Figure 1                                Figure 2                                Figure 3
                  Scale bar = 0.25 µm            Scale bar = 250 µm.             Scale bar = 100 µm.


                                             Type species: S. cristata, present designation

Diagnosis.

      The hibernaculum develops as an irregularly shaped, localized enlargement at the tip of a short stolon immediately adjacent to the zooid base. The structure takes on a roughly hemispherical or low conical shape with a well defined circular plug at its apex from which a new zooid will eventually emerge (Figure 3). Such a hibernaculum is fundamentally different in development and morphology from those of other kamptozoans. It is this difference, together with the freshwater habitat, that defines the new genus.

Description

      Colonies of zooids joined in linear series by slender stolons adherent to firm substrata throughout their length, each zooid capable of radiating 1-3 new stolons from the base; individual zooids with muscular, unsegmented stalks; calices oriented obliquely on the stalks. Hibernaculum low and roughly conical or hemispherical, with a central indentation ringed by a raised margin, the entire structure formed as the expanded tip of a short stolon adjacent to the base of a zooid.

Etymology

      The generic epithet honors Her Royal Highness Princess Maha Chakri Sirindhorn, who maintains a strong interest in the biological diversity of Thailand.

                                                           

                                                                    

                                                 Sirindhornella (S.) cristata, new species

Diagnosis

     The calyx shield bears a conspicuous, longitudinal crease, or carina, which extends along its entire length.

Description

      Zooids are relatively small and widely spaced (Figure 1), becoming crowded only where space is restricted, each zooid with 1-4 narrow stolons extending from the base. The stalk is roughly cylindrical and muscular throughout its length, tapering slightly from base to apex (Figure 2). When straightened, the stalk length of the largest zooids does not exceed 0.45 mm, and most zooids are slightly smaller than this. The stalk-calyx junction is wide and unconstricted. The calyx orients obliquely to the stalk, the oral side facing the general direction of next youngest zooid in linear series. A conspicuous aboral shield bears a medial ridge or carina throughout its length, especially pronounced in larger zooids. Also in larger zooids both the stalk and shield are minutely roughened with small tubercles or protuberances easily visible in clean specimens at 30x magnification. In older specimens the semi-opaque shield, together with adhering debris, obscure internal structures. The largest calyces measure 0.4mm in length. The tentacular shield is a wide band bearing 12-16 colorless tentacles. Stolons are relatively slender, about 30 micrometers in diameter, yet strong enough to be dislodged intact from most hard substrata. Septa within the stolons have so far been observed only at the proximal side of hibernaculae. The hibernaculae develop as irregularly shaped, localized enlargements at the tips of short stolons immediately adjacent to zooid bases. The wall of each hibernaculum thickens and darkens, and there is an expansion of the basement cuticle that adheres the entire structure to the substratum. The mature hibernaculum is a small mound measuring roughly 220 by 180 micrometers, with a well defined circular orifice at its apex about 75 micrometers in diameter (Figure 3). The orifice often appears plugged with a granular material. It is at this point where a new zooid eventually emerges to generate a new colony.

Etymology

      Sirindhornella cristata. The specific epithet cristata (crest) refers to the prominent longitudinal carina or crest on the aboral shield.