ZEESTERREN

← INHOUD GLOS E  (ECHINODERMATA )°

http://www.white-rabbit.jp/Echino/EchinodermataE.html

°

Ophiuroides – Brittle stars

SLANGSTERREN  

Overal ogen

Slangster heeft geavanceerde kijkers

De slangster Ophiocoma wendtii verandert ’s avonds van kleur, omdat hij zijn zonnebril dan afdoet.

Zijn beste vrienden leven onder water. Al veertig jaar bestudeert Gordon Hendler stekelhuidigen en daarbij ontdekte hij er een die bedekt is met ogen. Heel bijzondere ogen.

Tweehonderd jaar lang had niemand gezien dat slangsterren van kleur kunnen veranderen.

Gordon Hendler zag het wel, en ontdekte uiteindelijk hoe dat kwam. Hij was de eerste die een slangster diep in de ogen keek.  “Deze slangster heeft honderden ogen op elk segment van elke arm, en iedere arm heeft honderden segmenten. Dus hij heeft duizenden en duizenden minuscule oogjes die zijn bovenkant bedekken.”

De zeebeesten die bioloog Hendler bestudeert, zijn dus niet blind, zoals altijd werd gedacht. En ze hebben niet alleen ogen, maar ook nog evenveel zonnebrillen om te fel licht te blokkeren.

Vanaf het    piepkleine eilandje Carrie Bow Cay, voor de kust van Belize, is het goed zoeken naar slangsterren.

Hendler weet zo vijf soorten op te diepen tijdens een duik.

Deze groep stekelhuidigen kan in de wetenschap doorgaans op weinig belangstelling rekenen. En dat terwijl er veel fascinerends aan ze te ontdekken valt, aldus de zeebioloog.

Bijvoorbeeld die ogen.

 Joanna Aizenberg, onderzoeker bij Lucent Technologies, ziet wel iets in de ogen van de slangster.

Ze maakt beelden van de speciale lenzen waarmee de armen van het beest volzitten.

Die kunnen iets dat menselijke maaksels nog niet kunnen: ze zijn dik, maar toch focusseen ze het invallende licht perfect op een punt. Omdat zo’n perfecte focus het gevaar in zich draagt dat er te veel licht op de achterliggende zenuwcel valt, gebruikt de slangster een zonnebril.

Aizenberg doet die truc na, en hoopt de stekelhuidige op zijn eigen gebied te kunnen verslaan.

(Elmar Veerman)

　

sea stars text index | photo index

Sea star (Asteroidea)

Supersnelle evolutie: zeester ontstond binnen enkele  duizenden jaren

 25 juli 2012  Caroline Kraaijvanger 6

Ongeveer 6000 jaar. Zo weinig tijd had een Australische zeester nodig om zich tot een nieuwe soort te ontwikkelen. Het bewijst maar weer eens dat evolutie geen traag proces is: soms kan het héél snel gaan.

Dat schrijven wetenschappers in het blad Proceedings of the Royal Society B. Ze baseren hun conclusies op een onderzoek naar twee soorten zeesterren: Cryptasterina pentagona en C. hystera.

Boven: C. hystera. Onder: C. pentagona. Foto’s: Jon Puritz / University of Hawaii, Manoa.

Toch anders
De twee zeesterren zien er hetzelfde uit, maar ze leven in twee verschillende gebieden. Een ander groot verschil is het seksleven van deze zeesterren. Mannelijke C. pentagona laten sperma los in het water en vrouwtjes laten eitjes los. Bevruchte eitjes groeien uit tot larven en uiteindelijk tot volwassen zeesterren. C. hystera is tweeslachtig en doet het allemaal in zijn/haar eentje. De jongen ontstaan in het lichaam van de zeester en lijken bij de geboorte al op een zeester. Het enige wat deze jongen nog moeten doen, is groeien.

DNA
Maar wanneer zijn deze twee soorten nu van elkaar gescheiden? En hoelang hebben ze er over gedaan om echt tot twee verschillende soorten uit te groeien? De onderzoekers zochten dat aan de hand van het DNA van de zeesterren, uit. Uit het onderzoek blijkt dat de twee soorten tussen de 6.000 en 22.000 jaar geleden van elkaar zijn gescheiden en uit konden groeien tot twee verschillende soorten.

Razendsnel
Blijkbaar zijn de soorten niet heel geleidelijk aan door verschillende mutaties ontstaan. In plaats daarvan zijn enkele exemplaren (of misschien zelfs één zeester) naar een afgelegen gebied ten zuiden van het leefgebied van deze zeesterren getrokken. Een verandering in de zeestromen zorgde ervoor dat deze zeesterren geïsoleerd raakten: contact met de andere zeesterren viel weg. En daar ontwikkelde C. hystera zich in enkele duizenden jaren tot een nieuwe soort.

