Experimental Ecology of Larvae of New Zealand Marine Bottom-Dwelling Invertebrates with Special Reference to the Trans-Tasman Migration Theory

Abstract

This is an account of an experimental ecological study of the larvae of some New Zealand marine invertebrates, especially echinoderms, to obtain knowledge of larval ecology and larval life span in order to more closely examine the problem of whether New Zealand marine bottom-dwelling invertebrates could possibly be transported across the Tasman Sea from eastern Australia to New Zealand by means of the East Australian Current according to the “Trans-Tasman Migration Theory” which is described in the work. This work describes the results of a total of 37 experiments dealing with 15 species. Eleven experiments established the tolerance of survivorship that larvae of 8 different species had for certain limiting temperatures of 5 categories. Fifteen experiments provided similar details with respect to salinity for 13 species. Five experiments dealt with the effect of temperature on the development of larvae of 4 of the species, and 3 experiments dealt with the effect of salinity on development in 3 species. Two experiments describe the effect of salinity on hatchability and subsequent larval survival of 2 species, and one experiment describes the relationship of different concentrations of “Erdschreiber” medium to survival of a species at different temperatures. The time-distance factors regarding the East Australian Current and their bearing on the Trans-Tasman Migration Theory are discussed in conjunction with experimentally determined life-spans of larvae. The number of days larvae could be maintained alive in the experimental cultures approximating conditions of temperature and salinity considered to obtain in the East Australian Current are as follows: Pectinura gracilis, 154; Ophiomyxa brevirima, 138; Evechinus chloroticus, 116; Kirk’s Ophiuroid, 96; Polydora sp., 85; Coscinasterias calamaria, 63; Astrostole scabra, 62; Echinocardium cordatum, 62; Asterina regularis, 58; Pectinura cylindrica, 40; Lysiosquilla sp., 22; Mollusc sp. A., 18; Mollusc sp. B., 15; Benhamina obliguata (?), 14; and unidentified fish larvae, 11 days. The ecological evidence of temperature, salinity, and longevity characteristics of the species as revealed by experiment suggests that a trans-Tasman migration is possible for the larvae of the species in the above list which lived longer than 45 days, excepting the one, E. chloroticus. This latter species was the only one of the group surviving longer than 45 days which showed marked stenothermal and stenohaline characteristics. Certain of the experiments showed that the older larvae, those which have developed beyond the cleavage stages and have become truly pelagic, show much wider tolerance to environmental conditions of temperature and salinity than do younger larvae. The results of one of the experiments show that a complex relationship exists in connection with food, temperature, and longevity of larvae, and that this relationship is probably significant to the Trans-Tasman Migration Theory. The generally prolonged survival of under-nourished experimental larvae suggests that marine invertebrate larvae are more adaptable to inadequate food conditions than hitherto assumed. The wide toleration species showed generally under experimental temperature and salinity conditions suggests that the natural temperature and salinity conditions of the East Australian Current do not present, in general, a barrier to larval dispersal in respect to the Trans-Tasman Migration Theory. Results showed that the larvae of species which are widely distributed geographically are markedly eury-thermal and euryhaline. The geographical distribution of the genera represented is described. A part of the work describes the design of a new type of “Multi-temperature Incubator”. The conclusions of the work support the two theories now held by palaeontologists: (1) that a west-to-east trans-Tasman migration of larvae of certain New Zealand bottom-dwelling marine invertebrates has occurred during the past, since the late-Tertiary, and that it may be occurring now; and (2) that the New Zealand climate was slightly cooler during a period sometime between the early Pliocene and the Pleistocene than it is now.