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Ultrastructure of tapeworm-like parasites in marine dinoflagellates (Haplozoon) Angel, Philip


Haplozoans are intestinal parasites of a specific group of marine annelids, called maldanids. Haplozoans are dinoflagellates, yet distinctly abnormal. Dinoflagellates are traditionally considered free-living, photoautotrophs, unicellular, and have two flagella. Yet somehow, Haplozoans are parasitic, and possess a mysterious “multicellular” trophont stage, with no flagella. Their life cycle is also largely unknown; while there is a well-observed adult trophont stage, but our understanding of other life stages is speculative at best. The trophont possesses three different types of compartments that create a multicellular appearance: (1) a trophocyte at the anterior of the cell, (2) gonocytes that compose most of the body length, and (3) sporocytes at the posterior. To explore the unique characteristics of Haplozoon, and provide clarity into the life cycle, I collected LM, confocal, and SEM data. The LM and confocal fluorescent data revealed that haplozoans are in fact unicellular. They possess a single plasma membrane and have compartmentalized their cell using amphiesma. New compartments are added behind the most anterior compartment of the cell and become more mature as they are pushed towards the posterior of the cell. This is a striking example of convergent evolution with a group of “strobilized” multicellular parasites, the Cestoda (Platyhelminthes). This pattern of compartmental maturity suggests that the posterior-most compartment of the cell produces the subsequent life history stage. The confocal data demonstrated that haplozoans do possess flagellar basal bodies in the membrane of each compartment, evidence that supports the existence of a hypothetical free-living dinospore stage. A novel finding from fluorescent tubulin staining was the existence of a complex network of microtubules, concentrated in the trophocyte of the cell. These microtubules of the trophocyte, dubbed the microtubular basket, allows the trophocyte to manipulate its shape and provides the dexterity for the suction cup and stylet to function. These haplozoan ultrastructure discoveries provide novel understanding of this enigmatic protist and provide a foundation from which to continue future research.

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