Fig. 34
Sertularia pumila Linnaeus, 1758: 807.— Lindström, 1856b: 365, pl. 13, A, figs. 1–23.— Segerstedt, 1889: 18, 27.— Lönnberg, 1898: 53; 1899b: 17; 1903: 60.— Jäderholm, 1909: 96, pl. 11, figs. 1–3.
Sertularia (Dynamena) pumila.— Gislén, 1930: 346.
Dynamena pumila.— Jägerskiöld, 1971: 62.— Cornelius, 1979: 271.
Type locality. England: Sussex, Rottingdean (Cornelius 1979: 271).
Museum material. Kosterhavet, 58°49.953’N, 11°02.072’E, 26 m, 07.ix.2010, biological dredge, R / V Nereus, one colony, to 7 mm high, without gonophores, ROMIZ B3903.
FIGURE 35. Hydrallmania falcata: part of hydrocaulus with two branches and hydrothecae, ROMIZ B3900. Scale equals 0.5 mm.
Remarks. Taxonomic reviews of Dynamena pumila (Linnaeus, 1758) have been given in works including those of Cornelius (1979, 1995b), and need not be repeated here.
This well-known boreal and largely intertidal species is abundant on fucoid algae (especially Ascophyllum nodosum and Fucus spp.) and underlying rocky substrates on shores of northwestern Europe and northeastern North America. Hydroids of D. pumila are well-adapted to environmental stresses of life in the intertidal zone, and they occur in both wave-sheltered and wave-exposed areas (Rossi et al. 2000). The location of colonies on algal substrates varies depending on exposure, with those from sheltered areas occurring on mid- to upper parts of thalli and those from more exposed areas occupying areas towards the base (Cornelius 1979). On very high-energy shores, Cornelius noted that colonies may be restricted to crevices and to the undersides of overhangs. On very sheltered shores, they tend to occur in areas of maximum tidal flow. He also reported that hydrothecae of specimens from an exposed site had thicker perisarc and were notably smaller than those from an exposed site. Annulations on the stem facilitate bending, and basal articulating joints allow flexing of up to 180° in either direction (Hughes 1992). These skeletal structures facilitate feeding and minimize the risk of breakage or complete dislodging of the colony from its substrate under high-energy conditions. The ability of the stem to flex and bend also allows the colony to lie against the substrate during exposure at low tides, reducing the risk of desiccation.
At the northern limits of its range, D. pumila survives an extended period of the year as dormant tissue in stems and stolons. Colonies that I observed in southern Labrador during early summer (late June and early July, 2000) were just beginning growth. Further south in the Bay of Fundy region, eastern Canada, the species declines significantly in abundance during winter but nevertheless remains active all year (Henry 2001).
Intertidal colonies of Dynamena pumila were frequently observed in the study area during the workshop at Tjärnö in late summer 2010 on both attached and detached fucoids. Material listed above (ROMIZ B3903) was found in deeper waters but occurred on a detached algal thallus. At water temperatures of about 17º C, these hydroids were active but frequently overgrown by algae and diatoms. The species is common as well in Danish waters (Kramp 1935b) and in the Oslofjord, Norway (Christiansen 1972).
Reported distribution. West coast of Sweden.—Koster area to the Öresund (Jäderholm 1909, as Sertularia pumila).
Elsewhere.—Boreal waters of the North Atlantic from the Barents and White seas to Portugal in Europe (da Cunha 1944; Naumov 1960; Cornelius 1995b), and from southern Labrador to New Jersey in North America (Fraser 1944, as Sertularia pumila).