**1. Introduction**

Biological pigments (biochromes) can be defined as any material from biological origin that results in color. They can have many functions, among which mimicking and communication are but a few, and are normally products of complex and varied biosynthetic pathways involving a wide span of enzymes [1]. Pigments resulting from these metabolic processes can be broadly divided in two classes: those that are directly responsible for organism colors and those that are colored secondary metabolites, which may or may not be directly involved with the organisms' primary pigmentation [2]. One of the most abundant classes of natural pigments are tetrapyrroles, considered as the "pigments of life" [3] due to their role in photosynthesis, gas transport, and redox reactions. This class includes porphyrins, which are metabolites of heme.

Arguably, the best known porphyrinoids are bilins (also termed bilichromes or bile pigments) [2,4], whose coloration can vary between yellow, green, red, and brown [5]. They are secondary metabolites devoid of metal cores and are arranged in linear (chain) structures rather than in the customary cyclic configuration of porphyrins [2]. Biliverdin and bilirubin are notorious members of this group

that, despite resulting from the breakdown of heme, hold important biological functions, such as antioxidants, in humans and other animals [6]. Like many other tetrapyrroles, bilins are photosensitive, which provides them with a high interest in photodynamic therapy (PDT), since they can generate singlet oxygen when irradiated, thus triggering localized cytotoxic effects [7]. In fact, depending on their structure, porphyrinoids have a distinctive absorption spectrum in the UV-visible region, characterized by a strong band around 420 nm, named the Soret band, and a series of low-intensity absorption bands at longer wavelengths, typically between 500 and 650 nm, termed Q bands (refer to Arnaut [8] for an overview). However, photoactivation may occur when the pigments are subjected to light from the whole visible spectrum [9]. Porphyrinoids may exert light-independent toxicity, nonetheless, which renders mandatory safeguarding a high light:dark toxicity ratio to uphold a compound's value as photosensitizer [10]. Altogether, porphyrinoids are provided with a particular interest for biotechnological purposes as colorants, antimicrobial, biocides, or as biomarkers for the physiological status of animals [11–13].

Marine invertebrates, in particular, especially those of coastal environments, known for their bright and diversified coloration, seemingly have a wider span of porphyrinoids than their vertebrate counterparts. However, as for other natural products, the true diversity, nature, and function of these pigments in marine invertebrates remains largely unknown. Some of the first works on tetrapyrrole pigments from marine animals began with bonellin, a distinctive chlorin-like greenish pigment from the females of the Echiuran Polychaeta *Bonellia viridis* that is turned into a potent biocidal when photoactivated, protecting the animal from predators and biofoulants [14,15]. In turn, *Hediste* (*Nereis*) is a common intertidal worm that is known to have a diversified and seasonally changing pattern of pigments, in large part due to porphyrinoids resulting from endogenous heme breakdown [16].

Recently, we showed that an uncanny uniformly green Polychaeta, *Eulalia viridis*, an opportunistic predator of the rocky intertidal, owes its coloration to a multiplicity of endogenous porphyrin-like pigments whose abundance and distribution changes between organs [17–20]. These pigments, which seem to almost entirely replace common biochromes such as carotenoids and melanins, were found to be chiefly stored as granules allocated within unique specialized chromocytes [19,20]. The function and bio-reactivity of these pigments is not, however, fully understood. It was hypothesized that, more than mimicking the worm's surroundings, these pigments offer important protection from sunlight and that, being porphyrinoids, they may also have toxic properties modulated by light. Additionally, *E. viridis* secretes toxins that, delivered by its copious mucus secretion, are used to immobilize and partly digest prey (mostly invertebrates, especially mussels, barnacles, and other Polychaeta) before extracting a portion of flesh via suction with its jawless but highly muscular proboscis [21]. This ability also led us to hypothesize a strong investment in chemical warfare on behalf of the species that may extend to its pigmentation. On the follow-up of our preceding research, the current work aimed at a comparative screening for potential light-mediated toxicity of the novel porphyrinoid pigments extracted from *E. viridis* skin and proboscis in view of its potential biotechnological value.
