The first cleavage of a Pulsellum spec. (Scaphopoda, Pulsellidae)

The first cleavage of a Pulsellum spec. (Scaphopoda, Pulsellidae). Picture: © 2010, George von Dassow
The first cleavage of a Pulsellum spec. (Scaphopoda, Pulsellidae). You are able to see the polar lobe. Picture: © 2010 George von Dassow

Below you can enjoy a video of the first cleavage of a Pulsellum spec. dredged in Friday Harbor, San Juan County, Washington, United States. The dark region in the video is the pink cytoplasm, which gets sorted as the egg proceeds toward first cleavage into a prominent equatorial band. This video was taken by George von Dassow and originally published on his website about cell behavior during animal development on March 9th, 2010. The accompanying text is written by Katie Bennett and George von Dassow. All rights are reserved by George von Dassow.

Half-way through this movie you might ask yourself, what is that funny three-celled embryo?  How can it have three cells?  Is it normal?  If one found a single three-cell embryo in a culture of normal-looking two-cells, one would assume it was polyspermic or otherwise defective.  But what about a whole dish of them?  This apparent three-cell – or trefoil – stage is a normal feature of early development for many molluscs and annelids.  The scaphopod featured here, a predatory infaunal mollusc with a tube-shaped shell, exhibits classic polar lobe formation: the extrusion and transient partition of a large parcel of cytoplasm which is subsequently resorbed into one daughter cell.

As in this case, segregation of the polar lobe cytoplasm at the vegetal end of the embryo results in a protrusion so constricted that it appears to be a third cell.  This is remarkable for two reasons: first, the polar lobe is a mechanism for cell fate specification; second, it is a cytokinetic puzzle.  In animal cells, the cytokinetic furrow always crosses the midplane of the mitotic apparatus.  That’s how the cell ensures that the cell division plane passes between carefully-sorted sets of duplicated chromosomes.  In polar-lobe-forming cells, of course there is only a single mitotic apparatus, bisected by the “normal” furrow.  How is the furrow around the neck of the polar lobe induce?  There are several possible explanations, but all of them are speculative.

© 2010, George von Dassow
© 2010 George von Dassow
However it forms, what embryologist could resist the temptation of experimentally removing such a tidy bundle?  This experiment has been performed on several species, most famously by Clement on the snail Ilyanassa obsoleta.  The results show that the contents of the polar lobe endow its lineage with something that makes one of its descendant cells behave as the organizer for the rest of the embryo.  One interpretation would be that there is information essential for normal development contained within the polar lobe.  The nature of that information, however, remains elusive, and another possibility is that the polar lobe just makes some cells bigger, and consequently different.

Some molluscs and annelids make small polar lobes.  Others simply undergo an unequal first cleavage, without a polar lobe.  But there is good reason to think that the ancestors of both molluscs and annelids probably cleaved equally, instead of determining the organizer lineage in the first few divisions.  That means polar lobes have arisen several times during the evolution of existing molluscs and annelids.  Why?  Is there some reason, in the embryology of these groups, why many evolutionary lineages found it adaptive to speed up early cell fate specification?  And are these embryos, for some reason, predisposed to come up with this peculiar mechanism of unequal division?

Thanks George von Dassow for allowing me to repost his material. More information about this particular species and its habitat will I publish later! To be continued!

A nice day on the intertidal zone of Yerseke, or not?

Searching on Yerseke, © 2012 Frans de Jong

With the theme of the exotic species living in the Oosterschelde (the Netherlands), after the success of the ”Gorishoek excursion” (see: LINK) and the great stories we’ve heard about Yerseke made Frans de Jong and I decided to go on excursion to Yerseke on July 22th, 2012.

grey chiton (Lepidochitona cinerea) on Yerseke

After meeting each other on the parking lot next of Youth Centre ”De Zealte” on the Korringaweg we looked for a suitable location to go down of the sea dyke. When we arrived without sliding at the foot of the dyke, and skimmed the inventarisation area, it seemed pretty similar to Gorishoek. Only it is much dirtier and full of blue mussels (Mytilus edulis). If you look to the ground you can easily pick empty shells of the common limpet (Patella vulgata) and single valves of The queen scallop (Aequipecten opercularis). Still no Atlantic oyster drill (Urosalpinx cinerea) of the bristly crab (Pilumnus hirtellus), species for which we actually did come to Yerseke. A few meters to the water, in some pools, I saw impressive big examples of the

some of them are near the 3 centimetres. During the return to our car we saw a nice crab, not a bristly crab, but I think it is a brush-clawed shore crab (Hemigrapsus takanoi), an exotic species that repulse our aboriginal crabs, but I’m not sure about the determination. Collected that one and went to the car, and on to the other search spot near Yerseke …

… or not. The places we wanted to visit were not hardly accessible by car. If we would have walked, we would have risked missing the low tide for

Frans de Jong also prepared for a picture

the breakwaters. After a short discussion we decided to go to work island Neeltje Jans and continue our search on the North sea side of the island. There we only saw much common limpets, flat periwinkles (Littorina fabilis), a large grey top shell (Gibbula cineraria), some pullet carpet shells (Venerupis senegalensis), one living dog whelk (Nucella lapillus), a nice coloured crab, I think it is Japanese shore crab (Hemigrapsus sanguineus) and some smaller shells of the Cuttlefish (Sepia officinalis) at the south of the island.

