Amphidomataceae

by Koyo Kuwata, Mitsunori Iwataki, Mona Hoppenrath & Urban Tillmann (December 2025)

Below you will find general morphological characters that can be used for identification of the taxa of the family Amphidomataceae, the genera Amphidoma and Azadinium and its species.

Most species of Amphidomataceae are small (10–20 µm in length), with both varieties of Azadinium caudatum and a few species of Amphidoma being notable exceptions. Most species share an ovoid to elliptical cell shape and may exhibit variable size ratios between the hypo- and epitheca. Under light microscopy, the parietally positioned plastid is visible, as is the deep and wide cingulum, which accounts for roughly one-fifth to one-quarter of the cell length, along with a very distinct, pointed cell apex with the apical pore complex (APC). Additional features observable under light microscopy includes the large nucleus and, in some species, an antapical spine — although in certain species the spine may be very small and difficult to detect— and, where present, one or (in some species) multiple pyrenoids, which are recognizable by their characteristic starch caps.

All Amphidomataceae species have been reported to display a distinctive swimming behavior. Typically, cells move at relatively low speeds — for example, one strain of A. poporum swims at approximately 400 µm s⁻¹, while for Azadinium caudatum var. margalefii the speed has been recorded as 85 µm s-1. The swimming paths are irregularily interrupted by brief, rapid “jumps” in various directions. Some Amphidoma species are particularly slow in their overall movement, as e.g. reflected in the epithet of Amphidoma languida (from the Latin languida, meaning lazy, slow). High-speed jumps occur intermittently but are consistently observed, especially when cells encounter nearby objects, such as the glass bottom of the observation chamber. Occasionally, cells may cover larger distances at elevated speeds.

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All species of Amphidomataceae are photosynthetic and, as demonstrated for A. spinosum, are capable of growth in axenic culture. Each species possesses a presumably single, parietally arranged chloroplast that is lobed and extends into both the epi- and hypotheca. In several species, a stalked pyrenoid is visible under light microscopy due to its characteristic starch cup, and the presence, absence, or position of the pyrenoid provides a useful feature to support species-level differentiation within the Amphidomataceae. The nucleus is thick with multiple chromosomes, and exhibits species-specific shapes ranging from round to ellipsoidal, or rarely elongated, with slight variability in its position within the cell.

Transmission electron microscopy (TEM) observation of intracellular ultrastructure reveals general features of dinoflagellates, including the dinokaryotic nucleus, mitochondria with tubular cristae, parietal chloroplasts surrounded by three membranes, a round to oval pyrenoid, and fibrous bodies.

Three types of pyrenoids — stalked, bulged, and embedded — have been observed within the family.

Cytoplasmic invagination into the pyrenoid matrix has been observed in Amphidoma but not in Azadinium, suggesting that the presence or absence of this invagination may represent a genus-level diagnostic character.

Amphidomataceae possess a theca composed of cellulose plates; however, these plates are very thin and, in most species, are difficult to discern under the light microscope. When the protoplast leaves the theca under stress (a process known as ecdysis), the plates and the theca become visible even by light microscopy. The plates can also be visualized by fluorescence microscopy after cellulose staining and examined with respect to their arrangement. For a complete and detailed analysis of the plate pattern and further diagnostic features of the theca, however, the use of electron microscopy is recommended.

All amphidomatacean species share the distinctive combination of six plates in each of the precingular, cingular, and postcingular plate series, a feature not observed in other thecate dinophytes and thus representing a synapomorphy of the Amphidomataceae. Within the family, a major difference exists in the epithecal plate arrangement: all Azadinium species possess three or four apical plates and two or three anterior intercalary plates, whereas all Amphidoma species have six apical plates and lack intercalary plates entirely. All species share two antapical plates of different size as well as five sulcal plates.

The surface of thecal plates in most species is smooth, although some species exhibit rough or strongly reticulated plate ornamentation. In general, the plates are perforated by small pores, which may vary in density and distribution: they can be numerous and randomly arranged, as in A. caudatum; sparse and scattered, as in smaller Azadinium species; or particularly concentrated on the apical plates in certain Amphidoma species. In some Amphidoma, a distinct row of pores may occur below the lower cingulum list, and additional rows of pores can be present along the sutures between apical and precingular plates.

All species of Amphidomataceae possess a distinct pore located in the epitheca, historically designated as the “ventral pore” (vp) based on its position in the first described species, Azadinium spinosum. The amphidomatacean vp is larger than typical thecal pores, surrounded by a platelet-like structure, and exhibits species-specific positions on the ventral side of the epitheca. In many species the vp is located within or in contact with the pore plate. The position of the ventral pore serves as a highly informative diagnostic character for species-level identification within the family.