Biology 342 Fall 2014
Emily Merfeld & Nicolette Tapia
Phylogeny of Camouflage
To contextualize the phylogenetic development of color-changing behavior in cuttlefish, we examine both the phylogeny of the species and the cuttlefish eye.
Phylogeny of Cuttlefish
Cuttlefish are of the phylum Mollusca class cephalopoda, order sepioloida, and family sepiidae (Battler and Alcock, 2012). The first cephalopods appeared 500 mya with hard external shells and the ability to leave the ocean bottom- unlike the mollusks they evolved from. This was due to the evolution of air-filled chambers connected by the siphuncle, which allowed them to control buoyancy. The soft-bodied coleids, including cuttlefish, octopi and squids, developed during the Triassac period (Packard, 1972; Messenger, 2001). Shell reduction was a major diverging feature of the coleoid cephalophods, distinguishing them from a large number of shelled marine organisms (Figure 1; Kroger et al., 2011).
Figure 1. Cephalopod evolution [photo from Kroger et al., 2011].
Evolution of the Eye
Considering that visual cues are necessary for camouflage (see Mechanism), it is likely that the visual and chromatophoric systems came about through coevolution. In other words, each trait exerted selective pressure on the other to create a highly specialized system of polarized light sensitivity and color-changing skin cells. The polarization-sensitive visual system also plays a significant role in navigation (Wehner, 2001; Waterman, 1981, 1988) and detecting prey (Messenger, 1991). Unlike the round pupil of vertebrates, cephalopods have square pupils that become narrow rectangles in response to increased light. They have also evolved the ability to focus the lens to various distances by moving it inwards or outwards. Photoreceptors called rhabdomeres are unique to invertebrates but found in both cephalopods and mollusks, and allow for the perception of both unpolarized and polarized light (Shashar and Cronin, 1996).
Evolution of Camouflage
Dynamic body patterning and color change has been shown in a variety of cephalopods (Figure 2; Hanlon and Hixon, 1980; Lindgren et al., 2012). Most cephalopods have chromatophores (Messenger, 2001) and thus are able to change color. Furthermore, two distinct types of photophores- light-emitting cells- allow for bioluminescent color change in cephalopods. Photophores are gained and lost throughout cephalopodic evolutionary history, but are usually gained when organisms inhabit the upper levels of the ocean (Lindgren et al., 2012). Autogenic photophores are generally found in descendants of bathyteuthoids and oegopsids which inabit the upper levels of the ocean, whereas bacteriogenic photophores are found in demersal (ocean floor dwelling) species descending from sepiolods and loliginids (Lindgren et al., 2012). Cuttlefish and closely related species are in the latter category (Figure 3).
Figure 2. Cuttlefish-like body patterning in a closely related cephalopod species with chromatophores bacteriogenic photophores, myopsida. Photo from Richard Young and Michael Vecchione.
Figure 3. Phylogeny of two types of photophores: autogenic (green) and bacteriogenic (orange). Cuttlefish (Sepiidae) is marked with an arrow. Image from Lindgren et al., 2012.