non-ceph: starfish reproduction

[monty]

I pretty much have no clue, but I can confirm that most starfish (and echinoderms in general) reproduce sexually and have a planktonic larval stage. Starfish can regrow severed limbs, and I think some research was done in terms of how much of the middle you need to re-grow, but my recollection is that you need more than half of the middle to be viable. There was a paper in the last month or so about ago about sand dollar larvae being able to bud: http://www.columbusdispatch.com/liv...llar.ART_ART_04-01-08_B5_V29P5DQ.html?sid=101
Note, though, that the larva looks nothing like the adult animal, and is a whole lot smaller, so this wouldn't explain your starfish. Are you sure they're actually starfish and not some other sort of critter? I can't really tell from the picture. I think there are some weird mutant species of Starfish that has 6 arms, but almost all have a 5 or a multiple of 5 (like sun stars).

 

[cuttlegirl]

Looks like Asterina to me, a common hitchhiker... See if it matches these pictures.

http://reefkeeping.com/issues/2004-06/ac/feature/index.php

 

[cuttlegirl]

I pretty much have no clue, but I can confirm that most starfish (and echinoderms in general) reproduce sexually and have a planktonic larval stage.

I just found out that there is an Asterina species that is ovoviviparous, meaning that development takes place inside the adult and then the fully formed juveniles are released from the adult. I am not sure if that is the same species that you have.

http://www.jstor.org/pss/1542316

 

[monty]

I just found out that there is an Asterina species that is ovoviviparous, meaning that development takes place inside the adult and then the fully formed juveniles are released from the adult. I am not sure if that is the same species that you have.

http://www.jstor.org/pss/1542316

Weird/cool. I can only read the abstract, but I think I get the point. Very odd... I should really learn to never accept any generalizations in biology. And it looks like the Asterina found in aquariums commonly also has a very wide variation in number of arms, which is curious, since I found other references that made a big deal about how HOX genes and developmental weirdness account for populations of 6-armed starfish and 4-symmetric urchins, yet they didn't suggest that populations of starfish with a random number of arms from 3 to 6 would exist. I wonder if this is a developmental thing, or perhaps if, since apparently the abstract describes direct developing starfish (so I was wrong about the generalization about a larval stage) if they develop 3 legs first and then grow the rest, or something.

I thought that the "normal" developmental process for echinoderms was that a fertilized egg develops into a blaterian larva with some "set aside" cells that will become the adult, then the larva feeds to provide nutrients for the developing adult, until the adult wraps around (I forget if it's head-to-tail or left-to-right to transform into a radial adult) and eats the remaining larval bits. The sea urchin genetics people seem to not mention that there are weird outliers to this... although maybe that's just because there's plenty of controversial weirdness in the supposedly-well-studied sea urchins.

This seems interesting, too:

http://www.springerlink.com/content/n704g233g7431443/

apparently being an example where the "larva" is a "cilliated blastula"

I think I'll split the stuff not-so-relevant to the original question into physiology and biology to reduce (or induce?) confusion.

 

[Jean]

Some Stars are fissiparous, they reproduce by breaking apart. One of the ones in the pics looks to be doing just that, 3 arms on either side of a stretched out central disc. We have one here that we call the three and three star (Allostichaster insignis) because this is it's main mode of reproduction and we often have 3 armed stars (well actually stars with three big arms and three little ones!) in the aquarium tanks!

J

 

[monty]

I continue to be astounded at how many generalizations I read about or learned about in high school biology turn out to be completely wrong...

Do you know what happens morphologically as they split? I'm pretty sure (insofar as I'm sure about anything about echinoderms now) that the adult body plan is head-to-tail linked in a circle-- do they split midway into a new head-and-tail and make to daughter "loops"? Or unzip down the midline? or something else-- I can't think of a way to grow another whole proto-adult and pass 3 arms off to the child, but at this point I wouldn't be surprised by anything...

 

[Jean]

I continue to be astounded at how many generalizations I read about or learned about in high school biology turn out to be completely wrong...

