Monotheca hyalina (Bale, 1882)

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 : 80-81

Monotheca hyalina , measurements (µm)

Remarks. There is no known type material of Monotheca hyalina ; a microslide NMV F 59052 in the Museum Victoria Bale collection labelled in Bale’s handwriting ‘ Plumularia hyalina Queenscliff , February 1881 ’ was nominated as probable syntype by Stranks (1993). I select this microslide as lectotype of Monotheca hyalina .

Monotheca hyalina is very variable in height of the stem, and slenderness and robustness of the hydrocaulus, in the presence or absence of intranodal septa and size of the hydrotheca. In some specimens, the prehydrothecal chamber is so reduced that the hydrocladial median inferior nematotheca extends one-third the length of the hydrotheca.

Specimens from warm-temperate southern Australian waters are distinguished by their slender, flexuous stems with longer internodes, while those from the cool-temperate waters have sturdier stems with shorter internodes and smaller hydrothecae. Dimensions of these two groups are compared in the above table.

Despite the considerable structural and dimensional differences between the two morphological extremes, intergradation does occur, hence M. hyalina is recognised as a single but somewhat variable species. It is often difficult to obtain a good lateral view of the hydrotheca as they tend to twist forward in microslide preparations. M. hyalina is an algal epiphyte; some specimens from Tasmania have a large apical stolonal plate to securely anchor stems in a rigorous oceanic environment.

Examination of New Zealand material and a review of descriptions of Ralph (1961b) and Vervoort and Watson (2003) suggests that the New Zealand material is not M. hyalina because: i) the hydrotheca is pitcher-shaped with a short, concave free adcauline wall, ii) there is a partial intranodal septum below the hydrotheca, iii) there is an axillar nematotheca in the apophysis of the stem, and iv) as the New Zealand specimens usually have more than one hydrotheca on the hydrocladium it is excluded from Monotheca as defined here. It is likely that the New Zealand material is an undescribed species of Plumularia . It is noteworthy that Leclère et al. (2007) in their molecular analysis demonstrate that M. hyalina from New Zealand is outside the group, thus supporting the conclusion that the New Zealand species is not M. hyalina .