Phylactothecidae Stechow, 1921

Cantero, Álvaro Luis Peña, 2024, Fig. 4 in Positive association between PTN polymorphisms and schizophrenia in Northeast Chinese Han population., Zoological Studies 63 (17), pp. 141-149 : 17-19

Family Phylactothecidae Stechow, 1921 Hydrodendron sp.

( Fig. 10 )

Material examined: Stn 65, a few stem fragments up to 40 mm long, without gonothecae (NIWA 40219).

Description: Stem and branches strongly polysiphonic, with anastomoses. Only a few short hydrocladia left, with only a few hydrothecae visible. Hydrothecae on a short, usually adnate hydrophore (some with a short free part). Hydrothecae completely free, low, with a ring of desmocytes between diaphragm and aperture; abcauline wall straight, adcauline wall slightly adcaudally directed. Hydrothecal aperture circular, strongly tilted abcaudally. Secondary hydrothecae usually present, on a short hydrophore, also with ring of desmocytes, aperture circular, rim slightly everted. No nematothecae left, but their positions clearly visible. One nematophore per internode, on opposite side of hydrotheca (some internodes seem to have another on hydrothecal side). With accessory hydrothecae arising from accompanying stolons, short and completely free, with diaphragm and desmocytes.

Measurements (in µm): Internodes: length 480, diameter at distal node 310. Hydrophore: adnate part 230−260, free part 20−40. Hydrothecae: height 30−50, diameter at aperture 230−250, diameter at diaphragm 230−260. Accessory hydrothecae: height 40−50, diameter at aperture 220−230, diameter at diaphragm 210.

Remarks: Although there are no nematothecae left, their positions are clearly visible, indicating that this material belongs to Hydrodendron Hincks.

As mentioned above, there are short and completely free hydrothecae, provided with diaphragm and desmocytes, arising from accessory stolons, which are also present in Hydrodendron arboreum ( Allman, 1888) (see Fig. 2A in Peña Cantero and García Carrascosa 1995).

It is not possible to assign the present material to any species of the genus. It could belong to either H. arboreum or Hydrodendron tottoni Rees and Vervoort, 1987 , but the difference between these two species is not clearly established. According to Vervoort and Watson (2003), H. tottoni is characterised by the slanting hydrothecal rim, although the degree of tilt varies within and between colonies. They kept it separate from H. arboreum , suggesting that in Allman’s species the hydrophore appears to be considerably elongated, with the hydrothecal diaphragm a considerable distance above the node. Vervoort and Watson (2003) also noted that in H. tottoni the hydrothecal rim is not perfectly flat, but is laterally curved downward to varying degrees, and that desmocytes were not observed. In the present material, the hydrotheca is clearly tilted abcaudally and the hydrophore does not project the hydrotheca beyond the distal node, bringing it closer to H. tottoni . However, the hydrotheca is approximately straight and desmocytes are present, which would place it closer to H. arboreum . Apparently, H. arboreum and H. tottoni also differ in the size of the nematotheca, as it is larger in the former. Unfortunately, this character cannot be considered here, as nematothecae are missing in the present material.

The species described by Watson (2003) as Halecium sp. may actually be a species of Hydrodendron and be conspecific with the present material. In my opinion, it is plausible that the “gonothecae” she describes are actually nematothecae. They are “on proximal part of internode opposite a hydrophore” ( Watson 2003: 167), a typical position of nematothecae in Hydrodendron species (e.g., H. arboreum ). Watson herself described the “gonothecae” as “minute to small, base subspherical, perisarc very thin” and “arising without pedicel” ( Watson 2003: 167). These features are consistent with what could be the description of a nematotheca of Hydrodendron . Furthermore, the width of the “gonothecae” reported by Watson, around 120 µm, is similar to that of nematothecae in Hydrodendron species; for instance, Peña Cantero and García Carrascosa (1995) reported nematothecae 60−110 µm high and 50−80 µm in diameter at aperture in H. arboreum . Watson’s material also appears to have hydrothecae originating from accessory tubes; according to her description, “hydrocladium issuing from inside a hydrotheca on peripheral tube of stem or polysiphonic tube of branch” ( Watson 2003: 167).

Watson’s material is similar to the present species in having hydrophores with a short free part, but differs in that they are longer and have a larger free part (304−416 µm and 56−100 µm adnate and free part, respectively) and in that the hydrotheca, opposite or just above the node, slightly widens to a weakly everted rim. In addition, Watson’s material was collected at depths between 818 and 1158 m, significantly deeper than the present material (see below).

Ecology and distribution: Hydrodendron sp. was collected from Seamount 6, north off Macquarie Island, at depths between 119 and 125 m.

Figures

Fig. 10. Hydrodendron sp.: A, internode with hydrotheca; B, detail of primary and secondary hydrothecae; C, internode showing hydrothecae and position of missing nematothecae (arrows); D, accessory hydrotheca. Scale bars: A = 200 μm; B−D = 100 μm.

Fig. 2. Types of hydrothecae found in the collection studied and their main morphological characters. Abbreviations: aad, adnate adcauline wall; ab, abcauline wall; ad, adcauline wall; aint, ahydrothecate intermediate internode; c, cusp; d, diaphragm; des, desmocytes; fad, free adcauline wall; ht, hydrotheca; int, internode; lnt, lateral nematotheca; mnt, medial infrahydrothecal nematotheca; n, node; nt, nematotheca; ped, pedicel; pr, perisarc ridge; r, rim; ren, renovation; stn, stolon.