Caprella equilibra Say 1818

Caprella equilibra Say, 1818

(Figure 3–5)

Caprella equilibra Say 1818: 391–392; McCain 1968: 25, figs 12–13; McCain and Steinberg 1970: 19; Cavedini 1982: 500; Krapp-Schickel 1993: 782–783, fig. 533.

Caprella aequilibra Mayer 1882: 45, pl. 1, fig. 7; pl. 2, figs 1–11; pl. 4, figs 20–25; pl. 5, figs 16–18; Chevreaux and Fage 1925: 455, fig. 433.

St6: 300 males, 250 females, 500 juveniles; St7: 19 males, 6 females, 21 juveniles; St8: 50 males, 35 females, 100 juveniles.

The morphology of the specimens observed is in agreement with the description and figures included in Krapp-Schickel (1993) for Mediterranean specimens. This cosmopolitan species has a distinctive ventral projection between gnathopods 2, head with rectangular rostrum, gnathopod 2 basis robust and short with high and denticulate anterior carina. The largest male length found during the present study was 10 mm in length. Taking into account that Krapp-Schickel (1993) found specimens up to 20 mm, presumably our specimens are subadults. However, this variation could also be attributed to environmental variation between different geographical regions.

Three species with similar characteristics have been described from the North American Pacific coast: Caprella equilibra, Caprella mendax Mayer, 1903 and Caprella pilidigita Laubitz, 1970. All of them show the ventral projection between gnathopods 2. According to Laubitz (1970), C. pilidigita is clearly different from the others, especially based on the structure of gnathopod 2 (dactylus with hairs along inner margin, propodus three times as long as broad, basis one-third of total appendage) and the setation of antenna 1. Caprella mendax was described by Mayer (1903) but its distinction from C. equilibra is very tenuous (some authors such as Dougherty and Steinberg (1953) and Stoddart and Lowry (2003) consider C. mendax to be a synonym of C. equilibra). In fact, our specimens showed characteristics of both species as summarized in Table 2.

Widespread species such as C. equilibra can show natural morphological variation in their range or may encompass cryptic species. Disentangling species boundaries and/or cryptic species is a common issue for a great number of marine groups and especially for crustaceans (Cabezas et al. 2013, and references therein). In this sense, Cabezas et al. (2013) provided the first evidence for cryptic species in Caprellidae with a study of the cosmopolitan species Caprella penantis Leach, 1814, and supporting evidence that this species is a complex of at least four species. However, another study on the exotic Paracaprella pusilla Mayer, 1890 has demonstrated that this species exhibits great genetic uniformity throughout its distribution range (Ros et al. 2013). In the case of C. equilibra, further studies are necessary to clarify this issue. Due to this and the presumable absence of superadult males, the material examined has been assigned to C. equilibra despite a certain similarity with C. mendax, such as the length of pereonite 5 larger than pereonite 4 or antenna 1 length.

Caprella equilibra has been found on a great variety of substrates such as seaweeds, hydroids, bryozoans, sponges or ascidians from the intertidal to 3000 m deep (Krapp- Schickel 1993). In the present study, C. equilibra was more abundant on hydroids and bryozoans, scarce on gorgonians (Pacifigorgia cf. agassizii) and absent on seaweeds. Despite its worldwide distribution and ubiquity of substrates, it has been only found in Isla Isabel within our study area. Guerra-García and Tierno de Figueroa (2009) included C. equilibra as detritivorous predators based on a high percentage of detritus in the gut content (> 80%) besides hydroids (> 7%) and copepods (> 6%). More recently, Alarcón-Ortega et al. (2012) showed with material from the present study that the gut content of C. equilibra was approximately 40% of both detritus and hydroids, so that the substrate is not only used as habitat but can also be a source of food.

Type locality. South Carolina. Other records: cosmopolitan species (McCain 1968; Krapp-Schickel 1993; Guerra-García 2004).