Stephen C. McKillup

The decision to feed by a scavenger in relation to the risks of predation and starvation

Recent theoretical models have considered how nutritional status and predation risk affect the decision by an individual to forage. We consider the feeding behaviour of a scavenger under risk of predation. The intertidal gastropod Nassarius dorsatus Röding was used to test the following hypotheses: (1) a damaged conspecific should be a less preferred meal than a damaged predator or another species; (2) a scavenger should be able to discriminate between a damaged conspecific in the presence and one in the absence of its predator, preferring the latter; and (3) the decision by a scavenger to feed should reflect a trade-off between nutritional status and the above preferences. Results from the laboratory and field were consistent with predictions. Two hypotheses are suggested from the work: scavengers may generally be less willing to feed on carcases of conspecifics than on those of other species, and intertidal scavengers may commonly face a relative or absolute shortage of food.

Flies that are parasitoids of a marine snail: the larviposition behaviour and life cycles of Sarcophaga megafilosia and Sarcophaga meiofilosia

We report on the larviposition behaviours and life cycles of what appear to be the first known insect parasitoids of a marine mollusc. In the field, Sarcophaga megafilosia Pape, McKillup & McKillup and Sarcophaga meiofilosia Pape, McKillup & McKillup were seen depositing larvae close to live individuals of the intertidal snail, Littoraria filosa (Sowerby), which lives on the upper leaves and branches of mangroves. Once a larva had entered the shell, by wriggling between the operculum and the spire, it began burrowing into the foot of the snail. This caused the snail to retract violently and produce large amounts of mucus that entrapped and prevented other larvae present outside from entering the shell. Each snail died within an hour of larval penetration and its shell, within which the larva and pupa developed, became glued to a leaf or branch with dried mucus. S. megafilosia only larviposited on snails with shells 10 mm or longer, while S. meiofilosia only larviposited on those with shells from 4 to <10 mm long. Both flies were reared in the laboratory and had similar life cycles with a minimum generation time of about 50 days. Adults lived for up to 7 months and overwintered as adults; there was no evidence of a larval or pupal diapause.The larviposition behaviours of these flies are contrasted with a sarcophagid parasitoid of terrestrial snails. We suggest that other sarcophagid parasitoids of littorinid snails may have been overlooked, and that visual selection of hosts by S. megafilosia and S. meiofilosia may be at least partly responsible for maintaining the shell colour polymorphism shown by L. filosa.

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.

The importance of crevices to the intertidal snail Littoraria articulata (Philippi) in a tropical mangrove forest

The intertidal gastropod, Littoraria articulata (Philippi), is common on the trunks of mangroves at Coorooman Creek, Central Queensland. Individuals of shell length less than 3 mm were only found in empty tests of the barnacle Hexaminius popeiana (Foster). Larger snails were exposed on the trunk or occasionally in crevices such as knot holes. The relationship between shell length and shell strength differed between barnacle dwelling and larger L. articulata: in both cases it was linear, but the rate of increase of strength with increasing length was significantly greater for barnacle dwelling snails than for exposed ones. In contrast, there was an exponential relationship between shell strength and length for the weaker shelled Littoraria filosa (Sowerby), which occurs higher on the tree and does not inhabit barnacles.The survival of L. articulata was affected by the presence of crevices: significantly fewer disappeared from posts with artificial crivices than from those which lacked them, and some snails testhered to posts appeared to have been eaten by fish. The importance of crevices is discussed in relation to predation pressure, growth and habitat selection by larval L. articulata.

The use of sectioned otoliths to age barramundi (Lates calcarifer) (Bloch, 1790) [Centropomidae]

The relationship between the number of rings present in sagittal otoliths and the age of barramundi, Lates calcarifer (Bloch, 1790) [Centropomidae], was investigated by examining cross sectioned otoliths of 37 tagged fish of known age between 1 and 5 years from the Johnstone River, north Queensland. Concentric rings were clearly visible in all otolith sections and were validated as annual marks. The technique was then used to estimate the age and calculate von Bertalanffy growth parameters for 70 barramundi from the Fitzroy River, central Queensland. Growth appeared to be rapid but variable in the first year; the von Bertalanffy growth parameters for length versus age were L∞=690 mm, K=0.53, t0= 0.003 years. October 1 was designated as the birth date. Whole otolith length, width and thickness were also approximated well by the von Bertalanffy equation. We suggest that examination of otoliths is a useful technique for ageing barramundi but note that further validation of the ageing method is still needed for fish older than six years.

