Pachycordyle michaeli

Pachycordyle michaeli (Berrill, 1948)

Fig. 11

Aselomaris michaeli Berrill, 1948: 289, figs. 1–4.

Type locality. USA: Maine, Boothbay Harbor (Berrill 1948: 289).

Museum material. Tjärnö, floating dock at Sven Lovén Centre for Marine Sciences, 58°52’33.68”N, 11°08’43.65”E, <1 m, 07.ix.2010, collected manually, along edge of dock just below surface of water, two colonies, up to 0.5 cm high, with gonophores, ROMIZ B3891.

Remarks. Schuchert (2007) suspected that Pachycordyle navis (Millard, 1959), originally described from South Africa and subsequently discovered in northern Europe, was conspecific with P. michaeli (Berrill, 1948) from boreal waters along the east coast of the United States. There appear to be no noteworthy morphological differences between the two, and they are united here under the senior synonym. Clavopsella quadranularia Thiel, 1962 from the Kiel Canal in the Baltic Sea had earlier been included as a synonym of of P. navis by Millard (1975). Cordylophora inkermanica Marfenin, 1983 from the Black Sea was added to the the synonymy of P. navis by Schuchert (2004). Overviews of the taxonomy of this shallow-water species have been given by Schuchert (2004, 2007).

Berrill (1948) described Aselomaris michaeli from material found throughout the Boothbay Harbor region, Maine, USA. Spawning in the species appeared to be induced by light. Planulae were planktonic for about one day before settling to the bottom. By one day after settling, each planula metamorphosed into a hydranth with tentacles and with a bipolar stolon. Additional details of growth and form in this hydroid were described in a subsequent paper (Berrill 1949).

Pachycordyle michaeli is known from brackish water environments in Europe, tolerating salinities down to about 8 ‰ (Schuchert 2004). Likewise, the species was observed in brackish waters (c. 17–24 ‰) of the York River estuary, Virginia, on the east coast of North America (Calder 1971). Dense colonies were found there just beneath the surface of the water on floating objects. Hydroids were active from mid-October through early June at water temperatures ranging from 3.5° C to 20° C, and gonophores were observed from mid-November through early June. The species remained dormant during summer and early autumn. In the Baltic Sea, fertile colonies were observed by Thiel (1962) in spring, and again from late summer to autumn. Gonophores were present on colonies at Tjärnö during early September 2010, at water temperatures of about 17–18° C.

In addition to its occurrence in western Sweden, P. michaeli is known from the Isefjord, Denmark (Schuchert 2004, as P. navis).

Reported distribution. West coast of Sweden.—New record.

Elsewhere.—North Atlantic from Denmark and the Baltic Sea to southern England in Europe, and from Maine to Virginia in North America (Berrill 1948, as Aselomaris michaeli; Calder 1971, as A. michaeli; Schuchert 2004, as Pachycordyle navis); South Africa (Millard 1959, as Rhizorhagium navis); Black Sea (Marfenin 1983, as Cordylophora inkermanica).

FIGURE 12. Rhizorhagium roseum: part of colony with stolon, hydrocaulus, and hydranth, ROMIZ B3909. Scale equals 0.5 mm.