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Research study 1

Few investigations have been done on reproduction and embryonic development in west-coast sponges. Studies at Bamfield, British Columbia on the calcareous sponge Sycon coactum indicate that fertilisation is internal as in other sponges, leading to the formation of an unusual hollow blastula larva.  Oocytes (25-30µm diameter) are abundant in the mesohyl of the choanocyte chambers in June-July, while embryos are common in July-August. The larva escapes the parent sponge by squeezing directly through the choanocyte lining into the flagellated chamber, in the process inverting itself such that ciliated cells formerly facing inward into the blastocoel now face outward for propulsion. Interestingly, the choanocytes overlying the developing embryos become progressively smaller and flatter in shape as the embryo develops, perhaps to allow egress of the larva. After a short swimming period (not investigated by the authors) the larva “invaginates” (turns itself inside out), thus moving the ciliated cells to the inside to later form the choanocyte chambers.  Eerkes-Medrano & Leys 2006 Invert Biol 125: 177. Photos courtesy Sally Leys, University of Alberta, Edmontona

NOTE  the hollow part is the original blastocoel and it communicates with the outside via an opening; the larva is thus termed a stomoblastula

photo/schematic showing water flow through a sponge Sycon coactum courtesy Sally Leys, U Alberta Water flow in Sycon coactum: seawater enters from the outside through ostia, and thence to channels and pores that lead to the choanocyte (flagellated) chambers. The water flows through the chambers, then enters the spongocoel via apopyles (not shown) and exits from the osculum.
schematic showing route taken by a larva of sponge Sycon coactum to exit the sponge Route taken by larvae in Sycon coactum: after squeezing through the choanocyte layer the larvae are carried out of the sponge in the water flow
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photograph of some tropical tube sponges spawning taken from a video courtesy James Constable, Florida

CLICK HERE to see a video of tube sponges Aplysina archeri spawning. This is a Caribbean species but clearly shows the large volume of sperm and eggs emitted in sponges. The eggs are yellow and enclosed in a ropey adhering substance. The sexes cannot be differentiated by eye. Video courtesy James Constable, Florida.

NOTE the video replays automatically

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Research study 2

The process of cellular rearrangement in calcareous sponges, or “invagination”, as referred to in Research Study 1 above, has long been of interest to evolutionary scientists. A summary of the embryogenetic events in a calcareous sponge, as commonly presented to introductory classes on invertebrate biology, involves settlement of the amphiblastula accompanied by a cellular rearrangement whereby the flagellated cells comprising the anterior hemisphere move into the posterior half of the larva. The flattened space now becomes filled with mesohyl. An opening breaks through which becomes the osculum, and the tiny sponge begins to filter seawater for food. Interestingly, this intermediate “rearrangement” stage termed “invagination” by the present authors, has been photographed only once (about 100yrs ago) and so a search was commenced in Victoria, British Columbia using amphiblastula larvae from Sycon sp. collected at Bamfield, British Columbia. After 4 years, the elusive transitory stage has been seen and the processes leading to it studied.

drawing of cross-sectional view of an amphiblastula larva of a sponge

photograph of an amphiblastula larva of a Sycon sp. sponge courtesy Sally Leys, U Alberta

drawing of cellular re-arrangement (invagination) in a sponge larva as it settles to the sea bottom
X-section of amphiblastula larva showing flagellated cells at anterior end and an internal cavity, the blastocoel. These posterior cells now begin to grow upwards and around the anterior cells until they are completely enclosed (see drawings on far Right)
Amphiblastula larva of Sycon coactum
Photo courtesy Sally Leys, U Alberta
This growth, or invagination, encloses the flagellated cells so that they face inwards, and compresses the blastocoel. The latter later becomes filled with mesohyl (a proteinaceous material), which forms the body of the sponge. The growth of the large cells stops, and an opening, the osculum, is left. It represents the main exhalent opening of the sponge

The authors conclude: 1) that the “invagination”/ rearrangement process is not gastrulation, nor can the external opening be considered anything but an osculum, 2) that gastrulation does occur, but earlier in embryogenesis during the formation of the two cellular layers, and 3) that the rearrangement of these layers can be considered a part of metamorphosis. Now, to those of us who have long believed that gastrulation is a uniquely metazoan process that leads to the formation of two tissue layers (i.e., germ layers) and a gut with a mouth (the primitive blastopore), the idea seems controversial, to say the least. However, given new data that show that the Calcarea may be a possible sister-group to all other Metazoa, it may not be so far-fetched. The major implication of this is that, rather than sponges being a “dead-end” phylum, a sponge-like animal may have been ancestral to all metazoans. In concluding, the authors call for further study of germ-layer formation and the relationships of the primary germ layers among basal metazoan phyla.  Leys & Eerkes-Medrano 2005 Integr Comp Biol 45: 342.

NOTE  the present study actually has its origins in the classic works of Ernst Haeckel in the mid-late 1800s. In fact, much of the information on reproduction in calcareous sponges, leading to formation of a hollow amphiblastula larva and to an unusual rearrangement of the outer cellular layers during settlement/metamorphosis, stems from the work of Haeckel and others on the genus Sycon. Haeckel gives us such terms as blastula, gastrula, blastulaea, gastraea, ontogeny, phylogeny, and many concepts, most notably, the one most guaranteed to baffle undergraduate students..."ontogeny recreates phylogeny..."

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