Plumularia setacea

Plumularia setacea

Even macroscopically, P. setacea is a rather variable species. The size of a single plume in the examined material ranged from 4 to 70 mm, the maximum number of hydrothecae per hydrocladium, varied from 2 to 8 (Fig. 2). Plumes can be branched or not, hermaphroditic or gonochoristic (table 1). Even more drastically reduced colonies have been described growing on plumes of other P. setacea colonies (Millard 1973; Galea & Leclère 2007). These forms are so deviant, notably in having mostly only one hydrotheca per cladium, that they could even be classified in the genus Monotheca (but see Watson 2011). This variation is most likely environmentally induced, as in turbulent, exposed waters small colonies predominate (Gili & Hughes 1995, Rossi et al. 2000). However, this variability has been known for a long time and the mentioned characters have never been used for species discrimination. Species discrimination within the Plumularia setacea- complex relies mainly on microscopic characters, in particular of the cauline and cladial segments and their associated structures, and also the gonothecae. But dimensions vary considerably even at the microscopical level. This is even apparent for specimens from the same geographic region, e.g., the samples from the vicinity of Roscoff (France, English Channel coast) depicted in Fig. 4. The length and width of the cauline segments varies widely, paralleling plume size. An important character of the cauline segments is the number of nematothecae associated with the apophysis bearing the hydrocladium. While the vast majority of apophyses have only one nematotheca, occasionally there are two (Fig. 4D 1). In one sample (MHNG-INVE-70533), 4 out of 28 examined apophyses had 2 nematothecae instead of one only. The variability of this character in the population representing the "typical" P. seatacea is important for two reasons. On one hand, this character seems to show no or little variation in P. lagenifera of the NE Pacific (one nematotheca only, table 1), on the other hand some populations of P. setacea- like hydroids have been assigned to a distinct species based on the presence of two nematothecae per apophysis (Plumularia diploptera Totton, 1930; see discussion in Ansín Agís et al., 2001).

The hydrothecal segments show less variation in length than the cauline ones, but the lengths of the ahydrothecate segments (intersegments; Schuchert 1997) vary again rather widely (Fig. 4, second row). The number of nematothecae per intersegment is relatively constant, only rarely there are two instead of one only (table 1).

One important character that is rather invariable within the European population is the curvature of the outer wall of the hydrotheca (Fig. 4A 3 and rest of the same row). The wall is either straight or occasionally even concave (Fig. 4A 3, 4C 3). Convex walls were never seen in the present material and have also never been mentioned in the literature (for material from the NE Atlantic). The thickness of the abcauline wall is also quite invariable, but it is always more or less thin (maximal thickness seen as in Fig. 4B 3).

The size variation of the gonothecae is also remarkable, in particular of the male ones (Fig. 5). The female gonothecae are always larger than the male ones and when fully developed nearly always have an upright neck with a terminal opening. The development of the planulae takes place within the female gonotheca.

Using the criterion of "all hydrotheca with straight abcauline", then also a few samples from the NE Pacific had to be identified as P. setacea (Fig. 6). These samples appeared indistinguishable from Atlantic samples. One interesting aspect is the occasional presence of two apophysal nematothecae (Fig. 6A 1). The same variability is present in Atlantic P. setacea, but it was not found in any of the P. lagenifera samples (see below).

A re-examination of the syntypes of P. palmeri (USNM 018625, from off San Diego, California) showed that this material is indistinguishable from Atlantic P. setacea and also P. setacea from the NE Pacific (Figs 6, 7). All hydrothecae were either straight or even concave and the apophysis of the stem segments had 1 or two nematothecae. As already stated by Bedot (1914), P. palmeri must be treated as a synonym of P. setacea.