Monotheca flexuosa (Bale, 1894)

Watson, Jeanette E., 2011, Review of the genus Monotheca (Hydrozoa: Leptolida) from Australia with description of a new species and a note on Monothecella Stechow, 1923, Memoirs of Museum Victoria 68, pp. 71-91 : 78

Monotheca flexuosa , measurements (µm)

Hydrorhiza, width 50–80

Stem internode length 344–400 diameter at node 36–44 length of apophysis (adcauline wall) 48–52

Hydrocladium length of athecate internode 116–128 width at distal node (shoulder) 50–58 length of thecate internode (measured along 164–200

base)

Hydrotheca length of abcauline wall 132–148 length of adcauline wall 136–60 diameter of margin 140–152

Gonotheca length (excluding pedicel) 560–696 maxiumum diameter 280–320

Nematotheca length of base 45–55 diameter of cup 40–50

Remarks. All microslides of Monotheca flexuosa in the Bale collection of Museum Victoria are designated syntypes by Stranks (1993). I designate NMV F58754 as lectotype of Monotheca flexuosa .

The systematic importance of the presence, absence or position of the cauline nematotheca has been much discussed by authors (e.g. Medel and Vervoort, 1995). In many southern Australian specimens of M. flexuosa , cauline nematothecae are present in the proximal one-third to mid-cauline internode, suggesting precise position is of little or no diagnostic value for identification of the species.

Stems of M. flexuosa from sheltered habitat are usually much longer, and more flexuous and silky in appearance than those from more rigorous habitats. The longer stems may bear a single row of up to 15 female gonothecae whereas shorter stems have fewer gonothecae, usually near the base of the stem. No correlation with environmental conditions or habitat could be found to explain the presence, absence or length of the terminal neck of the gonotheca.

M. flexuosa is the most abundant and widespread species of Monotheca in Australia. In southern Australia, it is fertile from late spring to autumn (November to March) when colonies occur on many invertebrate substrates (mussels, ascidians, polychaete tubes), on some flabellate red algae and the green algae Caulerpa scalpelliformis and Caulerpa remotifolia .

An unusual endoparastic association between M. flexuosa and the red alga Mychodea carnosa was reported by Watson (1973). The stolons of the hydroid penetrate the outer medulla of young Mychodea fronds, producing external hydrocauli at intervals. As the alga grows, its tissue is gradually broken down by the hydroid stolons so that eventually the alga is attached to the substrate only by the hydrorhiza of the hydroid. Hydrocauli emerging from the algal medulla differ somewhat from the normal epiphytic form, comprising one or two short cylindrical basal internodes bearing one or two large nematothecae, followed by normal hydrocauline internodes.

Type locality. Mouth of Snowy River , Victoria, Australia .

Known distribution. Temperate and tropical Australia, New Zealand, Strait of Gibraltar. The species may prove to be cosmopolitan.