​Orientation

Crinoids, or feather stars and sea lilies, are unusual among echinoderms in that the opening to the mouth and anus are both located on the dorsal, or top side of the animal. This side is called the oral side, and the opposing side is called the aboral side. Both sides of A. pectinifera and the relative locations of the features discussed in this section are presented in the visual atlas (Fig. 2).

Fig. 2. False color visual atlas of Alloeocomatella pectinifera showing oral (A) and aboral (B) sides. A) The theca is the central body portion of a feather star. The fleshy central disk on the oral side is shown in royal blue. It is surrounded by numerous oral pinnules (pale yellow). The oral opening (purple) is near the margin of the central disk, and the anal papilla is a small protrusion in the middle of the central disk (orange). The unbroken arms near the oral opening of this specimen are called the anterior arms and are noticeably longer than the posterior arms on the other side. B) The aboral face of the theca is shown in pink. The centrodorsal plate (green) is surrounded by several cirri (yellow). The arms (sky blue) bear numerous pinnules (red).  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. 

Size

The maximum arm length is typically 20.0 - 27.5 cm, with the anterior arms up to 2.9 times longer than the posterior arms (Messing 1995). The cirri, hook-like structures on the aboral surface, are 1.4 - 1.9 cm long, and the diameter of the centrodorsal, the central plate on the aboral side, is typically 0.4 - 0.6 cm (Messing 1995). The original specimen acquired and described by Clark (1911) had arms approximately 9 cm long and cirri 1.4 cm long. An immature individual had anterior arms, posterior arms, and cirri 13.0 cm, 4.5 cm, and 1.0 cm in length, respectively, with a centrodorsal 0.21 cm across (Messing 1995). Six specimens collected at low tide from the shallow reef flats near the northern reef crest of Heron Island, Queensland, Australia (approximately 23º26'13"S 151º56'13"E) on 20 September 2012 had central disks (the fleshy central portion bearing the oral opening) ranging in size from 0.9 - 1.2 cm and arms 7.5 - 15.2 cm.


Color

Coloration varies from orange to bright red, pinkish, deep burgundy, or dark red-brown and can be solid throughout or patterned (Messing 1995). Patterned specimens typically have white or lighter colored articulations between the segments (Messing 1995). White or yellowish banding may occur on the centrodorsal, and pinnules may have a white aboral stripe and lighter colored tips (Messing 1995). The six specimens collected at Heron Island on 20 September 2012 ranged in color from bright red to deep burgundy-brown and were relatively uniformly colored throughout, although some had lighter coloring in the articulations between the segments.


Identifying Features

A. pectinifera is an unstalked feather star with ten arms arranged in pairs arising from five radials (Clark 1911; Messing 1995). Interestingly, this species feels less "sticky" than other comasterids, possibly due to having relatively few arms and less well-developed spines on the ends of the distal pinnules (Messing 1995). The anterior arms (the arms nearest to the oral opening) are 2.0 - 2.9 times longer than the posterior arms, which distinguishes it from the similar looking Alloeocomatella polycladia, with an anterior to posterior arm length ratio of 1.0 - 1.5 (Messing 1995). The articulations between the segments can be important diagnostic features and are discussed at length for this species in Messing (1995).

A. pectinifera attaches itself to the substrate with mobile, hook-like cirri arising from a pentagonal to circular centrodorsal plate on the aboral side (Messing 1995). The centrodorsal plate is the remnant of the stalk that more primitive feather stars have on their aboral side (Breimer 1978; Arguelles et al. 2010). Multiple pinnules branch off from the arm, giving it a feathery appearance. The oral pinnules, those pinnules nearest the central disk, bear comb-like structures on the outer segments–a characteristic feature of all feather stars in the family Comasteridae (Clark 1911; Messing 1995).