Wetenschappers zijn verbaasd over deze snelle aanpak. “Dat is ongelofelijk snel in vergelijking met de meeste andere organismen,” stelt onderzoeker Rick Grosberg.

Primitieve ogen van een zeester zijn veel beter dan wij

denken

 05 juli 2013  Tim Kraaijvanger 2

Voor het eerst zien wetenschappers dat zeesterren hun primitieve ogen gebruiken om visueel te navigeren door hun omgeving. Dit blijkt uit onderzoek van de universiteit van Kopenhagen. Blijkbaar kunnen zeesterren met hun primitieve ogen beelden zien. Dit betekent dat zeesterren een belangrijke rol spelen in de evolutie van het oog.

Het menselijk oog is erg verfijnd met een iris, lens, pupil en hoornvlies. Primitieve ogen waren dit niet. De meeste eenvoudige versies van ogen konden enkel licht en donker onderscheiden. Voor de platworm is dit goed genoeg, omdat dit organisme van licht af beweegt naar het donker.

Ook zeesterren hebben ogen. Op de punt van elke arm zit een plekje dat gevoelig is voor licht. Tot nu toe dachten wetenschappers dat zeesterren alleen maar licht en donker konden onderscheiden. Nu blijkt dat de ogen wellicht geavanceerder zijn.

Het oog van een zeester.

Experiment
Onderzoekers van de universiteit van Kopenhagen plaatsten zeesterren met en zonder ogen op een afstand van één meter van een koraalrif. De zeesterren zonder ogen bewogen zich willekeurig, terwijl de zeesterren met ogen doelgericht naar het koraalrif navigeerden. “Deze resultaten laten zien dat het zenuwstelsel van een zeester visuele informatie kan verwerken”, zegt onderzoeker Anders Garm.

Evolutionaire tussenstap
Garm en zijn team onderzochten de fotoreceptoren in de ogen van de zeesterren. Daaruit blijkt dat deze fotoreceptoren een soort overgangsvorm zijn van rhabdomerieke naar ciliaire fotoreceptoren.

Garm: “Vanuit een evolutionaire invalshoek is dit zeer interessant. Wellicht dat zeesterren de eerste dieren waren die beelden zagen.

Als dit zo is kunnen we uitvogelen waarom zeesterren deze evolutionaire sprong maakten.”

Bronmateriaal:
“Seeing starfish: The missing link in eye evolution?” – Universiteit van Kopenhagen

OOGEVOLUTIE →

Eleven armed sea star

The large Knobbly sea star
is an icon of Chek Jawa.
Chek Jawa, Jun 05

 Juvenile Knobbly sea stars are common on
Cyrene Reefs but not elsewhere.
Cyrene Reefs, Apr 08The Cushion star is more pentangonal
than star shaped.
Terumbu Ular, Apr 06

The Eight-armed sand star
has more than five arms.
Chek Jawa, Jul 07The shorter arm of this Sand sea star
is regenerarting.
Changi, Jul 03Long pointed tube feet of the Sand sea star
helps it move quickly over the sand.
Chek Jawa, Apr 05Underside of a Common sea star. Greenish stomach outside the central mouth, tube feet emerging from groove beneath the arms.
Chek Jawa, Jan 03Huge bivalved pedicellaria
(pincer-like structures) on the
underside of the Cake sea star.
Chek Jawa, Jun 04Tiny white snails are sometimes seen on the upperside of a Sand sea star.
Changi, Jun 05A sea star disintegrating, possibly
due to flooding and a drop in salinity.
Chek Jawa, Jan 07The Crown sea star can come in dull (above)
or bright colours (below).
Chek Jawa, Jan 03

Chek Jawa, May 05

 2012http://www.wildsingapore.com/wildfacts/echinodermata/asteroidea/asteroidea.htm

March 22,

Sea Stars (Phylum Echinodermata, Class Asteroidea) of Singapore

Sea stars (class Asteroidea) are animals with a somewhat star-shaped body, made up of several arms extending from a central disc. They belong to a bigger group of animals called the spiny-skinned animals (phylum Echinodermata – “echino” roughly means “spiny”; “derma” roughly means “skin”).