The responses of intertidal scavengers to damaged conspecifics in the field

Previous studies have shown that some marine scavengers avoid damaged conspecifics. Intriguingly, however, none of these species have responded unanimously to damaged conspecifics in the field; some individuals of each have shown avoidance, but others have fed. The generality of avoidance of damaged conspecifics among species, and the unanimity of this behaviour within each species, were assessed in the field for seven species of intertidal scavenging snails from four sites within latitudes 16–36°S and longitudes 137–180°E. The effect of starvation on the responses of four of these species to damaged conspecifics were examined in the laboratory. In the field, some snails of each species avoided damaged conspecifics, but the remainder fed. In the laboratory, snails readily fed on damaged herbivores but would only eat damaged conspecifics after at least 4 days of starvation. It is suggested that some individuals fed on damaged conspecifics in the field because their innate response to avoid such carcasses h...

The feeding behaviour of Thalamita crenata (Portunidae, Decapoda), a cannibalistic marine scavenger

Interactions between individuals of the mangrove swimming crab Thalamita crenata attracted to carrion were studied in the field at one site in the Cocos (Keeling) Islands. T. crenata was found to be an omnivore and a cannibalistic scavenger. Although several crabs were usually attracted to a carcass placed on the sandflat, only one T. crenata ever fed at a time. Larger individuals displaced smaller ones at carcasses and crabs of carapace width 2 cm wide or less were significantly under‐represented among those attracted and feeding. The results are discussed in terms of the costs and benefits of engaging in escalated contests with cannibalistic conspecifics. It is predicted that the duration of fights and the likelihood of escalated contests will be (a) a function of the size of a bait and (b) inversely related to the nutritional status of crabs.

Fire does not stimulate flowering of the grasstree Xanthorrhoea latifolia subsp. latifolia in central Queensland

Many species of grasstree, genus Xanthorrhoea, flower profusely after fire, but little is known about responses to fire by northern Australian species. After noticing flowering in an unburnt population of Xanthorrhoea latifolia (A.Lee) Bedford subsp. latifolia in central Queensland, we tested the hypotheses that (1) X. latifolia latifolia flowers annually in the absence of fire and (2) fire has no effect on the percentage flowering. Flowering in the absence of fire consistently occurred every February–April (late summer–autumn) from 2006 to 2008. A manipulative experiment with two treatments, namely (1) burnt in spring 2009 and (2) an unburnt control, showed that the percentage flowering did not differ significantly between treatments in 2009 (before the experimental treatment was burnt) or from 2010 to 2012. There was a significant negative correlation between the percentage flowering and rainfall before each flowering season. This population may be extremely resilient to fire. Flowering occurred with or without fire, plants resprouted after fire and mortality in the burnt treatment was 2.75%. Our findings emphasise that every species within a ‘fire-dependent’ or ‘fire-tolerant’ genus cannot be assumed to respond in the same way to fire; some may require fire management at the population level.

Relationships between morphometric variables and age for captive individuals may not accurately estimate the age of free-ranging juvenile koalas (Phascolarctos cinereus)

Abstract. Several studies report methods for determining the age of juvenile Queensland koalas (Phascolarctos cinereus adustus) but these are mostly based on data from captive populations, because observing the birth of koalas in their natural habitat is extremely rare. We identified the exact date of birth for two male joeys by initially observing one within minutes and the other within hours of their birth, at St Bees Island, central Queensland. Successive measurements of head length, as these individuals matured, were used to construct a growth curve for free-ranging juveniles. When tested, only one previously published growth curve (based on body mass) was able to accurately estimate the age of the two joeys. Both methods were then tested for precision using morphometric data for other juvenile koalas in the St Bees population. The estimation of age of juvenile koalas was considerably more precise when based on head length. These results demonstrate the inaccuracy that may be inherent in growth curves derived from captive animals and also show that estimates of age based on data from individuals in a particular population or locality may not be accurate throughout the range of a species.

Intra-specific host preference for Littoraria filosa by the dipteran parasitoid Sarcophaga megafilosia: the consequences of attacking individuals outside the preferred size range

1. The marine snail, Littoraria filosa, lives on the leaves and trunks of mangroves. Populations of L. filosa are annual when its dipteran parasitoid Sarcophaga megafilosia is present, but perennial in its absence.