Marine Planktonic Larvae

By Jean-Marie Cavanihac
France

 

Remember when we were young, we listened to marvellous stories in which, with a kiss, a princess metamorphosed an ugly toad into a "charming prince"! These stories were appealing for young girls, but young boys, of course, didn't believe these stories were real at all! ... However Nature seems to have dispelled this myth for millennia ... Everybody knows about insect metamorphosis in which a 'worm' - a caterpillar in fact - becomes a chrysalis, then a splendid butterfly. Isn't it a similar story? Maybe here, Nature is the princess!

The same process occurs for marine creatures and some larvae appear very different from the adults. But sometimes I think that the story works in reverse: larvae are often more beautiful than the adults! Here's an interesting observation: in many cases, only a larval stage is mobile; in fact this is a means to distinguish species when the adult form is sessile.

To illustrate my viewpoint, here is a gallery of some intriguing specimens. For some of them I have no pictures of the adult forms! Maybe this is preferred as the larva is often more pleasant to see than the adult itself.

First is one of my favourite specimens, the sea urchin. The adult is covered with harmful spines and it's not very sympathetic (especially to your feet if you walk on it!) but the pluteus stage larvae have a majestic appearance, reinforced by their slow movement.

Above is an Echinocardium cordatum pluteus larval stage, and below a Strongylocentrotus larva in polarised light. The larvae have a calcareous skeleton which is distinctive in polarised light. (The photo doesn't reproduce with fidelity the astounding colours you can see directly in the eyepieces!). The right hand picture is of an adult urchin, but you probably already know this.

Still staying in the echinoderm family: below is a larva of bristle star (pluteus stage) and animation of a young adult:  

Below is a sea cucumber larva (Holothuria). (There's no need to have an adult specimen image as its nickname is evocative enough!)

In another phylum, and not so distant relative to us are the tunicates. Tunicate larvae, which look like tadpoles, have a notochord in their "tail" which is an embryonic nervous system like our spinal cord. A larva of an ascidian (sea squirt), for example, is illustrated below.

The adult is less elegant; a sort of bag (in the sea squirts) with two siphons in which ciliated slits make the water circulate. Right-hand picture: ascidian colony.

Another tunicate: Botryllus below which covers rocks with a sort of soft carpet. Individuals are regrouped around a common excreting siphon. The incoming siphon is to the left of the right hand picture below showing one isolated specimen.

Another phylum: Crustacea containing the barnacles, class Cirripedia. You probably know that the barnacle is a pest on a boat hull, as it slows down the speed of the boat because they form a rough coating. It's harmful for us too and be cautious if you put your hands on rocks covered with barnacles because their calcareous shells are very sharp! I definitely prefer the larva which has two stages: the first is shown in the left hand picture below and looks like a copepod larva. The right hand picture below is the second stage; the cypris stage which can't feed, can live for a month, and must find a place to fix itself in this time. In the second row below is shown an adult and animation showing an adult feeding. (Note the elegant movement of the six pairs of long legs and cirri, which protrude from the barnacle body sweeping through the sea water to catch microscopic planktonic critters as food.)

Very intriguing too are crab larvae; the first stage or zoea is a swimming larva with some appendages helping it to float. Shown below is a porcelain crab larva.

Worms have larvae too. I have no picture of an adult horseshoe worm (phoronid) but I particularly like its umbrella shaped larvae. In fact a larva has metamorphosed itself into a young adult in my live box. The photograph below left was a little difficult to take because the specimen swims out of the field of view. You can see inside the body two large vessels in which blood cells circulate. In the worm larvae below right, the lophophore (a ciliated structure around the mouth) with the shape of a horseshoe can be seen.

Another worm larvae; of a ribbon worm, Phylum Nemertea. I only have a larval picture which looks like a Greek soldier's helmet!

Phylum Bryozoa: the larva below is a ciliated planula, dark colored, with a spherical shape. When it finds a suitable substratum, the adult polyp develops, growing by budding. (The adult here is a Bugula species.) In this case, I think the adult is more interesting to observe than the larva!

Probably many other intriguing larvae exist in the sea. The ten examples shown here are but a tiny example of Nature's infinite diversity and strategies for life!
 

  Comments to the author Jean-Marie Cavanihac are welcomed.

 

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All drawings and photographs Jean-Marie Cavanihac 2004

Published in the June 2004 edition of Micscape Magazine.

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