**1. Introduction**

Hundreds of venomous ray-finned fish species (Actinopterygii) are distributed in tropical and temperate marine and freshwater environments worldwide, being commonly associated with human injuries. Although this significant biodiversity translates into a promising source of bioactive compounds, the study of fish venoms is rather underrepresented in the literature [1]. Nevertheless, the number of accidents caused by venomous fish is by no means small, and some can have serious consequences and even be fatal [2,3].

Venomous fish species belonging to the order Scorpaeniformes, which includes the families Synanceiidae (e.g., stonefish and devil stinger), Scorpaenidae (e.g., scorpionfish and lionfish), and Tetrarogidae (e.g., waspfish) [4] have been frequently associated with envenomation events [3]. Weeverfish from the family Trachinidae, order Perciformes, are also worthy of note [5] (Figure 1).

**Citation:** Campos, F.V.; Fiorotti, H.B.; Coitinho, J.B.; Figueiredo, S.G. Fish Cytolysins in All Their Complexity. *Toxins* **2021**, *13*, 877. https://doi.org/ 10.3390/toxins13120877

Received: 12 November 2021 Accepted: 4 December 2021 Published: 9 December 2021

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**Figure 1.** Phylogeny of venomous ray-finned fish families. The families included in the tree were listed based on the minimum estimate of venomous species according to [1]. Families belonging to the order Scorpaeniformes are highlighted in red, and so is the family Trachinidae from the order Perciformes. Numbers refer to NCBI taxonomic IDs. The phylogenetic tree was generated using the Interactive Tree of Life (iTOL–https://itol.embl.de; last accessed on 3 November 2021) tool [6]. The images adjacent to the phylogenetic tree depict a specimen of the scorpionfish *Scorpaena plumieri*, a member of the Scorpaenidae family, zooming in its dorsal venomous spines.

The venom apparatus of these fish is similar, and so are the biological activities associated with their venoms. The apparatus invariably consists of mucous-covered spines that can be located in different regions of the fish, being more common in the dorsal region (Figure 1), although several species have pectoral fins modified as a venom injection system. The secretory system is found in the anterolateral cavities of the spines, and may comprise well-defined glands or a set of more primitive, specialized secretory cells. Envenomation occurs when the victim threads or touches the spines, tearing the mucous sheath by mechanical pressure and causing the venom to be released into the wounds [7–9].

Excruciating pain is the most prevalent symptom of envenomation by fish, although local inflammation and a number of systemic responses, of which the most severe are cardiovascular and respiratory disturbances, are also observed [3,9,10]. Although envenomation by fish is rarely fatal to humans, under certain experimental conditions fish venoms are invariably lethal to animals [11–15].

Fish use their venoms mostly for defensive purposes, and the molecular composition of these venoms—combined with the evolution of strategies, such as mimicry and aposematism—reflects this fact [16]. Several bioactive compounds, including enzymes, such as hyaluronidases and proteases, large proteinaceous toxins, lectins, and peptides, among others, have been identified in fish venoms [17,18]. However, these venoms are somewhat unique in the sense that the biological activities underlying the major symptoms of envenomation are all induced by members of a high-molecular weighted protein family present in these venoms.

When first described, these extremely labile toxic components were referred to as "lethal factors" [12,19]. Physio-pharmacological studies on these lethal factors confirmed their multifunctional nature. These toxins reproduce the pain, inflammation, neurotoxic, myotoxic, and cardiotoxic effects induced by the crude venoms (for review, see [20,21]). In spite of this multifunctionality, the lethal activity associated with these toxins is attributed mainly to cardiac collapse and respiratory arrest [15,19,22].

In addition to the aforementioned effects, the multifunctional lethal factors also show strong hemolytic activity in vitro—a common feature in scorpaenoid fish venoms [13,19,23–28]; therefore, they are more accurately called membrane-damaging toxins or cytolysins [25]. A few cytolysins have been successfully identified in fish venoms and had some of their structural and functional features described (Table 1).


#### **Table 1.** Venomous fish and their cytolysins.

(\*) purified protein; (\*\*) semi purified lethal/hemolytic large protein; (\*\*\*) identified at the cDNA level only; (#) also known as *Trachinus vipera*.

> The wide spectrum of biological activities displayed by fish cytolysins implies that they play a major role in the envenomation process, which makes them subjects worthy of investigation. In this review, we will present a detailed overview of the biochemical and functional features of the known fish cytolysins. These toxins will be compared as to their structural and functional differences and similarities, and what has been put forward so far regarding their mechanisms of action will be discussed.