Examples of other echinoderms include sea urchins (possibly where the term “spiny-skinned” come from?), sea cucumbers,

and feather stars.
Like other echinoderms, sea stars generally have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a sea star into 5 equal parts (imagine a star-shaped pizza…). Also, any keen follower of “SpongeBob SquarePants” will probably know that echinoderms are brainless (like Patrick Star). But despite that, they can still perform their daily functions – they can move, they can eat, they can shit, and they can reproduce – all these without a brain!
One other characteristic that makes the echinoderms so different from us, is that instead of blood vessels, they have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste. The sea star can also move by changing the water pressure, which extends or contracts the tube feet on its underside. The tube feet may also be used for breathing, sticking to hard surfaces, and feeding – they work like a conveyor belt by transporting food from the tip of an arm to the mouth in the middle.
Sea stars are also known for their amazing ability to regenerate lost body parts – such as an arm bitten off by a predator. Still, it would take lots of energy and resources to regrow lost body parts, and in the meantime, the sea star will be moving slower, making it more vulnerable to predation and less efficient in finding food.More than 30 species of sea stars have been recorded from Singapore, but unfortunately, many are threatened by loss of habitats due to reclamation and coastal development, and some are collected by poachers.
The Knobbly Sea Star (Protoreaster nodosus) is one of the bigger sea stars found in Singapore, and some can be more than 35 cm wide! This is perhaps also one of the prettier sea stars, as they occur in various colours, ranging from bright red, orange and pink to dull colours such as brown and beige. They have a hard, calcified body with large nodules on the top surface, which protects them from most predators except fish with sharp and powerful teeth, such as pufferfish and triggerfish. Indeed, every now and then I will see individuals with broken nodules or arms.
Despite the big size, this sea star feeds mainly on microorganisms, although it has also been observed to feed on snails, clams, soft corals and sponges. Like most other sea stars, they feed by everting their stomach over their food and digestion takes place externally. Observations suggest that the juveniles may live in seagrass meadows, and eventually move to the reef as they mature.
The biggest sea star in Singapore, based on body width, is the Maculated Sand Star (Luidia maculata) – the biggest I had seen was 60cm wide! Anyway, I used to call this the eight-armed sand star, until I started seeing a number of them that had 9 arms. This huge sea star burrows in soft sand and feeds on other echinoderms and molluscs.
The heaviest sea star we have is probably the Cushion Star (Culcita novaeguineae), which can weigh more than 5kg! Mature ones are more pentagonal in shape and appear somewhat bloated – like a cushion!
Juvenile Cushion Stars appear very different from the mature ones though. They are a lot thinner with more obvious arms. Juvenile cushion stars usually feed on encrusting algae, while mature ones preys on corals, other small organisms and tiny decaying particles (detritus). They usually feed at night, and in order to maintain its huge body mass, they are continuously moving in search for food. It can grow to about 40cm across.
The Mammillated Sea Star (Pentaceraster mammilatus) is another of the bigger sea star that can be found here. It’s about the same size as the closely related knobbly sea star, and is usually found in the same habitats. “Mammillated” means “having nipple-like projections”, and is certainly a rather appropriate description of this sea star. It was first recorded from Singapore only in 2008, and is likely an introduced species, possibly from ballast water.
Since then, we have been seeing sea stars that suspiciously appeared like hybrids between this sea star and the knobbly sea star. It’s probably too early to determine the ecological implications though.
Locally called the Cake Sea Star, Anthenea aspera can grow to more than 20cm across, making it one of the bigger sea stars too. I recently found out that “aspera” means “difficulties” – did the author have a hard time finding this sea star, or did he have a hard time identifying it, I wonder?
This sea star certainly comes in many colour variations – pink, red, orange, green, brown, black, and not to forget the many different patterns on its top side. Indeed, it also may not be easy to spot them among the seagrass or coral rubble if the colours happen to be just right and blend into the surroundings. This sea star has many tiny clip-like structures on its underside, and some on its topside called pedicellariae. Exact functions of these structures are poorly understood, but they are generally thought to keep the sea star’s body surface clear of unwanted stuff, such as dirt, algae or encrusting organisms.
Juvenile cake sea stars are often confused with Gymnanthenea laevis, a closely related sea star. Unlike the former, G. laevis often have spine-like structures (tubercles) on its topside though. “Laevis” means smooth, and I really wonder what’s so smooth about this sea star, as it often appears rather spiny. The name must have a reason, though. Not sure if anyone reading this can help? This species also has many pedicellariae on its body surface.
Mean while, we have a “rough” sea star as well. Goniodiscaster scaber (“scaber” means “rough”) is often called biscuit sea star locally, as it has a regular star shape and look as if it’s cut out using a cookie cutter. Smaller specimens are more commonly seen, but bigger ones can grow to more than 15 cm wide.
On the other side of the scale, the smallest and lightest sea star I have seen is a tiny one from the family Asterinidae. It is less than a centimetre wide, but we still have not been able to confirm what species it is. I am sure there are even smaller species in our waters though, just that I have not seen them.
A related sea star that we have identified is the Rock Star (Asterina coronata), also called the crown sea star, which grows to about 2cm wide. This sea star usually hides among crevices or under rocks in the day -and that’s why we called it the rock star! It emerges at night to feed, possibly on algae or detritus. Most of them have dull colours, but we often see brightly coloured ones as well!

