Thalassomyxa

1. Thalassomyxa australis Grell, 1985 (ref. ID; 4753, 4816)
2. Thalassomyxa canariensis Grell, 1994 (ref. ID; 4857 original paper)
3. Thalassomyxa jamaicensis Grell, 1992 (ref. ID; 4816 original paper, 4824, 4857)

Thalassomyxa australis Grell, 1985 (ref. ID; 4753, 4816)

Type locality

Thalassomyxa canariensis Grell, 1994 (ref. ID; 4857 original paper)

Descriptions

• Motile phase: The striking peculiarity of Thalassomyxa canariensis, if diatoms are used as food organisms, is the spongy appearance of the cytoplasm. It consists predominantly of variously sized lacunae, separated by dense cytoplasmic interphase. There is no clear distinction between lacunae and digestive vacuoles. Obviously, the spongy appearance is in some way induced by the use of diatoms as food organisms. After feeding with the cryptomonad Pyrenomonas sp., the cytoplasm shows only digestive vacuoles and small vesicles. From the priphery of the motile stages tapering extensions arise, sometimes connected by anastomoses. The so-called "captor strands", as described in Thalassomyxa jamaicensis, are missing. On a diatom lawn, the growing motile stages contact each other rapidly, so that large fusion complexes arise. They display extensive, rather transparent sheets. Individual complexes are often connected by rigid plasmodial strands. (ref. ID; 4857)
• Resting phase: In the stages of the resting phase, the "spongy" appearance, characteristic for the cytoplasmic structure after feeding with diatoms, disappears. Instead of "empty" lacunae there are only digestive vacuoles and the dense cytoplasm surrounding them. The latter is connected to a peripheral cytoplasmic rim of variable thickness. At the termination of every resting stage, irrespective of its size, all the digestive vacuoles join to form a common central bag. It is only in the very largest fusion complexes that some of the digestive vacuoles remain separate. (ref. ID; 4857)
• Multiplication: As in Thalassomyxa jamaicensis, two different modes of multiplication can be distinguished:
  • 1. The first mode entails only binary divisions. It is characteristic for the so-called "minimal plasmodia" and can be equal or unequal. In any case, the size of the plasmodia is probably only due to growth, not to fusion. As already described for Thalassomyxa jamaicensis, one of the division products transforms first into the motile stage. This is indicated by the accumulation of empty diatom shells, at the beginning surrounded by a membrane-like boundary. In Thalassomyxa canariensis, it could be observed that the motile stage sometimes detach from the substrate and float, probably for a short time only, in the seawater.
  • 2. The second mode of multiplication proceeds by tripartite or multipartite fissions and occurs probably always as a consequence of preceding fusion processes. Since the plasmodia of Thalassomyxa canariensis grow faster that those of Thalassomyxa jamaicensis, the final stage of plasmotomy is achieved earlier.

Remarks

Thalassomyxa jamaicensis Grell, 1992 (ref. ID; 4816 original paper, 4824, 4857)

Descriptions

• Motile phase: In contrast to stages of the resting phase, those of the motile phase often fuse with each other, a characteristic of all plasmodia known so far (Grell 1991). How many stages remain separate or fuse with each other depends upon their density. In a diatom lawn, for instance, where food is abundant and equally distributed, fusion occurs frequently. The initial stages of the motile phase are small specimens which settle upon the substrate (slide) or creep about. If food is available, phagocytosis starts immediately. In cultures with Dunaliella sp. or Pyrenomonas sp. as food organisms one can observe that the next step consists of a longitudinal stretching, usually combined with the formation of branches, pseudopodia-like projections and flat sheets at both ends. On a lawn of diatoms, the plasmodial strands form a wicker-work of variable appearance. More striking than in Thalassomyxa australis are the extensive sheets which may cover large areas and consist of transparent cytoplasm. The plasmodial surface is often endowed with groups of bundled, tapering projections that are also quite transparent. Such groups may be connected to the remaining plasmodium by a long and sturdy strand which can expand or contract. The bundles of tapering projections may serve for seizing diatoms which become ingested by the plasmodial mass from which the strands arise. If this interpretation is correct, it becomes clear why a certain clustering of the diatoms can be observed as the grazing of the lawn progresses. That the digestion of the engulfed diatoms begins already in the motile phase is indicated by the appearance of numerous vesicles which accumulate in the cytoplasm after phagocytosis. They have a yellow-brownish content and are continuously distributed by the plasmodial strands. This early separation of the digestible material from the diatom shells seems to be a peculiarity of Thalassomyxa jamanicensis that is not observed in the type species. (ref. ID; 4816)
• Resting phase: A series of micrographs illustrates that the smallest stages of the resting phase are more or less circular. Later on, they become irregular. The larger stages show a single vacuole, filled with digestible material. Probably, many of the empty shells become expelled from the cytoplasm before transformation to the motile phase. After feeding with Dunaliella vesicles of a green color accumulate in the cytoplasm. Their contents are transmitted to the digestive vacuoles during the resting stages. In contrast to Thalassomyxa australis, where a large central vacuole, surrounded by many smaller ones, appears, the digestive vacuoles of Thalassomyxa jamaicensis have nearly the same size and are equally distributed in the cytoplasm. If Pyrenomonas is used as a food organism, one obtains similar pictures, with the exception that the digestive vacuoles have not a green, but a red color, when digestion begins. (ref. ID; 4816)
• Transformation: In Thalassomyxa jamaicensis, the process can best be observed after feeding with one of the flagellates. During transformation, all vacuoles may fuse to form a larger one. After expulsion, the whole undigestible material is enclosed by an envelope which -at first glance- looks like a single membrane. It surrounds the material after its discharge for some time. The concept of exocytosis excludes, of course, that we are dealing with the original membrane of the digestive vacuole. What actually happens may be illustrated by a series of micrographs that show the transformation of a small resting stage. One part (right) has already transformed into the motile stage. As soon as the digestive vacuole touches the plasma membrane, a membrane-like elevation arises which forms a wrapping around the area where the digestive vacuole protrudes. Finally, exocytosis of the digestive vacuole occurs and the expelled material becomes completely surrounded by the afore mentioned "membrane". (ref. ID; 4816)

Type locality