Tuesday, April 30, 2013

Flukes in Cukes! Flatworm Commensals in Sea Cucumbers, Sea Urchins and Starfish!

Fantastic image by Chelsea L. Wood
Today we look at some neat examples of flatworms that live in echinoderms!

And to the flatworm and parasitic worm people reading this? YES, I know flukes aren't free-living flatworms. It rhymed! So go with it for now. thanks for your patience!

Flatworms aka the Platyhelminths (in Greek-literally the "flat worms") look like living carpets. They are mostly predatory, but may also feed on small organic particles and live all over the place. They can be parasites, such as tapeworms or free-living beasts such as the one featured in the collage below.

These include the familiar Dugesia-that you find in high school biology (brown with arrow shaped head and two weird eyes) to these big, colorful species that live throughout the tropical Atlantic and Pacific. There are some 4500 recognized species of free-living flatworms..
Polyclad flatworm collage
An awesome collage by Arthur Anker!
Papers that were used today:  this paper by George Shinn (1981)-Hydrobiologia 84: 155-162
another in Biological Bulletin 169: 182-198 (also by Shinn) and this one, from Canadian J. of Zoology 61(4): 750-760 which describes the species living in the sea cucumber

What's interesting about the ones that I'll be talking about is that none of these is exclusively parasitic (such as a tapeworm or a trematode). These are free-living species..but they live INSIDE the body cavities of echinoderms!  

Think of it as if you ended up living in the intestine or the body cavity of a whale. Lots of space there and potentially...a  lot of food. Plus protection from predators, the elements and a safe place to reproduce!

So, technically they aren't really parasites (where the host 'loses') they are commensals that are considered just kind of benign.
Image from WallaWalla University Inverts site! 
It makes sense. Sometimes, an animal with a huge internal body cavity can be a home. We've seen the classic pearlfish and even when clams that live inside the throats of sea cucumbers.

It turns out that there's something in the neighborhood of SEVENTY species of different free-living flatworms that live in echinoderms as hosts! A nice list of these can be found in this paper here.

Most of these hosts appear to be sea cucumbers with sea urchins and sea stars. Some in cold water but also in the tropics!  Crinoids and brittle stars seem to be among the minority as hosts for flatworms..probably because there's not much "living space" inside their body cavity. Or maybe they're just not as well studied?

Here are two well-documented worms from the North Pacific coast.. one that lives in several Pacific urchins and one in a North Pacific sea cucumber...

The Urchin as a "House" for flatworms! 
Urchins on the west coast of North America (in the Pacific Northwest) include the well-known purple sea urchin (Strongylocentrotus purpuratus)
Purple Sea Urchin - Strongylocentrotus pupuratus
Image by Joe McKenna
And the giant red urchin (Strongylocentrotus franciscanus)
Red Sea Urchin
Image by Dan Hershman
and the deep-sea Allocentrotus fragilis
Image by NOAA Photo Library

ALL of these are often inhabited by this beast! Syndisyrinx franciscanus
Image by Chelsea L. Wood
Syndisyrinx franciscanus lives in the digestive tract of its host and apparently, infested urchins have been found with up to 186 worms!!! (an average of about 29/individual)
Image by Chelsea L. Wood
This one is called Syndesmis dendrastrorum 
From the EOL page for S. dendrastrorum
and it lives in the common Pacific Northwest sand dollar Dendraster excentricus!! (seen below alive with spines)
Sand Dollar (Dendraster excentricus)
Image by Patrick Warren
or perhaps more familiar if seen like this? Spines removed...
Eccentric Sand Dollar (Dead) - Dendraster excentricus
Image by Cheryl Moorehead
Even the familiar Pacific Sand Dollars can HAVE WORMS!!!  Ya' learn something new every day!

What do they do in there? Mostly, these live in the intestine feeding on the host's intestinal lining (the tissue) AND apparently also like to eat on the symbiotic protists (the ciliates) that ALSO live in the intestine of the host.  But this apparently doesn't create any detrimental effects on the host. So-commensal rather than parasitic.