Do you know what happens morphologically as they split? I'm pretty sure (insofar as I'm sure about anything about echinoderms now) that the adult body plan is head-to-tail linked in a circle-- do they split midway into a new head-and-tail and make to daughter "loops"? Or unzip down the midline? or something else-- I can't think of a way to grow another whole proto-adult and pass 3 arms off to the child, but at this point I wouldn't be surprised by anything...

Ours unzip down the middle and then the side with no arms proceeds to grow new ones. I hadn't heard the head to tail linked in a circle description before. I understood them to have a non centralised nervous system and to be generally penta-radial around a central disc (some have secondarily modified this!), so that they can respond in any direction...............???????


J

 

[monty]

Ours unzip down the middle and then the side with no arms proceeds to grow new ones. I hadn't heard the head to tail linked in a circle description before. I understood them to have a non centralised nervous system and to be generally penta-radial around a central disc (some have secondarily modified this!), so that they can respond in any direction...............???????


J

Well, I'm starting to question everything I've read about them... this is particularly surprising, since I sat in on a class that used sea urchin embryos as the model system for gene regulatory networks for development... but it was more about the larval than adult form.

This random site on the intenet says:

Sexual reproduction involves the external fertilization of eggs by spermatozoa. The fertilized eggs develop into planktonic larvae. The larvae typically go through two stages, called bipinnaria and brachiolaria. They are bilaterally symmetrical and have bands of cilia used in swimming and feeding. As the larvae gradually metamorphose into adults, a complex reorganization and degeneration of internal organs occurs. The left side of the larva becomes the oral surface of the adult, which faces down, and the right side becomes the aboral surface, which faces up. The larvae settle to the sea floor and adopt their distinctive adult radial symmetry.

This is a bit more credible-sounding:

5) Echinoderm species exhibit a variety of developmental strategies, from maximally indirect development, where adults emerge from metamorphosis of a larva with virtually no similarity to the adult, to direct developmental transition from a fertilized egg into an adult. The bilaterally symmetrical larvae of indirect developing species are feeding, long-lived, and very simple in structure and cell number (in the most-used sea urchin models the pluteus larva has only about 2000 cells). A variety of intermediate developmental modes exist with, for example, non-feeding larvae or facultative larval feeding. For echinoderms, indirect development is primitive, and a dipleurula-type larva is found in all five living classes, as well as in the sister-phylum Hemichordata.[1-5]

This abstract would seem to indicate that it's complicated and still an area for active research...

and this one intimidates me with big words (pdf version at http://icb.oxfordjournals.org/cgi/reprint/38/6/965.pdf )

there's a glossary of echinoderm terms here: http://www.dnr.sc.gov/marine/sertc/Echinoderm glossary.pdf

I think I'll leave this at and stop trying to read enough that this post is coherent, 'cause the more I read, the more confused I get...

 

[Animal Mother]

I witnessed a spawning event a while back with my tiny brittle stars. Was quite interesting. They all came out at the same time, everywhere in the tank... like there was some sort of signal. Then they all climbed to the highest nearby spot and started spewing their reproductive juices into the water column. It was over in about 15 minutes.

I actually saw a larger 2 legged brittle star with a mangled disc climbing around in the seahorse tank a while back. Never saw the other half though. Don't know if it was intentional fission or if maybe I crushed the other half moving rocks around.

I don't think any two asterina stars look the same. Very rarely do I see a 5 legged one.

 

[monty]

from this overview (restricted access): http://linkinghub.elsevier.com/retrieve/pii/S0960982205014016

Are echinoderms really considered bilaterians? Yes.

While adult echinoderms are radially symmetric (usually
pentameric, but sometimes with higher-order symmetry),
phylogenetic analysis unequivocally shows this state is derived
from a bilaterian ancestor.

Analyses of hox gene expression and fossil evidence indicate that
the anterior–posterior axis of echinoderms runs along the axis of
the gut. Echinoderm larvae exhibit bilateral symmetry, with the
first ontogenetic signs of a pentameric form appearing in the
adult anlage (rudiment) in the advanced larva. Extinct Paleozoic
forms with bilateral adult body architecture are known from
fossils.

so it sounds like wherever I got the idea that the head-foot became a loop was wrong. I'm almost positive that some book I read in the UCLA medical library said this, though... maybe it was some old theory that turned out to be wrong...