GROWTH CYCLES IN THE TEMPERATE BRYOZOAN MELICERITA CHATHAMENSIS, SOUTHERN NEW ZEALAND: A NEW TOOL FOR PALEOSEASONALITY?

Growth checks (presumably annual/winter) can be identified in the erect, bilaminate, temperate cheilostome bryozoan Melicerita chathamensisbased on narrowing of the colony width, coinciding with thickened skeleton evident in x-ray. The purpose of this study is to document variation in the size of zooecia (lengths) within and among purported annual segments and among eight, complete, modern colonies (each 2-7 segments, up to 7years old) grown in shared environmental conditions (NW of Snares Islands, southern New Zealand, 47°51.052’S, 166°42.736’E, 168 m, collected by dredge on 7 Dec 11).

Boundaries between growth segments within colonies generally do correspond (± 1 generation) to major shifts in zooid size, spiked increase and/or decrease, between successive generations. Thus, outliers and changes in zooecium morphology through growth alone may allow for recognition of significant environmental events prior to or after (annual) growth checks. All segment boundaries (growth checks based on x-rays and colony width) correspond to zooecium size change, but not all zooecium size changes in growth series correspond to obvious growth checks. This may indicate that cryptic growth checks occur, suggesting that colonies are older than originally thought. Size trends in zooecia generations within segments are common (strong nearest neighbour effect within segments), but successive zooecia generally increase in size during growth of a segment, though they may decrease or remain constant. Number of zooecia per segment and size trends within segments can be correlated to some extent among colonies for equivalent growth years. The causal factors for the changes in zoecium sizes in these specimens are not clear but are likely to be related to seasonal and/or annual water temperature variations (9 to 12 °C) or food supply (both spring and autumn algal blooms) in this region, or a combination. Ongoing isotopic study (∂¹³C and ∂¹⁸O) of these same colonies at the zooecial generation level (see Key et al., this volume) will shed light on environmental controls over zooecium size and growth check/segment boundary formation.