Tuesday, March 26, 2013

Development of a hoplonemertean

On February 4, 2013, I did a plankton tow off a dock in Charleston, OR and found a few embryos like the one pictured here. The embryo was about 180 microns across, and was surrounded by an inner chorion and an outer chorion about 360 microns in diameter. I kept the embryos, and after two days discovered that they turned into planuliform larvae, hatched from their chorions and were swimming around in the dish.

The planuliform larva is uniformly ciliated and has a prominent bundle of cilia called the apical tuft at the anterior end (12 o’clock on the second picture). Such larvae are found in nemerteans (phylum Nemertea, a.k.a. ribbon worms) from the orders Hoplonemertea and Palaeonemertea. 

After a few more days these larvae developed features that allowed me to identify them as belonging to the order Hoplonemertea (e.g. several pairs of subepidermal eyes visible on the third picture). This picture shows an 11-day old individual. At this point they mostly crawled on the bottom of the dish, and had developed many of the adult structures, such as the brain and proboscis, so they can be considered juveniles rather than larvae. Hoplonemertean metamorphosis (the transition from planktonic larva to benthic juvenile) is inconspicuous. The transition from swimming to crawling is accompanied by changes in the epidermis. Apparently many hoplonemerteans replace the larval epidermis composed of large, ciliated, cleavage-arrested cells with intercalating smaller cells of the definitive epidermis (Maslakova and von Döhren, 2009).

To witness this epidermal transition, I fixed some of my specimens in paraformaldehyde (with a touch of gluteraldehyde) and stained them with fluorescent phalloidin to visualize the outlines of the epidermal cells using a confocal microscope.

The first confocal image shows a 2-day old hoplonemertean planuliform larva – the same age as the live larva pictured above. You will notice that large cells dominate the epidermis, but small cells are visible in between. These small cells are the intercalating cells of the juvenile epidermis. The bottom image is a 4-day old larva, and you can see that the large cells of the larval epidermis are farther apart from each other. The small cells of the juvenile epidermis occupy more space in between. Eventually, cells of the larval epidermis will be either resorbed or sloughed off, leaving the cells of the definitive epidermis to cover the entire surface. 

Maslakova SA, von Döhren J. (2009) Larval development with transitory epidermis in Paranemertes peregrina and other hoplonemerteans. Biol Bull 216: 273-292

Observations of a magelonid nectochaete larva

In a plankton tow taken in Charleston, OR on February 20, 2013, I found several polychaete nectochaete larvae of Magelona sp., which I was able to identify by their long anterior tentacles. The tentacles apparently develop as extensions of the prototroch (larval ciliary band) and are suspected to be used in locomotion and feeding  (Wilson 1982). I noticed the tentacles were relaxed and extended when the larva was hanging suspended in the water column, but during rapid sinusoidal movements of the body, the larva coiled up its tentacles and kept them close to the body.

This picture is a close up of one of the tentacles. You will notice that they are covered by papillae. The papillae are present along the entire length of the tentacle but only on the dorsal side. Pairs of papillae appear to be at right angles to each other as they each emerge from the surface of the tentacle. It is suspected that the papillae may serve a sensory function by detecting vibrations in the surrounding water (Jones 1968).

This is a close up of the larval mouth (ventral view) - note its distinct triangular shape. Two eyespots are also noticeable near the anterior end of the larva. Toward the posterior (down), bundles of bristles emerge from paired chaetal sacs on each segment of the body.

Wilson DP. 1982. The larval development of three species of Magelona (Polychaeta) from localities near Plymouth. J Mar Biol Ass UK 62: 385-401

Jones ML. 1968. On the morphology, feeding, and behavior of Magelona sp. Biol Bull 134(2): 272-297.

Feeding mechanism of the actinotroch larva

This picture shows an unidentified actinotroch larva of a phoronid worm that I caught in a plankton tow taken off a dock in Charleston, OR on February 20, 2013. The photo is a lateral view, and the anterior end of the larva is up. Actinotroch larvae have an anterior hood that covers the mouth (a lobe-like structure seen at upper left) as well as a crown of tentacles located posterior to the mouth. You can see that the tentacles are longer ventrally and shorter dorsally. This is because tentacle pairs are added progressively with new tentacles forming mid-dorsally, so the mid-ventral tentacles are the oldest. The number of tentacles varies between species. This larva had ~ 20 tentacles. Tentacles are involved in feeding. A posterior ciliated band, called the telotroch (at about 6 o-clock), helps in locomotion.

The second picture is a close up of the tentacles showing the cilia. The tentacles are covered by motile cilia, which create the water flow, but also bear a row of non-motile latero-frontal cilia. These stiff latero-frontal cilia apparently serve as a mechanical sieve, detaining food particles as they flow past the larva (Riisgård, 2002). Once caught on the upstream side of the tentacle, a food particle can be transported up to the mouth by the beating frontal cilia (Riisgård 2002), a tentacle flick (Strathmann and Bone 1997), or rapid lifting of the preoral hood which creates negative pressure (Strathmann and Bone 1997).

I gave my actinotroch some unicellular algae Rhodomonas (from a lab culture), which is a good food for many ciliated marine invertebrate larvae. This picture was taken a day after food was added, and it appears that the larvae fed on Rhodomonas, judging from the accumulation of the reddish pigment (the color of Rhodomonas) in the stomach.

Riisgård HU. 2002. Methods of ciliary filter feeding in adult Phoronis muelleri (phylum Phoronida) and its actinotroch larva. Mar Biol 141: 75-87.

Strathmann RR and Bone Q. 1997. Ciliary feeding assisted by suction from the muscular oral hood of phoronid larvae. Biol Bull 193: 153-162.