A NEW MODEL OF PODIAL DEPOSIT FEEDING IN THE SAND DOLLAR, MELLITA QUINQUJESPERFORATA (LESKE): THE SIEVE HYPOTHESIS CHALLENGED

Abstract

The feeding mechanism of Mellita quinquiesperforata (Leske) has been examined in detail. This sand dollar is a deposit feeder, ingesting particles mostly in the range of 100-250 µm. The particles are picked out of the substrate individually by specialized long barrel-tipped podia, which form a narrow palisade surrounding the geniculate spine fields on the oral surface. Selected food items are passed to short barrel-tipped podia, thence from podium to podium until they reach the food grooves where they are finally aggregated into mucus cords. The cords are passed to the mouth by the activity of food groove podia. At the peristome, the cord is passed between the circumoral spines by large food groove podia and steered into the mouth by five pairs of buccal podia. The lantern is powerfully muscled and has hardened teeth which crush diatoms and fracture many sand grains. For this reason, there is an apparent accumulation of fine particles (<50 µm) in the gut. Analysis of size frequencies of the material in the mucus cords and substrate indicates that no selection of fine particles occurs and, in fact, that they are virtually absent from the native sediment. An account of spine and podial morphology and distribution is included with descriptions and measurements of surface ciliary currents. It is shown that the formerly accepted sieve hypothesis of feeding cannot be entirely rejected on theoretical grounds. However, during feeding there was no evidence of the operation of any of the elements of the supposed sieve mechanism. Furthermore, the ciliary currents are not fast enough to account for the movement of most ingested material. Patterns of ciliary flow on the oral surface are not simply centripetal, but are much more complex than previously supposed.

On the significance of Bacillariophyta in feeding of clypeasteroids Scaphechinus mirabilis (Agazzis, 1863) and Echinarachmus parma (Lamarck, 1818) (Echinoidea, Clypeasteroida)

“…выбранные по размеру, весу и химии поверхности. Выбор минеральных и органических пищевых частиц и перенос их с любой точки поверхности тела осуществляет наружный орган, состоящий из множества микроскопических игл и амбулакральных ножек (Telford et al, 1985).…”

“…2012). Поскольку Bacillariophyta относятся к углеводному питанию клипестер наряду с мейобентосом, детритом из макроводорослей и морских трав (Telford et al, 1985), то при соблюдении стехиометрии углеводного и минерального питания предполагалась существенная доля Bacillariophyta в пищевом спектре S. mirabilis места обитания (Martin, Quigg, 2012). Результаты проведенного исследования в бух.…”

Evaluation of sand grain crushing in the sand dollar Mellita tenuis (Echinoidea: Echinodermata)

“…It is also suggested that at least one species that occupies quartz-rich (siliciclastic) sediments, Mellita isometra, alters the size of quartz grains during feeding by crushing grains with the teeth associated with a modified Aristotle's lantern (Telford et al 1985). If grain crushing is occurring to the extent described, then these sand dollars have a greater impact on nearshore substrates than previously thought, given the abundance and widespread distribution of many species in siliciclastic substrates (Birkeland & Chia 1971, Stanley & James 1971, Ghiold & Hoffman 1986, Mooi & Harold 1994, Takeda 2008.…”

“…Echinoid teeth have a material hardness equivalent to fluorite (Klinger & Lawrence 1985); the tensile strength of fluorite is 0.1397 kg cm -2 (Kukleva & Lodygin 1982), versus 1380 kg cm -2 for natural quartz (Bechmann & Parsons 1952). Telford et al (1985) suggested that there are inherent weaknesses (fracture planes) in quartz grains to explain how sand dollars may crush them; however, it is unlikely that such weaknesses in the grains reduce their tensile strength by 4 orders of magnitude. In addition, there are several hypotheses that suggest that the strength of a material, when measured per unit size, increases with decreasing size (Epstein 1948 and references within).…”

A new South American Miocene species of 'one-holed' sand dollar (Echinoidea: Clypeasteroida: Monophorasteridae)

“…collecting food particles according to the podial particle picking model described by Telford et al (1985). Alate morphology in which the test is strongly widened is now also known among unrelated North and South American species of Abertella and Eoscutella (Durham 1955;Parma 1985;Osborn & Ciampaglio 2010), suggesting that adaptations increasing the rate at which podial particle picking fields can encounter new food particles arose several times independently among scutellines.…”