eupentacta quinquesemita! (abstract here)

What is evisceration?

A sea cucumber behavior wherein they can EXPEL their "guts" THROUGH their body wall.

Yuck. AWESOME. The most awkward super power ever!

Its often associated with defense-but as we'll see, there is always more..

I limited my coverage here strictly to evisceration (the general behavior) but next week..I'll delve into a special sub-section of evisceration...

1. How it Works: Evisceration and REGENERATION!!

So when we refer to "evisceration" the first word that one sees is "viscera" which usually refers to the intestine. Such as what we see in this diagram in purple!

But what is often seen in evisceration is an expulsion of MOST or MANY of the various organs inside as we see here...
That includes gonads (for reproduction), their "respiratory tree" (basically gills for "breathing"), their hemal (i.e. circulatory) system and of course, their intestine!

Different kinds of sea cucumbers evert/expel different organs. These can also vary depending on certain times of the year (as we'll see).

Basically what happens inside the body this:
  1. The various attachment ligament/tissues which connect and anchor all of the various internal organs to the body wall all begin to rapidly soften. Sea cucumbers, like all echinoderms have a unique kind of connective tissue which permits them to toughen or soften their body texture at the animal's whim.
  2. The body wall itself begins to get soft at certain localized spots where the "evisceration" or expulsion will take place.
  3. Muscles in the body wall contract causing a rupture through the softened/weakened body wall and all of the various guts/viscera/what-have-you are SPEWED out through the hole!! Pew! Pew!
(NOTE: actual sea cucumber evisceration does not go BOOM!)

One tropical family of sea cucumbers, have special structures called Cuverian Tubules which are adhesive and used specifically for defense. I'll be blogging about those-hopefully next week..so I will hold off on saying much about those for now.

2. Who Eviscerates?

Interestingly, the extent to which evisceration occurs varies among different groups. Here is a generalized classification for sea cucumbers taken from the Holothuroidea Tree of Life page.
Only two of the major orders shown here have been shown to display evisceration behavior-The Dendrochirota (or Dendrochirotida on the tree) and the Aspidochirota (or Aspidochirotida) on the tree.

Evisceration has apparently not been observed in the others (which are curiously-mostly made up of deep-sea members). Its not clear if some deep-sea sea cucumbers-such as sea pigs eviscerate or not.

Some of these non-eviscerating groups (such as the "Apodida) DO have very potent powers of regeneration!! -which is something we'll discuss below.

A. The "Dendrochirotida" apparently eviscerate primarily through the ANTERIOR end!
Apparently the body wall plus muscles, nerves and the cloaca survive and ALL the "lost" parts are regenerated!

Some experiments dissected two dendrochirotidans-Sclerodactyla and Thyonella into "anterior" and "posterior" regions.

Apparently under lab conditions the anterior sections died. And not ONLY did the posterior regions survive-BUT the segment including the cloaca could REGROW the ENTIRE animal.

Recent experiments have suggested that relatively young animals can regrow a body from either region. So the potential for regeneration may be related to the growth of the animal in question.

B. In contrast-those sea cucumbers in the other major group -the "Aspidochirotida" eviscerate through the POSTERIOR end!
(From the Echinoblog Art Department!)

Apparently this group shows a different array of evisceration and regenerative abilities. The species that have been studied don't instantly die if cut into different regions but apparently several taxa are capable of asexual reproduction-and so its unclear to what degree this group shows any limiting factors to regeneration.

3. Why do sea cucumbers Eviscerate?
Since evisceration was discovered-one of the most apparent ecological interpretations of "why would it do that???" was explained as "for defense."

It often expels those viscera (which are often quite sticky, gross and inundated with toxic body chemicals) towards oncoming predators-which usually include fish and hungry starfish!

But as sea cucumbers were better studied-it turned out that these cases were not quite so clear cut! And defense was sometimes just not an option.

For example, one of the better studied sea cucumbers-Eupentacta quinquesemita
Eupentacta quinquesemita (HMB) (2)
(photo by Dougmason on Flickr)

and sure enough, tests with expelled viscera and a place for the sea cucumbers to hide was effective against one of their most aggressive predators..the sun star.. Solaster stimpsoni
Solaster stimpsoni (Striped Sunstar) (photo by jackson.chu on Flickr)

Interestingly though, Byrne found that these sea cucumbers would eviscerate ON THEIR OWN-and would do so SEASONALLY!

Evisceration would apparently begin in September and continue through November. Of the populations she sampled -76% at one site and 49% at another were eviscerated.

Regeneraton in these individuals can take about 2-4 weeks. Viscera were regrown in time for winter but apparently this "annual evisceration" event could vary from year to year...

Sometimes, parasites (usuallly certain kinds of snails) were associated with the evisceration-but for the most part, the regularity was often observed without parasites present.

Could Evisceration serve as a kind of "back up excretory purge?"
So, here's the thing-sea cucumbers, like most echinoderms directly lack a way to excrete wastes. They are essentially open to seawater the way starfish, sea urchin, and other echinoderms are..