Apparently, the worms produce egg capsules are released into the intestine of the host and released outside with the feces.  When the capsules are eaten by a new host, they become active.... probably a reaction to the intestinal fluid and proceed to live out their new life in the new host's intestine.

Flukes in Cukes! 
A free-living worm lives in the Pacific NW sea cucumber Parastichopus californicus.
California Sea Cucumber (Parastichopus californicus)
Image by T. Van Nunnery
This beast is called Anoplodium hymanae, a worm that is named for the famous Invertebrate Zoologist Libbie Hyman
Image by Chelsea L. Wood
These are a little more aggressive than the ones that live in sea urchins.

This species reaches the body cavity by penetrating the wall of the intestine..usually through the respiratory trees (feathery structures colored in blue in the pic).  I've briefly mentioned this area as where some sea cucumbers can feed via their butt!

The eggs are spread out via the anus with the feces until they are devoured by a host.

Similar to the ones in urchins, the larvae hatch in response to digestive fluids in the intestine of the host. Get into the intestine, move to the respiratory trees and then further move out into the body cavity of their new host!
Image by Chelsea L. Wood
Starfish got worms too!
So, there aren't a lot of records of flatworms that live in/among sea stars. Six were recorded in asteroids..and oddly enough, the one below was not included. So maybe its something new?

Description of this pic indicated the cold-temperate North Atlantic asteriid species Leptasterias littoralis. Is this a commensal flatworm moving within the tube foot groove? Moving around on the surface?  Something new? A convergence of two species by chance?
Starfish & Flat Worm
Image by Nick Sleptov
For more worm-starfish relations?

Go to this pic of Echinaster callosus and look closely at the short, striped things crawling on the swellings!    this one has a tighter shot that shows them a little more easily..note the brown squares with the white stripes.. (and includes a shrimp to boot!) WOO!  Acoel flatworms? 

How many remain to be discovered?? 


Kenna Wickman said...

Well finally Chris, you grossed me out.

Here is a question. When harvesting and eating Uni from sea urchins, would it be easy to inadvertently consume some of these parasites? Do any of these pose risks to humans?

I am going to go puke now....

ChrisM said...

The ones that live in sea urchins appear to live primarily in the intestines and apparently don't go wandering the way that the sea cucumber ones do. Uni are gonads and are of course a pretty separate organ, so, its hard to say how likely it is to find these outside of their primary "habitat". But that does seem to suggest it is unlikely to inadvertently consume any.

They are tiny-ranging fr. 0.5 to 2.0 mm..but noticeable.

That said, the important thing to realize also-these are not nematodes or even trematodes and aren't resistent to human digestive fluids. I don't think they would pose any risk to humans (other than perhaps stomach upset).

i've heard lots of reports in the news from nematode infections (a different distinct worm that parasitizes vertebrates) but never anything from urchin worms. So I'm going to say, it seems unlikely that anyone has anything to worry about from these...

Nick Slptsv said...

Hello Chris,

Thanks a lot for sharing my photo of the Leptasterias littoralis. It has been taken as a part of Invertebrate Zoology independent research to determine whether tube feet in Leptasterias littoralis follow the "phylogenetic route" aka are based on species taxonomy rather than being the most beneficial in the environment inhabited, or if the tube foot morphology type is determined by species environmental conditions rather than its phylogeny.

The unfortunate part is that I haven't noticed the flatworm until looking at photos and have only capture 2 almost identical pictures. The flatworm was most likely in a close relationship with the starfish, based on the fact that starfish long with other collected taxa were stored in the same "typical" aquarium, the specimen was collected from the bottom of the tank and the picture was taken immediately after

ChrisM said...

thanks for posting! Often times it is very familiar animals, such as Leptasterias which often get overlooked for commensals and other details. Or possibly the worm was already in the aquarium and was just cruising by the specimen as you were taking a picture of it. A survey of more animals in the wild and various other experiments, ie more observations and data, would be the way to go. Offhand I don't think L. littorialis is known to have flatworm commensals/parasites. If they do, its not a very well studied relationship. Definitely a project waiting to happen! thanks for catching it!