Morphology, Habits and Reproduction of Diplosoma migrans (Ascidiacea)
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| License | CC Attribution - NonCommercial - NoDerivatives 3.0 Germany: You are free to use, copy, distribute and transmit the work or content in unchanged form for any legal and non-commercial purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
| Identifiers | 10.3203/IWF/C-1855eng (DOI) | |
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| Production Year | 1992 |
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Transcript: English(auto-generated)
00:00
morphology, life history and reproduction in Diplosoma migrans. Diplosoma migrans is a benthic synocidian which inhabits the rocky shelf surrounding Heligoland Island. The preferred habitats are the holdfasts of laminarians
00:26
where it is not only found on the surface but also hidden deep inside the rhizoid tangle. But the colonies also occur on stones, red or green algae and bryozoa. Small colonies consisting of only a few
00:43
individuals measure only several millimeters in diameter. They frequently exhibit a rounded almost spherical shape like these Diplosomas on a laminaria rhizoid. This small colony has on the other hand
01:01
attached itself to the branches of a red alga and lacks a specific shape. From the edge of the colony three slender finger-like protrusions the stolons extend into the water. These structures serve to anchor the colony to the substrate.
01:20
The colorless and transparent colonies can occupy an area of one square centimeter or more. They have no regular shape, their contours being sinuous to lobed and following all the irregularities of the substrate. The structure consisting of individual animals
01:40
or zooids can clearly be seen in this colony spread out on the bottom of a petri dish. The mantle with its numerous stolons envelops the shared cloacal chamber. The numerous zooids occupy the cloacal chamber from the base to the apical layer of the mantle. To the right of the picture
02:01
a chimney-like protuberance of the upper mantle wall is visible. At the tip of it the common atrial or x-current siphon of the colony can be seen. Through this opening a constant current flows out of the cloacal cavity to the exterior. The current transports fecal particles
02:21
deposited by the individual colony members into the cloacal chamber and eliminates them through the atrial siphon. This time-lapse sequence illustrates the elongation of the vertically projecting stolons which can attain a length of almost two
02:41
millimeters, a process that takes several minutes. The stolons are produced by ampullae filled with hemolymph which can be extended causing protuberances in the mantle material. At the tip
03:05
the epithelium forms a glandular pad which is conspicuous by its whitish color. The ampulla can withdraw from the stolon at any time whereby the epithelial cells shorten by means of contractile filaments. Due to the elasticity of the mantle material
03:23
the empty tunic contracts as well but far more slowly. The contraction of the epithelial cells effects a simultaneous thickening of the ampulla epithelium.
03:42
Observation of a colony over a period of several hours as here shows that the stolons can be produced all over the surface. By retracting the ampullae the stolons disappear again only to reappear at other locations. Apart from the pseudopodia-like stolons
04:02
we can also observe a strong tunica strand protruding from the margins of the colony and serving as an anchor. How these stolons develop can be seen in the following colony. The tip of the long stolon at the top edge of the colony touches the bottom of the dish and adheres to it by
04:21
secretions produced at the glandular pad of the ampulla. Now the ampulla withdraws from the stolon leaving the tunica sheath behind to function as an anchor rope for fastening the colony to the substrate. A conspicuous back and forth movement of the mantle material indicates internal traction processes. Such movements can lead to the
04:44
relocation of individuals as these are connected to the mantle. With the aid of the stolons and the elasticity of the mantle small colonies can migrate slowly. In so doing the anchor strands at the rear
05:01
of the colony are stretched to breaking point and reabsorbed into the mantle material. If the two ends of the colony each take a different direction of migration the colony stretches increasingly and the mantle is constricted in the middle to a thin strand which ruptures to produce two daughter colonies. A deposit of fecal particles
05:24
remains behind. The mantle tissue consists to a major part of vesicular cells. These large spherical cells contain an all-occupying central vacuole which displaces the cytoplasm and nucleus to
05:42
a thin marginal layer. The cells can slide to a certain extent over one another and are deformable thus giving the mantle its flexibility. As further construction elements of the mantle we can observe slender elongated cells which are strongly branched and make up a loose network.
06:03
These myocytes are contractile and are responsible for the tensile forces occurring within the mantle. Further cellular bodies are small migratory cells with long pseudopodia
06:21
and granular cells. The cytoplasm of these cells is packed with granules which refract light. That is why even under low magnification they appear all over the mantle of the colonies as small white dots. The granular sites progress by amoeboid movement with the help of short broad pseudopodia. The individual members of
06:48
the colony measure about one millimeter in length. The rounded abdomen is distinctly set off from the conical thorax in which the apical cilia crown and the dorsal brain are accommodated. From the brain
07:03
ganglia extend in various directions. Underneath lies the neural gland. As the focal plane changes the cilia funnel and cilia crown come into view. The thorax consists mainly of the pharyngeal basket the wall of which contains four rows of gill slits.