So, the intestines in Eupentacta will often build up metabolic waste and associated "wear and tear" type chemicals. Byrne speculated that a regular evisceration could conveniently and economically "excrete" or purge these chemicals (along with the intestine and other viscera).

AND do it regularly!!

4. And how does Regeneration Fit into all of this?

The part of this process that has the MOST interest from the rest of the scientific community is actually the second, more understated part of the evisceration process.

That is to say, it was not the expulsion of the viscera but the REGROWTH of those guts and organs!! Sometimes in a surprisingly short period of time.

Regeneration time in sea cucumbers apparently varies on which organs were lost and in what species..Regrowth time varied from 7 to 145 days!! Yes, that means that at the shorter end of the range, there were some species for which regeneration of an organ could take as little as a WEEK!

How amazing is sea cucumber regeneration? One study of a sea cucumber called Leptosynapta crassipatina found that a simple disk of tissue containing part of the calcareous oral ring, the nerve ring, and the mouth could regenerate the COMPLETE animal!!

The physiology of regeneration is complicated-but essentially, lost parts form from either the "re-differentiation and remodeling of exisitng tissues" or from having a generalized "a mass of undifferentiated, proliferating cells that forms at the wound site, eventually giving rise to the regenerated structure".

So, basically, this has implications for understanding cell life and death (implications for cancer), regenerating body tissues, and for better understanding of stem cell research.

I've often thought of this as a great example of how something that started out as essentially a neat story in the Natural History of Marine Invertebrates has evolved into research that likely has the broadest implications and with an unforseen but likely important impact!

That said-next week, MORE on SEA CUCUMBER DEFENSE!!

Tuesday, January 24, 2012

How to tell apart the "knobby stars" Protoreaster from Pentaceraster

Knobbly sea star (Protoreaster nodosus)  and Pentaceraster sea star (Pentaceraster mammilatus)

Greetings! Happy 2012 and Gung Hay Fat Choy! A prosperous Year of the Dragon to all of you!

To kick off this new year: an experiment. I get a lot of requests from my international colleagues to assist them in identification of the many kinds of starfish they observe in and around their study areas.

A lot of my colleagues (and my readers) are spread out across the Indo-Pacific, so not only the tropical Pacific-Australia, Singapore, Indonesia, Philippines, Japan, etc. but also in the Indian Ocean! including the coasts of India, Africa and so on.

One of the most frequently encountered sea stars in shallow water areas are members of the Oreasteridae. You've seen them on this blog in various places. This includes the big cushion sea star Culctia which occurs across the Indo-Pacific.

But two of the most commonly encountered genera of oreasterids are called Protoreaster and Pentaceraster!

What's so important about these species that we need to tell them apart? I have blogged about how various oreasterid species (here is the post), including Protoreaster nodosus is often heavily fished (this is the other).

Other genera of oreasterids often look similar and are fished just as heavily, typically because they co-occur. Telling them apart is, I think the first step. Which species are people seeing? Which ones are being fished?

How do I tell oreasterid starfish apart? Well, probably one of the most useful monographs on this group was written the famous German taxonomist Ludwig Doderlein when he described them as part of the famous Siboga Expedition.

And you know what's FRAKKING amazing? You can get this formerly rare monograph ONLINE thanks to the Biodiversity Heritage Library! Go here and see! (oreasterids begin just before pg. 296 with "text")

Some of the easier ones to distinguish include

Poraster superbus-which occurs across the Indo-Pacific. This species is identified by the long arms and the prominent rows of large, round knobs that occur down the radius of each arm. Poraster superbus is seen in and around tropical Asia (China and Japan), but are known primarily from the Indian Ocean.

Pentaster obtusatus
-another widespread species that occurs mainly in the Philippines and central Pacific area. This species lacks spines or any prominent tubercles on the surface. It has kind of a speckled appearance with lots of small, low tubercles..
a maroon star
(photo by "Happy Via" on Flickr)

There seem to be two genera of oreasterids that seem to confuse people the most -Protoreaster and Pentaceraster- both described as "knobby stars" and both with very similar appearance.
Unfortunately, between the similarities in appearance, the variation, and a lack of scientific work on these groups, the distinctions of these very abundant and frequently encountered species is pretty poor. Add to that the natural variability of features and size differences? Well..understanding which species of Pentaceraster is which, can be complicated..

So, that's what i'll be trying to show the difference between below..

Protoreaster! Let's start with the easier one! (it has fewer species)

There are 3 species.

Protoreaster is pretty common and as I've indicated, one species- P. nodosus is getting attention because of how heavily fished it is for tourist baubles and the like..

Generally speaking, one identifies Protoreaster based on the lack of spines on the marginal plates present between the arms (i.e., interradially). Marginal plates are the two paired rows of plates that form the perimeter around the edge of the body.
Speaking as a generality, there is a LACK of spination on the edge of species in Protoreaster..

The taxonomic definition of Protoreaster isn't terrific and further study will likely result in somebody's PhD thesis!

