Aspects of the morphology and general ecology of the infaunal, deposit-feeding irregular echinoid apatopygus recens (echinodermata: echinoidea)

Abstract

Aspects of the morphology and general ecology of the infaunal, deposit-feeding, irregular echinoid Apatopygus recens were examined in two parts. Part I examines the morphology of the A. recens. A literature review was carried out regarding the general external morphology of the species. A previously unidentified pedicellariae was located and the internal morphology was examined and described (not previously carried out). A juvenile grazing theory was hypothesized, proposing that the juvenile A. recens grazes, unlike the deposit-feeding adult, and that the presence of the Aristotle's Lantern (feeding apparatus) in juvenile specimens is necessary for survival prior to its resorption upon reaching adult status. A number of related morphological features were observed, namely the presence of specialised buccal spines around the invaginate and anteriorly positioned, subpentagonal mouth of the adult, as compared with the centrally located, circular mouth of juvenile urchins, lacking both the buccal spines and the invagination of the surrounding test. Part II examined aspects of the ecology of A. recens. The coarse sediment environment and the associated species assemblage of the Wellington Harbour entrance were examined. Physical characteristics (organic matter content, median Phi and inter-quartile distance) were relatively homogenous, and although the physical characteristics determined the general presence/absence of the species within the sediment, those species present were heterogenous in distribution. The species assemblage was characteristic of the coarse sediment environment, with A. recens dominating biomass and contributing significantly towards total numbers. Burial experiments were carried out in different (Phi) grades of sediment, upon different size-classes of urchin. Burial rate was negatively correlated with urchin size, and was fastest in the sediment grade Phi 0 (1mm), followed by Phi 1 (0.5 mm). While Phi 0 constitutes the optimum sediment grade for rapid burial, it is not necessarily the optimum sediment for feeding. Larger urchins exhibited increased difficulty or inability with burial as the sediment became finer. The length-frequency of 152 urchins indicated a normal distribution, as was reflected by the 47 urchins sampled for gut content analysis. The gut contents were sieved into their component Phi values, demonstrating that the proportions of the sediment Phi values ingested were significantly different from the proportions evident in the urchin's natural habitat. Particle-size selectivity (disproportionate feeding) was thus apparent within A. recens. Gut weight increased with increasing urchin size, and particle size-preference was shown to be partly a function of urchin size.