Mycale macilenta (Bowerbank, 1866)

Fig. 23

Hymeniacidon macilenta Bowerbank, 1866: 176 .

Hymeniacidon macilenta – Bowerbank 1874, pl. XXXIII fig. 7–13.

Mycale macilenta – Arndt 1935: 48, fig. 82. – Ackers 2007: 112–113.

Material examined

SWEDEN • 1 spec.; Saltbacken; 59.0832° N, 11.2242° E; 28m depth; 28 Oct. 2019; Mats Larsson leg. [MM-191029-2]; SCUBA; LAR-191029-PA290222, 0224; GenBank no.: OM415625 (28S D3-D5); voucher: GNM Porifera 1082 .

Description

The specimen was 5 cm in diameter but we only collected a 4 mm fragment. Its morphology is thin, encrusting with a single osculum at the centre in a small erect tube. The surface presented several pores regularly scattered and the aquiferous canals were partially visible. The specimen alive presented a translucent pale pinkish colour. However, given that the substrate was partially visible under the sponge, its true colour is difficult to ascertain (Fig. 23A). The specimen is white when preserved in ethanol.

Skeleton

Unfortunately, the fragment was too small to section, thus we have not observed the skeleton conformation. The megascleres are straight subtylostyles/tylostyles (Fig. 23B) measuring 193.6– 272 – 308.7± 23.28 ×2.2– 3.5 –5.7 ± 0.98 µm (N= 33). The microscleres are palmate anisochelae (Fig. 23C) with three size classes: a) 8.9– 12.2 –16.8± 1.31 µm (N =107), b) 17.4– 22.4 –24.4 ± 1.78 µm (N=25), and c) 27.1– 32.9 –40.6 ± 3.39 (N=22). Sigmas (Fig. 23D) came in two size classes: a) 18– 23.7 – 33.4 ±4.18 × 0.3– 0.8– 1.4 ±0.33 µm (N=14), and b) 67.5– 84.5 –110.0 ± 11.36× 3.2– 4.6 –6.7 ±1.02 µm (N=25). Finally, there are smooth toxas (Fig. 23E) in a single size class, measuring 56.2– 75.5 – 103.4± 13.13 (N= 11).

Remarks

This is a common temperate species found in the northeast Atlantic (NEA) and the Mediterranean Sea, the type locality being the Guernsey Islands in the English Channel. This is the first published report from Sweden, but an unpublished specimen from Sweden has previously been identified by Ole Tendal, it was found in the collections at Naturalis Biodiversity Centre, Leiden. Recording M. macilenta in Swedish waters means it can withstand colder waters than those of the English Channel. On the Swedish west coast, the average sea surface temperature (SST) varies between 2°C and 8°C in winter and spring, reaching 23°C in the summer, but with an overall average of 10°C (SMHI data). In contrast, the English Channel in the winter and spring has a SST present temperature between 9°C and 11°C reaching up to 22°C, and with an overall average of 13°C (Morris et al. 2016).

There are reports of Mycale macilenta in the NEA and Mediterranean Sea, from the Canary Islands up to the Celtic Sea. However, the spicules in this species seem to vary, especially when it comes to the chelae and sigma size classes (e.g., Pestana 2018; van Soest 2014). Nonetheless, when comparing our specimen with the description based on the re-examination of the type (van Soest 2002), we note that our specimen presents the same three size classes of chelae (sizes reported for the type: 11–15 µm, 17–24 µm, and 33–59 µm). Furthermore, we observe that the smallest size of chelae is far more common than the other two size classes. This was also a general pattern noted both in the original description (Bowerbank 1866) and later re-examination of the type (van Soest 2002). Regarding sigmas, our specimen presents the same two size classes as the holotype (65–115 µm and 21–28 µm) (van Soest 2002). Our specimen presents a variable size of toxas but we do not find toxas longer than 103 µm while those in the holotype are 60–250 µm long. Finally, our specimen does not seem to present the same colour alive than what was reported in the original description (bright red) (Bowerbank 1866). Despite slight differences between our specimen and the holotype description, namely pale colouration and slightly smaller toxas, we consider our specimen to be conspecific with the holotype of M. macilenta .