Another small sea star found in rocky areas is the Cryptic Rock Star (Cryptasterina sp.). This sea star blends in very well into the surrounding rocks with its mottled patterns, and this certainly helps it to avoid predation. The ones I have seen are about 3cm across. So far I have only seen them on some of the Southern Islands. During low tide, it clamps itself tightly against the rocks to prevent desiccation. It feeds on algae and tiny animals on the rocks.

Moving on from the rocky shore to the sand bank, a number of burrowing sea stars are found here. The Sand-sifting Star (Archaster typicus) is possibly the most common sea star in Singapore, and hence many nature guides also call it the common sea star. It has many common names though, but I personally prefer to call it the Sand-sifting Star, as I thought the name itself tells the story of how the sea star behaves – it sifts among the sand to avoid predation and to forage for detritus to feed on.

Unlike most other sea stars, the Sand-sifting Stars have a rather interesting reproduction behaviour – males are often found stacked on top of the females, and the pairing may last for up to 2 months before the eggs and sperm are released into the water. The reproductive organs do not meet, and hence this behaviour is termed “pseudocopulation” – in other words, “fake sex”. This behaviour apparently increases the chance of fertilisation though.

Sand Stars (Astropecten spp.) are also rather common in Singapore. They usually hide in the sand in the day, and emerge when it’s cooler and darker to hunt small prey such as clams and snails.

A few different Sand Stars have been seen in Singapore, but it’s a challenge to determine the exact species. Unlike most sea stars which digest their food externally, sand stars swallow their food whole and digest them internally.

This sea stars may be a Astropecten indicus

Similar to the sand stars, the Penang Sand Star (Luidia penangensis) and the related maculated sea star mentioned earlier also digest their food internally. The former usually comes with arms.

The Galloping Sand Star (Stellaster equestris) is occasionally seen on sand banks near coral reefs. As the name suggests, it can indeed “gallop” by moving on the tips of its arms.

In the coral reef, divers often come across the Icon Star (Iconaster longimanus) on lower reef slopes and seabeds. They feed on algae. While most sea stars which broadcast lots of eggs, icon stars produce small numbers of large eggs. The eggs are orange in colour and apparently contain chemicals that deter fish predators.

In some of of our seagrass meadows, a few Nepanthia spp. can be found.

Lots of other sea stars can be found in Singapore, but unfortunately I do not have photos of them. Here are a few other interesting facts about sea stars:

1. They have an internal skeleton (endoskeleton) made up of calcium carbonate (ossicles), somewhat like us.

2. Most sea stars have an anus on the upperside, but not the Astropecten and Luidia species.

3. Sea stars have light-sensitive eyespots at the tips of their arms, and they often keep the tips slightly tilted upwards so that they can sense their surroundings better.

4. Like most other marine organisms, the sea stars in Singapore are threatened by loss of habitats and collection.

Afternote:

Quoting from Romeo & Juliet: “A rose by any other name would smell as sweet.”

As such, unlike many other local nature guides, I never insist that my participants stop calling the asteroids “starfish”, and switch to calling them “sea stars”. I guess that’s as good as asking them to call a cuttlefish “sea cuttle”, a horseshoe crab “sea horseshoe”, a jellyfish “sea jelly”, a hermit crab “sea hermit”, or even a feather star “sea feathers” (and in fact some people do call them as such).

To me, these are just common names, and so it’s a matter of personal preference or habit. In my case, ironically though, I personally prefer to call the asteroids “sea stars”, not for any scientific reasons, but more because I think it sounds nicer. Haha…

As long as the guide explains clearly that a starfish is not a fish, and a horseshoe crab is not a crab etc, I think it is sufficient. If anyone is so anal about getting it scientifically correct, then they might as well go and lead a scientific group and use scientific names – common names are for the general public and they were never meant to be scientific in the first place.

Posted by Ron Yeo

Sea stars commonly encountered on Singapore’s shores and reefs, including some special ones.

653 photos | 1,350 views

items are from between 01 Jan 2000 & 14 Jul 2007.

← prev 1  2 3 4 5 6 7 8 9 next →

 

starfish

Cryptasterina

Red Knobbed starfish

FOSSIELEN

  zoekplaatje

starfish, brittlestars, and sea-lilies.    http://www.humboldt.edu/natmus/lifeThroughTime/Silurian.web/index.html

Fossil  sea stars