Physical Description
Creseis acicula has two morphological forms; Creseis acicula acicula and Creseis acicula clava. Morphologically distinct ‘formae’ are defined by van der Spoel (1967) to denote subspecies. Both forms have similar life histories and ecology, and were observed at Heron Island. This website focuses on Creseis acicula at species level, though it is important to understand the intraspecific morphological differences. All Creseis acicula possess long, narrow, smooth shells with two distinctive ‘wings’ which protrude from the shell aperture. The shell is transparent and the internal visceral mass can be observed through the shell (Figure 1).
Creseis acicula acicula
C. acicula acicula has a needle-like shell, and compared to C. acicula clava, can grow quite large (up to 33mm long and 1.5mm wide). The rear angle of the shell is very small, at around 13-14°.
Creseis acicula clava
C. acicula clava has a shorter, tube shaped shell, reaching a maximum size of only 6mm (aperture diameter 1mm). The adults do not grow as long as the acicula form. The wings have a wing protrusion and are opposite a wing gland. The wing gland touches the base of the wing protrusion, which does not occur in C. acicula acicula.
Figure 1: Image showing comparison of the shell shape of two Creseis acicula formae, C. acicula acicula and C. acicula clava.
Useful references for field identification can be found at the Marine Species Identification Portal:
C. acicula acicula
C. acicula clava |