A “Loose-lying” Form of the Brown Alga Hormosira
Botany Division, Department of Scientific and Industrial Research, Wellington
[Read before the Wellington Branch, October 26, 1948; received by the Editor, November 4, 1948: issued separately, February, 1950.]
Landward of the Picton wharf is an artificially formed tidal lagoon that receives a fresh-water stream at its upper end and discharges through a narrow gap bridged by road and railway. At an angle to the main embankment a lower filling some 150 metres long runs out into the lagoon in such a way that currents have built out a hook-shaped spit from its free end. (Plate 8, Fig. 1.) The basin so limited has no through current and receives its tides through the ever-narrowing opening between the end of the spit and the embankment. It is almost drained at low-water spring tides and there is a tidal range at Picton of 1·1–1·8 m.
The “beach” on the filling side of the basin is gently sloping. Plants of Salicornia australis covering the sides of the filling are succeeded by 4–6 metres of almost bare gravel, with the barnacle Elminius modestus, sand, mud, and some shell. Scattered in a narrow belt about low-water neap tide on 21st March, 1947, were clumps of Hormosira of peculiar form, up to 3 decimetres in diameter and 7–10 cm. deep. These were without holdfasts and quite unattached. Similar clumps were even more abundant lower down, in places meeting one another over as much as twenty square metres. Above the upper limit of the living Hormosira were various-sized fragments, dying or dead, that had apparently been carried above the critical level for survival. Here obviously was a “loose-lying” form comparable to those known in other countries in many fucoids. Visits on 9–10th October and 17–18th November, 1947, and on 12th February and 1st September, 1948, showed the community to be fairly permanent and worth describing.
In many cases the material of a whole clump belongs to one complicated branching system, although the great weight of the plants and the fragile nodes make it difficult to lift a large clump intact. The colour is striking, the upper parts, whether young or old, being pale ochre, the portions covered from direct light, again both young and old, dull olive green to blackish. Under-parts of clumps are often partly embedded in black, evil-smelling mud, and in such places segments die and decay, causing fragmentation within the clump.
The most striking morphological features are the following:
Holdfast. No sign of holdfast or attachment has been seen in any plant growing naturally within the basin.
Branching. The branching of Hormosira is normally though not strictly dichotomous. Figures 1A and 2A (Pl. 9) show the contrast between branching of attached and “loose-lying” plants; characteristics of the latter are:
Fig. 1—Aerial view near Picton showing lagoon, railway embankment leading to wharf, and filling cutting off Hormosira basin; the shingle spit that almost closes the basin outlet can scarcely be seen. Photo. White's Aviation Ltd. Fig. 2–Single, rather open, loose-lying Hormosira plant in situ, November, 1947. Decaying Ulva on channel side at right; Enteromorpha and Gracilaria and scraps of Hormosira amongst shells on left. Fig. 3—General view of densest bed of loose-lying Hormosira at very low tide on November 11, 1947; old Ulva fronds here and there on top of Hormosira.
Branching from almost every hollow segment.∗∗
Multiple branching much more common than dichotomous; a dozen buds may arise from one segment.
Branching from solid stalks as well as from hollow segments.
Incomplete division of hollow segments giving odd shapes and some flattening.
Renewed branching from mature parts, both solid and hollow.
No equivalent of the spirality seen in salt marsh fucoids (Baker, 1912).
Vegetative multiplication. With such close polychotomous branching, the decay of a hollow segment as when embedded in mud, or the breaking of any one of the solid nodes owing to the great weight of the proliferating thallus, leads to division into at least two parts, often more. Detached pieces continue to grow, at times showing some tendency to bipolarity, as when young segments develop from the broken end of a solid node (Pl. 9. Fig. 3).
Floats. The cavities in the thallus contain watery mucilage, but in normal plants a number of them contain also some gas, and float easily. No part of the loose-lying plants shows any tendency to float up in sea water and only a few of the cavities contain any gas. This lack of buoyancy combined with prolific budding and crowded branching results in the formation of plate-like clumps thicker at the edges and with the majority of the young buds round the periphery, facing upwards. Outwards and downwards (Pl. 9, Fig. 2).
Size. Hollow segments of 7 × 6 mm. to 13 × 12 mm. fall well within the ranger of size usual in Hormosira banksii; occasionally an old flattened central segment is as much as 25 mm. across. Plants, however, reach a much greater bulk than is usual. A portion of one clump, probably less than half of it, weighed more than 6000 grams when drained; its single branch systems measured up to 25 cm. or 22 segments long. The number of segments increases of course in geometrical progression from within outwards. And this portion alone had several thousand budding tips.
Sterility. Hair-pits are visible in some, but not all hollow segments. They are few in number, small, with narrow cavity, small ostiole, and few hairs arising in part at least from the bottom of the cavity (Pl. 9. Fig. 2B). They probably represent the “cryptostoma” stage in the development of the conceptacle described by Osborn (1948), No well-developed conceptacles were seen, and the plants appear to be sterile.
