**1. Introduction**

*Dicathais orbita*, commonly known as the Australian Dogwhelk or Cartrut shell, is a predatory marine gastropod in the family Muricidae. This family of marine molluscs is well known for the production of the ancient dye Tyrian purple [1,2], which was the first marine natural product to be structurally elucidated by Friedlander in 1909 [3]. Over a century later, there remain major gaps in our knowledge of the ecological role and biosynthesis of this secondary metabolite [4,5]. However, significant progress has been made by Australian researchers over the last five decades [1,6–15], thus providing a foundation for using *D. orbita* as model species in natural products research.

As a common and relatively large gastropod on rocky intertidal reefs, *Dicathais orbita* is an important educational resource and has been the focus of study by a wide diversity of Australian postgraduate research students. Investigations into the natural products of *D. orbita* first commenced with the Ph.D. thesis of Joe Baker in 1967 [9], who established the ultimate precursors of Tyrian purple from the biosynthetic organ, the hypobranchial gland (e.g., Figure 1). This work was continued in the Ph.D. thesis of Colin Duke [16], who identified the intermediate precursors and synthesized a range of structural analogues. After a twenty year gap, my Ph.D. study into the antimicrobial properties of Australian molluskan egg masses identified the precursors of Tyrian purple from *D. orbita* as interesting lead compounds for bioactivity studies [17]. This initiated an ongoing program of research focused on *D. orbita* and their bioactive compounds, resulting in the completion of a further four Ph.D.s [18–21], one Masters of Biotechnology [22] and eight Honors theses [23–30], with an additional five Ph.D.s currently in progress.

**Figure 1.** (**a**) The development of Tyrian purple in the hypobranchial gland of *Dicathais orbita*; (**b**) The transfer of reduced precursors from the capsule gland of females to the egg capsules and the oxidation of precursors in the prostate gland of male *D. orbita*.

Around the same time as the research on *D. orbita* natural products chemistry commenced, Australia research students began investigating the ecology and life history of this species. The first in-depth study into the biology of *D. (aegrota) orbita* was undertaken by Bruce Phillips in Western Australia, whose Ph.D. thesis was published in 1968 [31]. Several additional student theses investigating the life history and ecology of *D. orbita* have been recently undertaken in South Australia [24,28,29]. *Dicathais (Thais) orbita* was also the major focus of a Ph.D. thesis by Gibson investigating imposex caused by TBT pollution on the east coast of Australia [32]. This established *D. orbita* as one of the first Australian invertebrate model species for ecotoxicology and an important indicator for environmental monitoring [32]. *D. orbita* was also included in the Ph.D. thesis of well known Australian ecologist Peter Fairweather, who investigated interactions between predators and prey on intertidal shores [33]. *D. orbita* has been subsequently included as a model species in several other student theses investigating environmental stressors and human impacts [34,35]. These insights into the ecology and life history of *D. orbita* have greatly facilitated ongoing natural products research, through interesting biological insights and population assessments, which help ensure sustainable collection.

To be suitable as a model system for innovative natural products chemistry research, a wealth of biological data is required on the organism, along with extensive familiarity with secondary metabolism system to be studied. *Dicathais orbita* is a candidate model species for the biosynthesis of brominated indoles, as these natural products and the associated biosynthetic glands in this marine mollusk are relatively well known (Figure 1). Useful biological traits for the selection of model species also include availability and life history features that make them easy to manipulate and maintain in the laboratory, as well as genetic knowledge and potential economic benefit [36]. Indeed *D. orbita* is a relatively large, long-lived gastropod that is common on rocky reefs in temperature Australian waters [33,37–39] and it also occurs as a pest predator on some molluskan aquaculture farms [40]. This species produces benthic egg capsules that each contain thousands of embryos that can be studied through several stages of larval development [41] and the reproductive cycle and anatomy of the adults is well documented [15,42,43]. *D. orbita* is resilient to environmental fluctuations [pers. obs] and both broodstock and juveniles can be easily maintained in laboratory aquaria [44]. The taxonomy of this species is well resolved [45], as is its systematic position within the Rapaninae subfamily of Muricidae [46] and the Gastropoda [47–49] more broadly. Genetic information on this species is also accumulating [5,50], with preliminary genome sequencing currently underway. A significant transcriptome database exists for a related species of *Rapaninae* [51]. As highlighted by Rittschof, and McClellan-Green [36], the power of model organisms could increase exponentially with input from multidisciplinary research teams that work from the molecular level, through the various levels of biological organization, to the ecosystem level. The combination of natural products chemistry and biological research undertaken on *D. orbita* to date establishes this species as potentially useful model for future studies on the evolution and biosynthesis of marine secondary metabolites, as well as for new method development e.g., [52].
