The Genus Codium in New Zealand
Part II: Ecology, Geographic Distribution.^*

By Vivienne Dellow, Department of Botany, Victoria University College

on the average, 3 plants were completely removed each month. The figures in Table I indicate a sharp reversal of growth relations between the months of July and September. In July erosion of thalli was proceeding at a relatively rapid rate; 54 per cent. of the original 37 plants showed a decrease in surface area. With the advent of early spring in September came a burst of fresh growth in 22 out of the remaining 26 thalli, representing an increase in size in 85 per cent. of the population. In 13 of the 22 plants the growth was secondary after a greater or lesser portion of the first-formed plants had worn away. A slackening in growth was again apparent in October.

The intense competition for space between Elminius modestus and Codium adhaerens becomes evident after mature thalli of the latter have been partly or wholly eroded, when the sharply demarcated outline of their original shape is seen to be formed by a dense crowding of barncles (Plate 52, fig. 3). In July, up to 40 young plants of Colpomenia sinuosa were counted among and just below the belt occupied chiefly by Codium. All these had disappeared by September. Towards the end of October, Laurencia thyrsifera suddenly increased in size and numbers, there being as many as 65 plants on the rock face. Those in the left-hand corner were entering into severe competition with the remaining Codium plants.

Despite the short period during which the Codium population has been closely watched, the figures at least illustrate the extreme importance of vegetative regeneration in adhaerens. It would appear that the length of life of the average plant is conditioned primarily by the severity of the habitat conditions (chiefly the mechanical effects of wave action) in both early and later stages of growth. Increase in size of the thallus is often balanced by fragmentation of small portions, but no matter how great a percentage of a thallus is removed, if the remaining portion is not entirely senescent it may give rise to a plant of dimensions equal to or even greater than those of the original plant (cf. Plates 51–52, plant no. 20).

Epiphytes

It has been argued that hairs are formed below utricle apices primarily as a protection against invasion by epiphytes. On the surface of all the New Zealand Codiums, however, there may be a dense covering of smaller plants or attached animal growths such as hydroids or bryozoans. Hairs and hair scars appear to be just as abundant on these Codium plants as on those free of invaders. The degree of epiphytism on a particular individual may prove to be correlated with habitat conditions rather than with morphology of the thallus; for example, a situation in relatively calm, shallow water or in tide pools is more likely to favour the establishment of epiphytes than one in which the mechanical force of continual wave action is tending to remove all tiny particles from the thallus surface. A possible exception is provided by Codium adhaerens var. incrassatum, where the firm exterior may be less congenial as a bed for sporelings than the more laxly compacted, outer surfaces of the other species.

Perhaps the most characteristic epiphyte on New Zealand species of Codium is Ceramium apiculatum J. Ag., which grows chiefly on C. fragile and on the northern variety of adhaerens. North Island specimens of Ceramium apiculatum are mostly small (not more than 2 cm. long); but those from more southern latitudes grow considerably larger, e.g. a plant from Timaru measured up to 7 cm. long.

The soft “megistophyse” apices of C. cranwelliae utricles provide a suitable attachment substrate for small algae such as Ulvella lens Crouan and species of Ectocarpus. The occurrence of Caulerpa hypnoides (R. Br.) C. Ag. together with Codium cranwelliae indicates that the latter may grow in a fairly deep-water community, since Caulerpa hypnoides is known to occur in the Hauraki Gulf at a depth of 24 metres. It is probably more correct to say that Codium cranwelliae is epiphytie on Caulerpa, as the rhizomes of the latter were found at the point of attachment of the Codium sphere on the under surface.

No epiphytie species were found on any of the specimens of Codium dichotomum or C. cuneatum that were examined These species, and also C. gracile, on which a crustaceous Lithophyllum is the only recorded epiphyte to date, are probably denizens of deeper water.

A remarkable instance of consortism is furnished by the small dark-green nudibranch Elysia maoria Powell (1937, Pl. 7. no. 5), which coils up in the

cracks between lobes of the thallus of C. adhaerens var. convolutum. It can scarcely be detected from the almost identical shade of the adhaerens thallus, and is particularly abundant in the more sluggish, turbid reaches of the Auckland Harbour.

B. Geographic Distribution

It may be noted that in New Zealand waters there occurs at least one representative of each of the four major subdivisions of the genus (Dellow, 1952, p. 120). This fact indicates that wide geographical separation is not one of the evolutionary factors involved in the formation of the different groups, that is, providing the groups are “natural” categories. Records from many of the outlying islands are incomplete, and large stretches of coastline on the mainland remain unexplored in respect to their algal populations. Drift records are included in spite of their lesser value, because in many cases they provide the only indication of the presence of a species in a particular locality. This applies especially to records for C. dichotomum, C. gracile, and C. cuneatum.

Since Codium is a characteristic inhabitant of warmer seas, it is not surprising to find that in New Zealand the greatest number of species is recorded from the north in the Bay of Islands. All the species save one (C. dichotomum) are listed from this locality; 37 degrees S latitude appears to be the southern limit for C. cranwelliae, while C. gracile drops out at 12 degrees. A third critical latitude is 44 degrees, south of which C. dichotomum has not been found, and also south of which C. adhaerens var. convolutum is replaced by var. incrassatum. C. fragile ranges throughout, and is evidently more widely tolerant of varying environmental conditions than the other species C. dichotomum provides a somewhat discontinuous pattern of distribution, being recorded to date from the Kermadecs, Wellington, the Sounds-Nelson district, Lyttelton, and the Chatham Islands. In this case paucity of records may be due to a lack of sufficiently detailed exploration of other parts of the coast.

Some discussion is necessary at this point concerning the proposed division of the New Zealand region into marine algal provinces (Moore, 1949, pp. 187–189) with particular reference to Codium. Those recognised are the Auckland, Intermediate, Central, Forsterian, Rossian, Chatham, and Kermadec Provinces. Miss Moore compares critical boundaries in a text figure with those defined by Cockayne (1928) for the land vegetation of New Zealand, and the faunal provinces of Powell (1937. p. 10) for the New Zealand Mollusca. Here the chief differences lie in Powell's recognition of an Aupourian Province comprising the North Auckland Peninsula, and in the lack of a faunal boundary near the centre of the North Island.

The distribution of C. cranwelliae and C. cuneatum agrees quite well with Miss Moore's Auckland Province, and the boundaries of both Auckland and Intermediate Provinces coincide reasonably with the known distribution of C. gracile. The correlation is again close for the southern C. adhaerens, which is confined to latitudes south of Timaru on the mainland, this marking the northern limit on the east coast of the Forsterian Province. recognised by both Powell and Moore (l.c.). The occurrence in the Chathams of C. adhaerens var incrassatum, together with a form which has been doubtfully assigned to C. dichotomum, is an instance of the mixed component in this region, due to mingling of the cold waters from the Antarctic Drift with the warmer waters from the East Australian Current. Var. incrassatum is found within the limits of both the