Echinodermata (Sea Urchins, Sea Stars, and Others)

Echinoderms are well-known marine invertebrates that evolved a unique body pattern organized into five symmetrical parts (Hyman 1955). Because of this unorthodox pentamerous body symmetry and the difficulty to identify the position of the ancestral anteroposterior axis (reviewed in Mooi and David 2008), echinoderms are rarely mentioned or often categorized as non-segmented in previous discussions about the evolution of segmentation (Sedgwick 1884; Masterman 1899; Hyman 1955; Minelli and Fusco 2004; Tautz 2004; Couso 2009)—with a few exceptions (Beklemishev 1969a; Balavoine and Adoutte 2003). But echinoderms indeed exhibit a conspicuous set of repetitive serial elements, not along the anteroposterior axis, which is obscured, but through the main axis of each of the five individual rays that make the echinoderm body (Mooi, David, and Wray 2005). Despite not being arranged along the anteroposterior axis, and thus a clear exception to the definition of “segmental” used in this chapter, these traits are worth mentioning given their clear segmental nature.

Adult echinoderms have five growth zones, one at the distal tip of each ray, where new skeletal plates are deposited in a regular series by terminal addition (Mooi, David, and Marchand 1994; Mooi and David 1998). Different than other segmental structures described in this chapter, the skeletal elements are not paired but have an alternating biserial distribution resulting in a zigzag pattern (Mooi, David, and Marchand 1994; Mooi and David 1998; Mooi, David, and Wray 2005). This segmental arrangement is evident by the skeletal plates and is also reflected in the circulatory system, the podia, neurons, and muscles (Beklemishev 1969a). Interestingly, the alternating growth pattern has changed in brittle stars and sea stars, and the elements have lined up side by side forming paired serial structures along the ray (Figure 9.2D) (Mooi and David 2000).

Although genetic interactions have been extensively studied in echinoderm embryos and larvae (Cary and Hinman 2017), the genes involved in axial terminal growth remain less known mainly due to the challenges of obtaining and raising juvenile stages (Byrne et al. 2015). Of the few investigated candidates, the gene Engrailed, known to pattern arthropod segments, is expressed in a segmental manner in the arms of juvenile brittle stars and sea stars (Lowe and Wray 1997; Byrne et al. 2005). However, the gene is likely associated to neurogenesis rather than the generation of repetitive structures (Wray and Lowe 2000; Byrne et al. 2005). Fortunately, a complementary approach to investigate echinoderm terminal growth through arm regeneration experiments is gaining traction, and the molecular and cellular processes patterning these segmental elements are being unraveled in brittle stars and sea stars (Bannister et al. 2005; Czarkwiani, Dylus, and Oliveri 2013; Czarkwiani et al. 2016; Ben Khadra et al. 2017; Ferrario et al. 2018).

Finally, echinoderms have also evolved segmental traits outside the five terminal growth zones. A notable example is the stalk of crinoids, which is built on a repetitive series of internal skeletal elements that can reach 50 cm in extant species (Figure 9.2E) (Hyman 1955). The ossicles that give rise to the stalk appear during larval metamorphosis as five sequential elements (Haig and Rouse 2008; Amemiya et al. 2016), but unlike the pattern of terminal growth in the rays (Mooi, David, and Wray 2005), these elements grow at the proximal region near the main body (Thomson 1865; Lahaye and Jangoux 1987) or by the intercalation of skeletal elements (Amemiya et al. 2016).

 
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