07:23
The pharyngeal basket opens apically into a wide buckle siphon which is crowned with six pointed flaps. The margins of the gill slits are provided with long cilia producing a constant current of water which enters through the buckle siphon into the pharynx and passes via the gill slits
07:44
into the cloacal chamber from where it is expelled from the colony through the atrial siphon. The water current serves both respiratory and feeding purposes simultaneously. The cilia beat metachronically giving the impression of rotary movement.
08:03
The direction of beating is the same in all the gill slits seen from outside it is counterclockwise. Diplosoma migrans like most tunicates
08:21
is a filter feeder. Fine suspended particles are wafted into the buckle cavity by the cilia and checked for their nutritional value by the sensor cells of the tentacles. The inflowing particles are caught in a fine filter of mucus which moves along the walls of the intestine in front of the gill slits
08:41
and are rolled into a sausage-shaped bolus on the dorsal side. If particles prove to be unsuitable the buckle siphon is immediately closed. If the stimulation is increased the whole thorax may contract. When viewed from above
09:03
an individual colony member displays the buckle opening with tentacles protruding into it. On shifting the focal plane further down on the dorsal side one after another the three layers of hook-shaped dorsal folds come into view. On the interior face of these folds
09:28
the mucus filter shapes the food bolus and passes it to the esophagus. The mucus filter is secreted by glandular cells of the endostyle
09:41
to the left of the picture. This structure is shaped as a longitudinal fold extending along the entire ventral face of the pharyngeal basket. The secreted mucus is distributed to both sides of the intestine by long cilia of the endostyle and then follows special cilia pathways like the cilia crown
10:02
towards the dorsal face. The food passes from the pharyngeal basket into the esophagus and from there to the stomach. The adjacent mid-gut is covered here by the stomach but the hindgut is again visible. Deep down in the abdomen
10:24
the beating heart can be recognized. The tunicate heart periodically reverses its beat. Here the waves of contraction pass from right to left and now from left to right.
10:54
Now the process of beat reversal is seen in another specimen. On a level with the bend in the intestine
11:04
two young buds can be seen. The larger one will develop into a new thorax, the smaller one will later become a new abdomen. Vegetative reproduction occurs with the fusion of the old thorax to the new abdomen and conversely the old abdomen
11:23
to the new thorax. On an older thorax bud the rows of gill slits are already clearly recognizable. By focusing on the median plane of the bud the endostyle and the lumen of the pharyngeal gill basket appear.
11:44
Above is the still closed aperture of the buccal siphon. Budding often fails to initiate reproduction but the new thorax rather serves as a replacement for the obsolete old one here to the left of the picture. Its material is later
12:02
resorbed. In the final stage of atrophy the thorax has become a drumstick shaped appendage of the esophagus. Zoids do not only reproduce vegetatively they are also capable of sexual reproduction.
12:22
The ovary of the hermaphrodite tunicate contains only a few oocytes which are conspicuous with their large germinal vesicle and nucleolus. Development from the fertilized oocyte up to hatching of the lava takes place inside the colony so Diplosoma is ovoviviparous.
12:45
The egg cell passes into the mantle and eventually rests at some distance from its parent zoid. These three eggs already contain advanced larval cells.
13:01
The long tail of the tadpole lava is wrapped around the body. It is permeated longitudinally by a large vacuole. On the dorsal side of the still opaque body one can also distinguish the sensory papilla with the eye and the statocyst as well as at the interior end
13:21
the three stolar papillae. The fully developed lava frees itself of the egg membrane and mantle and emerges free-swimming first into the cloacal chamber. It propels itself by means of the tail.
13:42
Short periods of swimming are followed by prolonged resting phases. The lava has left the colony via the atrial siphon. It has a typical tadpole-like appearance and for a short period only an hour or even less it lives a free-swimming existence.
14:10
The meanwhile transparent body is now conspicuous for its two pharyngeal baskets. One of them belongs to the primary individual the oozoid. The other one belongs to the blastozoid.
14:25
The blastozoid has already been differentiated vegetatively at this early stage of development. The lava therefore contains two zooids, hence its generic name, diplosoma.
14:40
The lava attaches itself to the substrate with the secretion from its stolar papillae before undergoing metamorphosis. The adhesive is produced by cells in the center of this organ. The sessile lava then draws the tissue of the now superfluous tail inside its body,
15:01
a process that takes only a few minutes. Only the empty tunical case of the tail remains behind. The sensual papilla is also resorbed in the same way as the stolar papilla. Soon the stolons are protruded and the former larval body now begins to move around as a primary colony. The buccal siphons of the twin zooids
15:24
open up to the exterior so that independent feeding can be initiated. The primary colony multiplies the number of zooids vegetatively resulting in an increase in the size of the colony.