The effects of two parasitoids on the life history and metapopulation structure of the intertidal snail Littoraria filosa in different-sized patches of mangrove forest

Abstract

We describe differences in life history of the intertidal arboreal snail, Littoraria filosa, among patches of mangroves ranging in size from isolated trees to large stands several square kilometres in area. Recruitment of L. filosa occurred from mid spring (October) to early winter (June), recruits grew rapidly and copulating adults were found during the following September-April. Populations within large patches of forest were annuals; all or most individuals died between October-January (spring-midsummer). In contrast, those in smaller peripheral patches were more likely to survive over the summer but survival differed among patches and years. These differences in life history were caused by a parasitoid fly (genus Sarcophaga) that attacked L. filosa 10 mm and longer and was present in all large patches, but absent from, or rare, in smaller peripheral patches. Experimental introductions to isolated trees confirmed that the fly could kill L. filosa. Another sarcophagid parasitoid that attacked L. filosa from 4 to less than 10 mm long was also found in every patch. The combined effects of these parasitoids appear to determine the metapopulation structure of L. filosa. Most adults in large patches were killed by the larger fly during early summer. Summer recruits were often killed by the smaller fly within a month of settlement and when this happened effective recruitment of L. filosa was reduced to autumn. The planktotrophic larval stage of L. filosa lasts less than 1 month, so the source of autumn recruits to all patches must have been adults that survived the early summer, most of which were in small patches or on isolated trees. Consequently these "peripheral sources" are likely to be important for persistence of the metapopulation of L. filosa. The results of this study demonstrate that metapopulation structure may be determined by complex interactions and that common models cannot be assumed to apply in all habitats.

Flies that attack polymorphic snails on coloured backgrounds: selection for crypsis by a sarcophagid parasitoid of Littoraria filosa

“…While searching mangroves on the coast of Central Queensland, Australia for Littoraria filosa at sites listed below and described in McKillup & McKillup (2000), S. megafilosia and S. meiofilosia were infrequently seen larvipositing near live L. filosa . Attacked snails were watched for the next 10 minutes to determine whether a larva succeeded in penetrating the shell.…”

“…While studying the life history and metapopulation structure of L. filosa on the Central Queensland coast (McKillup & McKillup, 2000) we found that this snail was attacked and killed by two parasitoid flies, Sarcophaga megafilosia McKillup, McKillup & Pape and Sarcophaga meiofilosia McKillup, McKillup & Pape (McKillup, McKillup & Pape, 2000; Pape, McKillup & McKillup, 2000). Larviposition behaviour by both species was similar, except that S. megafilosia only attacked snails with shells ≥ 10 mm long, while S. meiofilosia only attacked those from 4 to <10 mm long.…”

“…Since these flies cause considerable mortality (McKillup & McKillup, 2000), we decided to investigate whether they might be important in maintaining the shell colour polymorphism in L. filosa by (a) selecting for crypsis, and (b) exerting apostatic selection. These snails climb to avoid submersion by the tide (Reid, 1986) and often follow the falling tide downwards, thus moving frequently between brown branches and yellow or green leaves on the same tree, and are also active when the trees are wet with rain or dew (McKillup & McKillup, unpublished observations).…”

Phylogeny ofHeteronychia: the largest lineage ofSarcophaga(Diptera: Sarcophagidae)

“…According to Keilin (), Sarcophagidae associated with snails are either saprophagous or ‘doubtful parasites’; Villeneuve () hypothesized that saprophagous or necrophagous life habits may have evolved from an original parasitic strategy. Possible true parasitoids of snails may well exist amongst the flesh flies, as in the cases exemplified by Rostand () for Sarcophaga ( Heteronychia ) filia Rondani, by Lopes () for Titanogrypa ( Sarconeiva ) fimbriata (Aldrich), by Muma (, ) and Reeves, Pape & Adler () for Lepidodexia ( Johnsonia ) elegans (Coquillet), by Stegmaier () for Helicobia morionella (Aldrich) and Peckia ( Sarcodexia ) lambens , by Lopes () for Lepidodexia ( Notochaeta ) malacophaga (Lopes), by Coupland & Baker () for Sarcophaga ( Heteronychia ) villeneuveana (Enderlein), by McKillup & McKillup (, , ) and McKillup, McKillup & Pape () for Sarcophaga ( Sarcorohdendorfia ) megafilosia Pape, McKillup & McKillup and Sarcophaga ( Sarcorohdendorfia ) meiofilosia Pape, McKillup & McKillup, by Pérez‐Moreno () for Sarcophaga ( Heteronychia ) javita (Peris, González‐Mora & Mingo), and by Sinha & Nandi (, as Liosarcophaga choudhuri [sic]) for Sarcophaga choudhuryi .…”

Species composition, comparative size and abundance of the genus Littoraria (Gastropoda: Littorinidae) from different mangrove strata along the East African coast

“…The distribution of aquatic predators and tree-climbing grapsid crabs in the mangrove have not been studied in detail, but they are believed to be most frequent at the seaward edge of the mangrove, and therefore cannot account for the prevalence of some Littoraria species at the seaward edge. Predators of terrestrial origin include birds and flies (Cook & Garbett, 1992;McKillup & McKillup, 2000), but the extent of their influence is unknown.…”