Scurria scurra

Scurria scurra is a limpet of the family Lottiidae, described by Lesson in 1831 ^[1]. It measures between 5 and 21 mm and lives almost always on the stipes and in the holdfasts of kelp beds, above all on Lessonia nigrescens in northern and central Chile and on Durvillaea antarctica in the south ^[2]. It is a highly specialised herbivore: adults dig cavities in the stipe of their kelp and feed on its tissue ^[3]. The range recorded by the 2004 review runs from 24° S to 42° S in Chile, much shorter than the one described before, which reached north to Tuquillo, in Peru (10° S) ^[2].

The shell is conical and tall, with a subcentral apex and a size of 5 to 21 mm ^[2]. In specimens living on Lessonia nigrescens and Durvillaea antarctica the shell is thick, yellow and relatively round at the margin, with very fine concentric secondary ribs; in juveniles the apex is brown on the outside and the interior is whitish, with a narrow, well-defined margin ^[2]. Growth lines are prominent and episodic, and the shell of individuals living in stipe cavities is often covered by algae of the genus Ulva ^[2].

In the body, the mantle tentacles are similar in size and there is a gill cord; the basal plate of the third pair of lateral radular teeth is mineralised and opaque white ^[2]. The authors also detected, by molecular analysis, a transitional form with a tall conical shell, the apex in the anterior quarter and an irregular aperture, probably because it lives on mussels; the samples of this form came from valves of the mussel Perumytilus purpuratus ^[2].

Similar species

• Scurria araucana: Scurria araucana lacks a gill cord (as does Scurria ceciliana), whereas Scurria scurra has one; in addition Scurria araucana lives on intertidal rock, from the upper to the lower zone, and not on the stipes of brown algae ^[2].

According to the review by Espoz and colleagues, Scurria scurra was recorded from Caleta El Cobre (24° S) to Pirulil, on the island of Chiloé (42° S), a range of 2066 km ^[2]. It had earlier been described from Tuquillo, in Peru (10° S), to Tierra del Fuego, in Chile (56° S), about 5000 km; the authors note that this is one of the largest range reductions among these limpets, that it occurred at the northern limit and that it could be related to El Niño events such as that of 1982-1983 and to extraction of the host kelp ^[2]. The range recorded in that study does not reach Peru ^[2]. A later paper gives the same range of 24° S to 42° S ^[4].

It lives in the low intertidal zone, always where kelp beds occur ^[3]. Adults occupy the proximal parts of the stipe, in cavities they dig themselves, and juveniles shelter in cavities inside the holdfasts ^[3]. According to Espoz and colleagues, citing Muñoz and Santelices, individuals under 5 mm live in small galleries dug in the holdfasts, and larger ones in bare, open cavities in the stipe, before the first fork ^[2]. It also occurs in holdfasts of Durvillaea antarctica and Lessonia trabeculata, and rarely on other substrates such as Perumytilus purpuratus or on rock ^[3].

In northern Chile Scurria scurra lives exclusively on Lessonia berteroana, the species of the Lessonia nigrescens complex found north of 30° S, and in central and southern Chile it is associated with Lessonia spicata ^[3]. This association is explained by the limpet's specialisation for the habitat and for the resources the kelp provides ^[3]. At an experimental site in Lagunillas (30° S), limpet recruitment on stipes increased with the size of the kelp, and shell size varied together with stipe diameter ^[3].

The authors note that very little is known of its life cycle and that, like Lessonia, it has low dispersal potential, with genetically structured populations and phylogeographic breaks between 27° S and 30° S ^[3]. Its mitochondrial genome, of 18482 bp, has 13 protein-coding genes, 2 ribosomal RNA genes and 24 transfer RNA genes, and in the phylogenetic tree the species is sister to Lottia goshimai and Lottia digitalis ^[4].

Large individuals bore into the stipe as they feed on Lessonia tissue, and the authors credit the limpet with a compensatory role: according to earlier work, by pruning the oldest stipes it lowers the risk that the plant is detached and raises its life expectancy ^[3]. Espoz and colleagues note that, in the presence of the sea star Heliaster helianthus, the limpet shows no active escape response ^[2].

El Niño events cause local extinctions and recolonisation of kelp beds in northern Chile, affecting this association, and recolonisation by the limpet is not necessarily coupled with that of the kelp ^[3]. Beyond its role with the kelp, its ecological relevance is that of a herbivore representative of the communities associated with Lessonia beds ^[3]. Its temporary loss from some beds also contributes to the geographic dynamics of the kelp populations ^[3].

There is no threat category for this limpet that we can cite here, but the literature treats it as very sensitive to exploitation of the host kelp ^[3],[2]. Vega and colleagues show that, in areas open to the black kelp fishery, Scurria scurra occurs at low abundance and with a population dominated by juveniles, a situation not detected in management areas or in marine protected areas, where large adults are frequent ^[3].

At an experimental harvest site in Lagunillas (30° S), after 30 months the mean density per plant reached levels similar to the initial ones, but only 20 % of the population reached the larger size classes that were present at the start ^[3]. On the basis of these results they propose using the association as an indicator of the integrity of exploited kelp beds ^[3].