Seasonal and ontogenetic changes in tissue size in the Antarctic brachiopod Liothyrella uva (Broderip, 1833)

Abstract

Tissue ash-free dry mass (AFDM) was investigated in relation to animal size (length) and season in Liothyrella uva (Broderip, 1833). Brachiopods were divided into digestive gland, gonads, lophophore, other internal tissues (“other tissues”) and shell. Exponents relating AFDM to length were significantly different from 3 for all tissues, except the “other tissues” and total AFDM. Males had 21 % larger gonads than females in a late summer sample (3 February) but not at the end of the winter (13 September). Females had 27% larger lophophores than males and this difference was apparent throughout the year. None of the other tissues studied were different between sexes. Data on percentage of total AFDM located in a tissue indicated that there was a change in the allocation of assimilated material from the digestive gland and lophophore to gonads in older animals. Shell also declined as a percentage of total AFDM but this was because of the way it scales to animal size. Seasonal changes were assessed by solving equations of tissue AFDM to length for large juvenile animals (20 mm length) and large adult individuals (45 mm length) at each of six dates between September 1982 and March 1983. The pattern of gonad AFDM indicated a spawning period in late September–October with a build up of material during the summer and also through the winter. There was evidence for differences between males and females, with the replenishment of female gonad possibly being delayed because of the requirements of brooding larvae. In juveniles, “other tissues” and shell were identified as sites of storage of overwintering reserves, accounting for >90% of the variation in total AFDM. For adults, the situation is complicated by reproductive events and the pattern of shell AFDM (in the caeca) is changed dramatically compared to that for juveniles. The cycle for “other tissues” in adults is important for both overwintering and reproduction while the pattern for digestive gland is similar in juvenile and reproducing animals. There was no evidence to suggest a storage function for the lophophore and changes in AFDM reflected feeding activity although slight differences between adult and juvenile cycles were probably related to spawning and brooding. Seasonal changes in total AFDM were recalculated in terms of O2 required to metabolise this amount of substratum and figures produced were within 10% of previously published winter respiration rates.