In this photo, there are big spines on the ends of the arms on the top series (the superomarginal series) but not on the lower series (the inferomarginal sereis). (This is Protoreaster lincki)

In some species, spines are absent from the marginal plate series on the arm altogether! (this species is Protoreaster nodosus)
Protoreaster lincki is primarily an Indian Ocean species and it is often seen on the coasts of India and Africa... Most of the ones I've seen are this striking red and white color...
red starfish

(photo by "felt tip felon" on Flickr)

But sadly, it seems to be as heavily fished as its sister species P. nodosus..
(photos by "Sparky the Neon Cat" on Flickr)

Perhaps the best known species of Protoreaster is Protoreaster nodosus which, in some regions is so well-studied that there was actually a blog entirely devoted to it! Click here to see! Sadly, it has not been updated in many years.

P. nodosus occurs primarily in the tropical Pacific but apparently there are also records from the Indian Ocean. It is very abundant in places but there are of course concerns about how long that will last..

Diversity and variation of P. nodosus is bewildering...
Knobbly sea stars (Protoreaster nodosus)
(photo by Ria Tan, Wild Singapore on Flickr)
Knobbly sea star (Protoreaster nodosus)

(photo by Ria Tan, Wild Singapore on Flickr)

Here's more color variation in individuals from Sulawasi..
Protoreaster nodosus
(Photo by Ben Naden on Flickr)

Want to see more? Go to Flickr and just type "Protoreaster nodosus" and see HOW many you get!

I should note that there is a THIRD species of Protoreaster-Protoreaster nodulosus, but it is limited to a relatively isolated area in western Australia.

Basically, it looks like this... but given its really limited range, most folks outside of Australia won't be to worried about running into it..

Okay, so THAT was the easy part! Pentaceraster is more complicated and I won't really be able to get into as comprehensive an account.

Okay-so remember how I said earlier that spines were absent from the marginal plates between the arms?

Spines are OFTEN present on ALL if not MOST of the marginal plates in Pentaceraster. Unfortunatley-not ALL of them show this consistently..but most do.

Again...this is a group that doesn't show a lot of good solid scientific definition.

Here are the spines in Pentaceraster alveolatus
Sometimes, they aren't big spines..but more like short stubby knobs or cones such as in Pentaceraster mammilatus
or in Pentaceraster horridus.
One of the other features you often see in Pentaceraster? There are often really prominent, tubercles or large features present on the disk. Its not a great character for diagnosing species-but there it is.

Pics are from Pentaceraster horridus

Here are some pictures of various species of Pentaceraster... Honestly, there are about 14 species of Pentaceraster, but they overlap and are all closely related.. here are some that I think I can put a name on..

Many species of Pentaceraster are present in the Indian Ocean but many occur in the central/South Pacific.

Pentaceraster cumingi from Hawaii and the East Pacific.. In Hawaii, these can be quite big, but also occur in deeper water. Shallower water populations are found closer to places like Baja California and Panama..
Pentaceraster cumingi - a sea star from the Pacific coast of Panama
(photo by Artour a from Flickr)

Pentaceraster mammilatus from the tropical south/central Pacific and the Indian Ocean
Pentaceraster sea star (Pentaceraster mammilatus)
(photo by Ria Tan, Wild Singapore on Flickr)

with some brilliant variation in color-even among those only from a single region (these images hail from Singapore..)
Pentaceraster sea star (Pentaceraster mammilatus)
(photo by Kok Sheng on Flickr!)

..ahere are some less certain identifications... This one looks to me like Pentaceraster sibogae (and not P. alveolatus as indicated in the description). A species with a large number of short spines covering the surface.. from North Sulawasi!
054 (1)
(photo by Ular Tikk on Flickr)

This looks close to Pentaceraster alveolatus (from Cebu in the Philippines)
Pentaceraster alveolatus
(photo by Ben Naden on Flickr)

Possibly Pentaceraster tuberculatus (again in contrast to the name in the description-pic is from Cebu in the Philippines)
Pentaceraster alveolatus
(photo by Ben Naden on Flickr)

Here's some that just leave me scratching my head...All are Pentaceraster but which species? Or are they possibly new?

This one looks close to P. horridus.
Starfish, Zanzibar, Nungwi Beach
Photo by "mkabroad" on Flickr from Zanzibar

Starfish, Zanzibar, Nungwi Beach
Photo by "mkabroad" on Flickr from Zanzibar
Photo by Paul Schaffner on Flickr. Image of species in Tanzania.
Photo by Paul Schaffner on Flickr. Image of species in Tanzania.

hmm...So...maybe that wasn't all that much help afterall? But it does outline how complicated understanding a "species complex" can be even if they are BIG and OBVIOUS. A lot of them are similar to one another and distinctions can be very complex.

But who knows? Maybe this blog will inspire some intrepid new student to pursue the study of oreasterids!!

If ya' think about it, you gotta go diving and snorkeling in some pretty exotic places to get some DNA for these guys...