The Bivalvia: Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge

GILL AND PALP MORPHOLOGY OF TELLINA TENUIS ANDT. FABULA IN RELATION TO FEEDING

The Bivalvia — Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. GILL AN D PAL P MORPHOLOG Y O F TELLINA TENUIS AN D T. FABULA I N RELATIO N T O FEEDIN G James G. Wilson Environmental Sciences Unit, Trinity College, Dublin 2, Irish Republic ABSTRACT Internally as well as externally, there is little difference between Tellina tenuis and T.fabula. However there is a noticeable difference i n the size of the gills and palps, and it has been suggested that this difference ma y lead to different feeding habits and hence to the species distribution. In Tellina fabula,th e outer demibranch and inner demibranch are some 28% and 43% smaller respectively than those in a comparably-sized T. tenuis. As if to compensate for the smaller ctenidial size, the palps in T.fabula are almost twice the size of those in T.tenuis. It has therefore been suggested that the higher palp/gill ratio may be an adaptation to a deposit feeding existence. As regards feeding, Tellina fabulaproduce s faeces of a higher organic content and in addition has a higher index of selectivity for the fine ( 0.1). However, although there was no significant difference i n the relationship between size and palp ridge spacing measured at the edge of the palp (Table 2) there was a significant difference i n the mean inter-ridge distance between 7. tenuis (x = 165.6 urn, sd = 27.9) and T.fabula (x = 190.2 urn, sd = 37.7) (t = 2.10, p = 0.04). These findings reaffir m the importance of the relative sizes of ctenidia and palps in the treatment of food and other particles, but the palps are very mobile in normal activity and the ridge spacing can be altered considerably. The organic content of the faeces was 4.19% for Tellina tenuis and 5.34% for T.fabula. This argue s eithe r tha t the y d o a considerable amoun t o f filte r feedin g o r tha t ther e i s preferential selection of matter from the sediment. It is interesting to note that the comparative data for Nuculaturgida, which, as aprotobranch with small gills, is an obligate deposit feeder, was faeces of 5.4% organic content from sediment of 1.075% organic content (Wilson and Davis, 1985). Attempts to 'fingerprint' th e ingested material using chlorophyll/phaeopigments rati o were inconclusive. The ratio for sedimen t was 1:0. 7 an d for wate r 1:55. 8 with the fres h stomach contents of both Tellina tenuis (1:21.5) and T.fabula (1:11.0) somewhat in between. Trevallion (1971) commented on the large number of diatoms in the gut, but there was little evidence of diatoms in the guts of any specimens of either species examined over the period January - July , with the overwhelming bulk of material being sand grains and detritus. There was, however, a significant selectio n of particles by both Tellina tenuis and 7. fabula, an d Figure 1 shows clearly the much longer tail of fine particles in the faeces a s compared with the natural sediment. The coefficient o f selectivity (E) (Ivlev, 1961) for the <63 um (+40) fraction for 7. tenuis was +0.61 and for T.fabula +0.73 . Again, this can be compared with Nucula turgida, for which the same fraction had a coefficient o f selectivity (E) of+0.77 (Wilso n and Davis, 1985). Yonge (1949) made careful notes on the sizes of particles found at different stages in the ingestion process: 400 um for pseudofaeces, 320 |im in the stomach itself and 80 urn, with the majority around 40 um, in the faecal pellets of the mid-gut. These sizes led him to suggest 148 JAMES G . WILSO N selection or trituration of particles in the stomach with the postero-dorsal caecum acting as a buffer o r reservoir, but there are problems with this hypothesis. How would particles be selected and rejected by the stomach? Would it be physically possible for the crystalline style to grind down granite sand particles even allowing for the exceptionally large and strong gastric shield? From the work presented here, it appears more likely that large particles are merely relatively rare in the gut, and in both Tellina tenuis and T.fabula some 70% of particles (numerically) are less than 80 um. In other respects the data for both species correspond well with Yonge's (1949) observations of pseudofaecal particle sizes of up to 300 um, although the mean size for both species was around 150-200 um, and faecal pellet sizes of 225 x 350 urn (7. tenuis) and 300 x 250 um (7. fabula). Th e faeces themselves are easily dissociated and tend to fall apart on the slightest disturbance, due to the relatively larg e san d grain s o f whic h the y ar e composed . Thi s ca n b e regarde d a s a consequence of the habitat rather than of the mode of feeding, as similar observations have been made for Nucula turgida (Wilson and Davis, 1985). The spacing of the palp ridges (150-200 um) and the sizes of the particles rejected (150300 um) closely matched suggesting that it is the palps which determine the size of particles ingested and impose upper limit. The lower limit may well be set by the size of the ciliary gaps in the ctenidia, which may be of the order of 1 um (Moore, 1971) although M0hlenberg and Riisg&rd (1978) largely discount this simplistic 'filter' theory . It is clear that both Tellina tenuis and T.fabula can function as filter feeders, although Trevallion (1971) has suggested that filter feeding alone would not suffice to meet the energy demands of 7. tenuis. The influence of deposit on filtration ability and assimilation efficiency in bivalves has been demonstrated by several authors. M0hlenberg and Kiorboe (1981), for example, have shown a positive effect of suspended bottom material on the growth of Spisula subtruncata. Sediment may therefore be an essential item of the diet for both 7. tenuis and T.fabula under certain conditions. It is clear also that there can be no simple division between 'filter' and 'deposit' feeders, but that it is a continuum with an optimum somewhere along the gradient for each species. It may therefore be more accurate to call the Tellinacea 'opportunist ' rather than 'deposit' feeders, which can vary their diet and intake according to conditions and availability of food resources. Given then that both species ingest sediment and that it is the palps which handle the sediment, it is not surprising that the species with the larger palps, Tellina fabula,i s the one which is found in the finer, largely sublittoral sediments (Wilson, 1976a). In this situation 7. tenuis may be excluded either by its inability to handle large quantities of fine particles or by an inability to complete with T.fabula unde r such conditions. REFERENCES Ansell, A.D . 1961 . The functiona l morpholog y o f th e Britis h specie s o f Venerace a (Eulamellibranchia). Journal of the Marine Biological Association of the United Kingdom. 41: 489-515. Ansell, A.D., Barnett, P.R.O., Bodoy, A. and Masse, H. 1980. Upper temperature tolerances of some European molluscs. I. Tellina fabulaand 7. tenuis. Marine Biology. 58: 33-39. Atkins, D. 1937a. On the ciliary mechanisms and interrelationships of lamellibranchs. Part II: Sortin g device s o n th e gills . Quarterly Journal of Microscopical Science. 79 : 339-373. GILL AND PAL P MORPHOLOG Y O F TELLINA TENUIS AND T. FABULA 14 9 Atkins, D. 1937b. On the ciliary mechanisms and interrelationships of lamellibranchs. Part III: Types of lamellibranch gills and their food currents. 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