Infestation of Polydora rickettsi ( Polychaeta : Spionidae ) in shells of Crepidula fecunda ( Mollusca : Calyptraeidae ) in relation to intertidal exposure at Yaldad Bay , Chiloe , Chile *

Polychaetes include free-living, tube-dwelling and substrate-burrowing species, the latter developing relations with other invertebrates which may include strict commensal associations (Martin and Britayev, 1998). A large portion of the burrowing polychaetes belong to the family Spionidae, particularly the genera Polydora and Boccardia, and typically cause great damage to the mollusc shells that they colonize SCI. MAR., 69 (1): 99-103 SCIENTIA MARINA 2005

Crepidula fecunda Gallardo, 1979 is a common sedentary filter-feeding snail occupying boulders from the intertidal to subtidal zones bordering the shores of channels and interior bays of the southern Chilean archipelagos.Intertidal individuals are exposed to physical and chemical changes regulated by exposure and immersion cycles (Sanders, 1968;1969;Slobodkin and Sanders, 1969;Newell, 1976).The burrowing polychaetes inhabiting shells of intertidal molluscs feed on the organic fraction of sedimentary particles, as well as on planktonic and meiobenthic organisms, and are considered to be both filter and sedimentary feeders (Daro and Polk, 1973;Fauchald and Jumars, 1979;Dauer, 1980;Owen, 1957).In both cases, the polychaetes are limited to feeding during immersion.
The objective of the present study was to study the infestation of Polydora rickettsi Woodwick, 1961 on Crepidula fecunda and whether or not the period of tidal exposure affecting C. fecunda influences the populations of Polydora rickettsi inhabiting their shells.

MATERIALS AND METHODS
Samples of polychaetes were from shells of C. fecunda, a mollusc inhabiting a boulder beach of Yaldad Bay (Chiloe Island, Chile; 73°43'15"W; 43°07'30"S).Five sampling stations were established from the subtidal zone (Stations 1, 2) up through the intertidal zone (stations 3, 4, 5).The stations were located at 5 different levels and each level was nine metres apart.The station's exposure at low tide was estimated using a 1:12 method (Guías Glénans, 1995), which is calculated as the difference between high and low tides, divided by 12.A total of 175 snails (35 snails per station, collected along the beach) were collected and all specimens were individually fixed in 10% formalin-seawater and stored for subsequent examination in the laboratory.
Each snail collected was photographed in the laboratory using a Pixera model # 100c, using an ATI "All in Wonder" card ®.These photographs were processed using Scion Image 3.0 ® software to estimate the shell's surface area available for polychaete settlement.The shells were then decalcified by maintaining them for 12 h in 5% HNO 3 .These preparations were neutralised for a further 12 h with a 5% of sodium sulphate solution.After intense washing with tap water, the polychaetes from each snail were collected and counted.
The polychaetes were filmed using a Sony video tape recorder, and all images were digitalized in JPG format using the ATI "All in Wonder" card for subsequent measurement using the Scion Image 3 software.Lengths of incomplete individuals were estimated based on an exponential relation between the width of the fifth setiferous segment and the total length of 25 previously calculated individuals (y = 1.1191e 4.5402x ; R 2 = 0.9107).
The level of infestation of C. fecunda was expressed as a percentage of all individuals sampled at each station which had polychaetes in their shells.
Data were checked for normality, independence and homogeneity of variances before one-way analysis of variance was conducted.A Tukey HSD a posteriori test was applied to discriminate between paired samples when significant differences were detected.The taxonomic analysis was done following the key presented by Blake (1996).
The abundance of P. rickettsi in C. fecunda showed significant differences (F 4,170 = 4.78, P = 0.001) between the stations sampled (Fig. 1).Station 3 was found to be responsible for this effect, having a significantly greater abundance than the remaining stations (Tukey HSD P< 0.05), whereas there were no significant differences between the remaining stations.
Infestation of P. rickettsi in relation to shell surface of C. fecunda did not vary significantly with regard to sampling stations.The highest polychaete infestation occurred at shell surfaces ranging between 5 and 30 cm 2 .Polychaete densities were not POLYDORID INFESTATION OF CREPIDULA SHELLS 101 significantly related to C. fecunda shell surface (F 4; 170 = 1.90, P = 0.1) (Fig. 2).The polychaete size with the greatest abundance of individuals was 3 to 6 mm at all the stations, except at station 3, which showed a greater size variety, including the longest polychaetes.The size distribution of P. rickettsi showed a significant relation (F 19; 80 = 5.04, P = 0.0001) with their abundance (Fig. 3).

DISCUSSION
Densities of P. rickettsi were significantly higher in station 3.This suggests that polychaetes prefer a given zone of the intertidal that coincides with the highest abundance of Crepidula fecunda (Segura 2001).Furthermore, beneath a depth of 3 metres, the bottom changes from boulders to mud (pers.obs.).Schleyer (1991) suggested that the greatest density of polychaetes infesting the oyster Striostrea margaritacea were found in those occurring subtidally.Our results disagree with that, as we found the highest level of infestation at a two hour tidal exposure level.
Tidal changes and wind action produced sediment resuspension in the study area, serving as a food source for suspension feeders.Polydora spp.and Crepidula dilatata (like C. fecunda, Chaparro et.al., 1998) feed on plankton and suspended food particles, and thus benefit from local resuspension of fine sediments (Daro and Polk, 1973;Fauchald and Jumars, 1979;Dauer, 1980;Owen, 1957).This resuspension of sediments and organic material probably benefits P. rickettsi, and may be limited by exposure at low tide.
Our statistical analyses show that there is no significant relationship between polychaete density and snail surface area.That result suggests that P. rickettsi does not prefer shells already heavily occupied by conspecifics, although there is some degree of aggregation in surfaces ranging between 5 and 30 cm 2 .
Occurrence of small sized P. rickettsi (3-6 mm) across all the stations, suggests that juvenile stages of this species are not limited by tidal exposure.In contrast, the distribution of larger (older) worms was restricted to station 3 only, suggesting that exposure to desiccation (among other factors) might limit late stages to a more discrete tidal range.The presence of older stages of P. rickettsi at station 3 partially differs from observations by Zajac (1991), Stephen (1978) and Wisely et al. (1979).These authors suggested that salinity and desiccation exerted a strong impact on Polydora populations and reduced their densities, especially in rainy seasons and at stations with greater exposure times.
Our results suggest that intertidal exposure alone does not limit the infestation of C. fecunda by young stages of P. rickettsi, but may limit the distribution of older stages to a fringe located in relatively low intertidal levels.

FIG. 1
FIG. 1. -Percentage of C. fecunda shells infested with P. rickettsi (open bars), and rate of infestation per individual shell (line), at the five stations.