[Footnote] ∗ “Hollow segment” is the term used by Fritsch (1945) in contrast with “solid tissue.” As pointed out by Mollet (1880) node and internode are not very appropriate names for the parts. The swollen portion has been referred to as the receptacle (Osborn. 1948). But this term is hardly suitable where no functional conceptaeles are present.
Only odd pieces of Hormosira have been seen in the main lagoon, and in the basin it occurs only on the filling side, where it occupies the middle region, reaching neither to the upper end nor to the spit. In November, 1947, the community extended discontinuously over 120 paces and was best developed over an area 44 paces long with a maximum width of about 10 metres (Pl. 8, Fig. 3). The vertical range is about 15 cm., the deepest part of the basin being about 15 cm. lower still. At the lowest tides seen most of the bed was exposed for at least two hours (5–7 a.m. in October, 3–5 p.m. in February), but at other times the water level remained stationary for an hour while the highest Hormosira was covered by about 22 cm. of water.
Mostly above the Hormosira is a band of Enteromorpha ramulosa that grows a metre long in spring. Zostera beds occur, often between Enteromorpha and Hormosira, though Hormosira does lie on Zostera in places. Gelidium caulacantheum apparently grows only attached, e.g. to stones and mollusc shells. Gracilaria secundata f. pseudoflagellifera is common, mostly loose-lying, and is specially abundant within the curve of the hooked spit; in the lagoon outside it is attached to molluse shells, etc., in shallow channels, but is loose-lying a little higher up on mud-sand flats. Loose-lying Ulva lactuca var. latissima is abundant and in the deeper parts of the basin in spring the thalli reach a huge size, 2 metres or more square. Whereas previously only scattered Ulva plants had been seen amongst or partly on the Hormosira, in early September, 1948, much-fragmented green thalli covered most of the area previously dominated by Hormosira.
The firm gravelly bottom has a cover several inches deep of soft mud of high organic content. Salinity probably varies somewhat; one set of buoyancy tests showed it slightly less in the basin than in the bay outside. From the railway embankment and in smaller degree from the filling, there is seepage water flowing into the basin at least until the tide is dead low, but the distribution of the Hormosira shows no direct correlation with this.
Water movement must be very gentle. That clumps remain stable over rather long periods is shown by (1) size and weight attained, (2) marked colour difference from top to bottom, and (3) extensive rotting of under surfaces where the whole top is healthy; the plant photographed on 18th November (Pl. 8, Fig. 3) had not moved at all by the following February.
Hormosira banksii is a fairly common species on the rocky shores of Waitohi Bay (Picton Harbour) and detached pieces, such as are often seen in drift on the open sandy foreshore, could be carried into the basin by the flowing tide. While no portion of the loose-lying plants will float, fixed plants from outside have usually, but not always, a sufficient number of buoyant segments to hold the frond erect; when the tide is at a suitable level the top one or two segments characteristically break the surface and project freely into the air. Many pieces taken from the bay floated in the basin and would be likely to be carried away by the following tide; but there were a number of “bay” pieces that sank altogether in the basin. A few of those tested just
floated outside and just sank inside, and it is from such pieces that a free-living community might be derived.
To test whether “bay” pieces or normal fixed plants could be induced to take on the characters of the loose-lying form, some were collected from the eastern side of the bay on 10th October, 1947. Several pieces 10–15 cm. long were tied to a convenient anchorage and placed in the loose-lying bed together with a piece of rock about 10 cm. across with a fixed plant growing on it. On 17th November, 1947, they were still there, but showed little change. On 12th February, 1948, though the “anchor” was still in place, the Hormosira pieces had disappeared, probably because of the tied nodes breaking. The piece of rock had not moved. The plant on it had grown considerably, but was still easily distinguished from the loose-lying thalli and had conceptacles on almost every segment; one typical branch had about forty growing points where one of equal length from an adjacent loose-lying plant had about a thousand.
Nearly seven months later, on 1st September, 1948, when not much Hormosira was visible amongst the abundant Ulva, the plant on the experimental rock was conspicuous because the gas-filled segments held the fronds erect above the Ulva. It was now perhaps five times its original bulk, and bore well-developed male conceptacles on all but the youngest and oldest segments; on branches picked (Pl. 9, Fig. 4) the antheridia were extruded in normal fashion. The number of main branches had not increased since February, but they had grown in length and bushiness. Maximum lengths were in October about 6 cm. (9–10 segments), in February 12.5 cm. (12 segments), in September 15 cm. (15–16 segments). The numerous short branches from the disc holdfast did not appear to be actively growing, which is perhaps not surprising since they would be covered with either mud or Ulva or both.* The plant had in no way taken on characters of the loose-lying form, although two plants of the latter grew within a few decimetres, one of them anchored down by the byssus threads of a mussel (Mytilus canaliculus).
In February, 1948, loose Hormosira plants were taken out into the bay and there pegged down at about their original tide level. It was hardly to be expected that the easily broken nodes would hold the branches together in the more actively moving water and the whole had disappeared by the next September.