Afterwards we enjoyed a delicious barbecue at the campsite where I stayed. Altogether it was not a bad day.

References:

  • Bruyne, R.H. de (2004): Veldgids Schelpen. — KNNV Uitgeverij, 234 pp.
Another picture of the grey chitons (Lepidochitona cinerea) on Yerseke © 2012 Frans de Jong

A Sunday Morning on Gorishoek (on Tholen, the Netherlands)

The collecting group of the day going to the location

It’s Sunday morning 6. may, 9 o’clock in the morning. A group of 7 people walk from the parking behind restaurant ‘de Zeester’ to the already dry mud banks of the Oosterschelde. Full of hope we defy the cold (8°C) on our way to our collecting spot. One thing catches the eye immediately, everywhere we look lie empty doublets of Venerupis philippinarum (Adams & Reeve, 1850). We were already informed in advance about this phenomenon by Frans de Jong, who was there a month ago (see: LINK). We don’t know the reason of the mass beaching of these shells but it was a good start of the day. Between the  V. philippinarum we found a striking number of recent single valves of Glycymeris glycymeris (Linnaeus, 1758) a species with the Channel for it northernmost limit of its range. But there lives a small population in the Oyster wells of Yerseke. After collecting a sample of both exotic species, we walk further.

Urosalpinx cinerea (Say, 1822) with eggs

Then we come to the place with the species that Gorishoek is famous for. After turning some stones we found many living Urosalpinx cinerea (Say, 1822) and fewer but still large parties of Ocenebra inornata (Récluz, 1851). The nice yellow eggs are a sight for sore eyes and very fascinating. Between these two species of Muricidae are lots of living Littorina littorea (Linnaeus, 1758), Gibbula cineraria (Linnaeus, 1758) and Patella vulgata Linnaeus, 1758. Of all the five species I collect some. In the next month I try to prepare the radula of my collected P. Vulgata.

Nucella lapillus (Linnaeus, 1758) with eggs

A little further we come to an area with Nucella lapillus (Linnaeus, 1758). This is the species of Muricidae which belongs to this place but it is slowly supplanted by exotic species of that family. But there is some hope. This species has laid eggs too, so we can enjoy the beauty of this animal for one more generation. We hope it will be more. I’ve also found an unusually big specimen of N. lapillus, which was around 50mm.

© 2012 Frans de Jong

Another eyecatcher are the lots of empty shells of P. vulgata which are spread throughout the searched area. I’ve collected around 30 empty shells. That’s very different from the small samples of empty shells I’ve collected at Hoek van Holland and Borssele. Also there laid many single valves of Aequipecten opercularis (Linnaeus, 1758) and doublets of Ostrea edulis Linnaeus, 1758. So that is a indication that Crassostrea gigas (Thunberg, 1793) has not completely displaced O. edulis. But I don’t think it could be saved. Maybe they were the last Dutch doublets of the shell I’ll see in my life.

Some other interesting finds are a single valve of Arctica islandica (Linnaeus, 1767) and that, at the end of the Oosterschelde looks like a great specimen. A recent right valve of Pecten maximus (Linnaeus, 1758) with a size of 115mm is uncommon, which lives on higher depths in the Northsea. Shells of of adult animals beach infrequently.

After some two hours searching, we return to the cars. Over a cup of tea we talk about our finds and then we leave the sea and go back home. When I arrived home I inspectedt my shells. And found on the shells three Lepidochitona cinerea (Linnaeus, 1767). I’ve searched for that species there but I haven’t found it.

Later I will attach a full species list of this day.

References:

  • Bruyne, R.H. de (2004): Veldgids Schelpen. — KNNV Uitgeverij, 234 pp.
  • Faasse, M.A. & Ligthart, A.H.M. (2007): The American oyster drill, Urosalpinx cinerea (Say, 1822), introduced to The Netherlands – increased risks after ban on TBT? — Aquatic Invasions 2(4): 402-406
  • Faasse, M.A. & Ligthart, A.H.M. (2009): American (Urosalpinx cinerea) and Japanese oyster drill (Ocinebrellus inornatus) (Gastropoda: Muricidae) flourish near shellfish culture plots in The Netherlands. — Aquatic Invasions 4(2): 321-326