**1. Introduction**

Topshells are marine gastropods that inhabit the rocky shores. These marine snails together with limpets and winkles are the most successful algal grazers present in the intertidal of the Northeastern Atlantic and Mediterranean Sea [1]. Topshells occupy the rocky sea shores from the supratidal to the subtidal, one of the most extreme, heterogeneous, and dynamic environments in nature that expose these organisms to different levels of thermal and hydric stresses [2, 3]. These unpredictable environmental conditions are therefore responsible for many of their peculiar morphological and biological characteristics that can be perceived as adaptations to the intertidal environment [4]. The marine snails of the genus *Phorcus* are ecologically important algae grazers that play a major role in regulating the ecological balance of their habitat and have often been used as biological indicators in evaluating the consequences of anthropogenic impact on this ecosystem [4, 5].

known to respond to climate change through alterations in biogeographic distributions following a latitudinal gradient, from warmer towards cooler regions. *Phorcus* species are bioindicators and changes in their distribution have been successfully linked to hypothesis of climate change on Northeastern Atlantic shores, particularly in species presently at their northern limits, which may be expected to move further north as the coastal waters continue

Marine Snails of the Genus *Phorcus*: Biology and Ecology of Sentinel Species for Human Impacts…

http://dx.doi.org/10.5772/intechopen.71614

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The aim of this work is to compile and review a wide array of subjects related to *Phorcus* species biology and ecology, comprising anatomy, growth, reproduction, mortality, behaviour, and ecological role and also to evaluate and discuss the consequences of anthropological impacts such as fisheries, pollution, and climate changes on these keystone species and their potential as bioindicators of the effect of human activities on coastal marine environments.

Gastropods are comprised essentially of two main parts: the shell and the body. These asymmetrical molluscs have a twisted, spirally coiled shell around its body, which protects them from biotic and abiotic factors present in their environment, and a corneous or calcareous operculum, a flat plate that rests on the upper dorsal side of the foot that acts as a supporting pad for the shell. When the snail actively moves or blocks the aperture, the body withdraws, protecting the animal from predators and preventing water leakage in exposed rocky shores

In topshells, the shell is complete and usually pyramidal, moderately large, conical to globose in shape, with rounded to angular body whorls and often with a flattened base and an interior consisting of mother-of-pearl. This structure is formed in the embryonic stage, with the secretion of protein fibres from the outer skin of the visceral mass and from the mantle, while they are free-swimming larvae and they are followed by the secretion of calcium carbonate from the same cells. Posterior to the embryonic phase, the shell continues to grow through the addition of a protein mesh and calcium carbonate mostly on its margins but also on its interior. Shell growth is not continuous and it frequently leaves different growth lines since maturity and adverse environmental conditions may cease growth. The shell offers refuge both from predators and from desiccation being impervious to gasses and liquids and resistant to crushing [12–14]. Colour patterns of the shell are usually highly variable in topshells and are mostly

The soft body consists of two compartments connected by a waist and present a dark ash colour with a greenish tint [15]. The lower compartment encompasses the muscular foot and the head. The foot is used for locomotion over the substrate, swimming, jumping, and returning the animal to an upright position when overturned. Also, it helps to detect food. The upper compartment is used for respiration, digestion, excretion, gamete production, and shell secretion. The body of these organisms comprises a head with a short snout, a pair of conical

to warm, as has happened in the last decades.

**2. Biology and ecology of topshells**

related to diet rather than to genetic control (**Figure 1**) [12].

**2.1. Anatomy**

[12, 13].

The diversity and ecological importance of the genus *Phorcus* prompted intensive research over the past years. Recently, this genus was redefined by Donald et al. [5] to include species previously under the genus *Monodonta* Lamarck, 1799, or *Osilinus* Philippi, 1847, allowing to trace the biogeographic history of this genus' origin to 40–20 Ma, prior to the closure of the Tethyan Seaway.

Intertidal invertebrates' life history traits vary inter- and intraspecifically because of genetic differences and environmental effects. Growth, reproductive strategy, and mortality depend on a complex combination of selective forces and are fundamental to understand the distribution and abundance of these species along the intertidal [6, 7]. As such, knowledge of life history traits of *Phorcus* populations provides important information required to understand how these species adapt to an ever-changing environment, whether because of human activities, such as fisheries, habitat disturbance, pollution and climate change, or natural causes.

One of the main causes of disturbance in the intertidal ecosystem is the harvest of gastropods in the rocky shores, which has occurred since prehistorical times, resulting in shifts in abundance and/or size structure of these species. Another cause of disturbance is the contamination of coastal waters, by the presence of unnatural chemicals, as a result of industrial spillage and sewage discharges among others. Gastropod molluscs are frequently used as bioindicators to assess the health status of the coast and determine the effect of marine pollution [8]. Walsh et al. [9] recorded that these sentinel species have the potential to act as a useful biomonitoring system of pollutants in the marine environment. As such, they act as pollution indicators by tracing metals, providing information required for the establishment of protective measures of the ecosystem.

*Phorcus* species are recognized as good bioindicators of water quality due to their reduced mobility, easy sampling, adequate size for tissue analysis, widespread distribution, abundance all year-round, and ability to accumulate high metal concentration in their shell and tissues, reflecting heavy metal availability in coastal waters [10, 11].

Global climate change also causes disturbance in the intertidal ecosystem that results in changes in the geographical distribution of marine gastropods. Intertidal invertebrates are known to respond to climate change through alterations in biogeographic distributions following a latitudinal gradient, from warmer towards cooler regions. *Phorcus* species are bioindicators and changes in their distribution have been successfully linked to hypothesis of climate change on Northeastern Atlantic shores, particularly in species presently at their northern limits, which may be expected to move further north as the coastal waters continue to warm, as has happened in the last decades.

The aim of this work is to compile and review a wide array of subjects related to *Phorcus* species biology and ecology, comprising anatomy, growth, reproduction, mortality, behaviour, and ecological role and also to evaluate and discuss the consequences of anthropological impacts such as fisheries, pollution, and climate changes on these keystone species and their potential as bioindicators of the effect of human activities on coastal marine environments.