Less healthy loose-lying Hormosira plants had been collected earlier (28.4.44) in small numbers in Shakespeare Bay, immediately west of Waitohi Bay, where there is considerable pollution from meat works. An unconfirmed observation at Ngunguru, near Whangarei, suggests that similar forms may be found in suitable places entangled amongst pneumatophores of mangroves. Mr. J. A. Carnahan has reported (private communication, 4.9.47) that on Rangitoto Island “massive free-living forms of Hormosira banksii occur under certain peculiar environmental conditions; the internodes may be very large and there is repeated branching in three dimensions.” This locality was briefly
[Footnote] * This plant in April, 1949, still had fertile portions 15cm. long; shorter more freely branched parts up to 8 cm. long were mostly not obviously fertile.
visited under the guidance of Professor V. J. Chapman; features common to this and the Picton basin include reduced water movement and marked seepage from spaces in adjacent rock walls. It was not determined whether the mud at Rangitoto was highly organic and the large globular segments there could not be matched at Picton.
At Denham Bay. Raoul Island, Kermadec Group, after a southwest gale, Mr. J. H. Sorensen collected from drift (3.8.44) a Hormosira specimen which, though very fertile, had young buds both on old hollow segments and on the broken end of the lowest solid tissue, the latter showing that it had certainly begun to grow after being detached from its parent plant.
In a salt-marsh form of Fucus ceranoides, Skrine et al. (1932) describe proliferations arising from hair-pits. In the Picton Hormosira the conceptacles are represented by a sort of hair-pit, and anatomical investigations may prove that proliferations sometimes arise from such places. That this is by no means always the case is shown by the quite frequent origin of a new segment from the solid tissue where no equivalent of the hair-pit occurs. Development of new apical meristem here should afford interesting comparisons with its origin in the germling.
Levring (1940), writing of loose-lying forms of Fucus vesiculosus permanently submerged off the south Swedish coast, points out that they occur especially on muddy bottoms (Schlickboden) in still inlets. He suggests that, besides the influence of low salt content, the development of both the vegetative parts and the sex organs might be affected by the oxygen content of the water, which can sink to almost zero over muddy bottoms, and by a certain poisoning effect of decaying matter. Both these factors might be involved at Picton. If the shortage of oxygen or any other factor retards respiration without having an equally depressing effect on assimilation, increased growth could be expected.
The transplanted Hormosira was able to hold its fronds erect and above the mud surface, and thus probably the upper growing tips at least avoided the most unfavourable situation. The experiment has shown that conditions in the basin are quite favourable to normal growth of fixed Hormosira thalli with buoyant fronds. Using plants fixed to rocks and either pruning off all long fronds or choosing young specimens with growing tips only at about mud surface level, it might still be possible to develop the characters of the loose-lying form in fixed plants.
Another loose-lying brown algal community in New Zealand is that dominated by Macrocystis pyrifera, permanently submerged in quiet arms of Paterson Inlet (Moore, 1943) and Pegasus Harbour (Lindauer. 1947), Stewart Island. This laminarian agrees with the aberrant. Hormosira in lack of holdfast, excessive branching, lack of buoyancy, vegetative multiplication instead of normal reproduction, and in the muddy substratum on which it lies.
Another laminarian Ecklonia brevipes, apparently loose-lying or vaguely fixed in loose sand, has excessive haptera formation with reversed polarity, and is not entirely sterile, though fertile specimens are said to be “somewhat rare” (Lindauer, 1945). This species is recorded from Bay of Islands and Stewart Island “in somewhat
shallow water, gregarious on a sandy substratum” (Lindauer, 1947). It was found in drift and apparently unattached in dredged material from 7 fathoms in Breaksea Sound and Chalky Inlet by Dr H. H. Allan (February, 1946).
The Hormosira here recorded provides New Zealand with an example of a free-living fucoid, comparable to the unattached forms of Fucus serratus, Ascophyllum nodosum var. mackaii, and Pelvetia caniculata described from the west coast of Scotland by Naylor (1928) in one of the few papers dealing with such plants in intertidal as distinct from both salt-marsh ‘and sublittoral habitats. For convenience the Picton plant may be referred to as ecad libera and a brief description follows.
Hormosira banksii (Turner) Decaisne ecad libera
Massive plants not attached in any way, but often remaining stationary so long that the healthy upper branches can be distinguished from the more or less decaying lower branches. Multiple branching from almost every hollow segment, sometimes also from solid portions and from mature parts. No portion floats in sea water. Vegetative multiplication by breaking up of clumps. Hair-pits develop sparingly, but fertile conceptacles not seen. Hab., Picton, in shallow almost land-locked portion of tidal lagoon. Specimens in herbarium Botany Division, D.S.I.R.
A loose-lying form of Hormosira, here referred to H. banksii (Turner) Decaisne ecad libera, grows in an almost land-locked tidal basin at Picton. Lack of holdfast, excessive branching and great size of individuals are associated with sterility and vegetative multiplication. No part floats in sea water. Habitat and accompanying species are briefly described. Normal Hormosira plants attached to a stone experimentally introduced into the basin remained fertile with buoyant segments for more than a year.
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