The physiological ecology and life history strategies of the nudibranch molluscs 'Adalaria proxima' (Alder & Hancock) and 'Onchidoris muricata' (Müller) (Gastropoda: Opisthobranchia)

Gastropoda;Snails--Juvenile literature;Slugs (Mollusks)--Juvenile literature

Havenhand, Jonathan Neil ; Todd, Christopher David ; Todd, Christopher David

University:University of St Andrews

Department:Biology (School of)

关键词: Gastropoda;    Snails--Juvenile literature;    Slugs (Mollusks)--Juvenile literature;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/2708/JonathanHavenhandPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Thisstudy investigated the physiological ecology, larval biologyand populationgenetics of the nudibranch molluscs Adalaria proxima(A & H)and Onchidoris muricata(Müller). These two species are annual, simultaneous hermaphrodites and are ecologicallyvery similar with the exception that A. proxima reproduces bymeans of pelagiclecithotrophic larvae whereas Omuricata has long-term planktotrophic larvae. The aim ofthe study was therefore to determine the selective pressures which resulted in the evolution ofdifferent larval types in these two species, and to ascertain the ecological and populationgenetic consequences thereof.Simpleenergy budgets comprising the major components (consumption, growth,respiration and reproduction) were constructed for laboratory populations of each species. Inboth A. proxima and O. muricata,feeding rate displayed an asymptotic increase withbodysize.Mean feeding rates ofA. proxima were greater than those of comparable O. muricataindividuals,and overall assimilation efficiency was higher in A. proxima than in O. muricata.This difference was reflected in the somatic growth rates which were correspondingly greaterin A. proxima than in O. muricata.Netgrowth efficiencies were broadly comparable betweenthe two species, however, growth ofA. proxima was approximately linearover' time whilstthat ofO. muricatadisplayed a curvilinear, almost exponential, pattern.This is interpreted asdemonstrating that some formof constraint (possibly feeding rate) operated on the growthrates ofA. proximabutnot on those ofO. muricata.Respiration rates were found to be relatively constant within given animals, butsignificant differences were found between individuals. Theallometry of respiration ratewas not constant; Omuricata demonstrated a more rapidincrease inrespiration rate withincreasing bodysize than did A. proxima.Individualvariationsinrespiration rate didnotreflect variations in the energy partitioned to either growth or reproduction.Reproductive patterns in the two species were dissimilar. A. proximalaid fewerspawn masses containing fewer, larger ova than those laid by O. muricataindividuals. Inaddition, the spawning period ofA. proxima was shorter than that ofO. muricata(60 daysand105 days respectively). Bothspecies exhibited a similar(proportional) degree of somaticcatabolism over these periods. Theconsequently more rapid"degrowth"ofA. proximaisinterpreted as the necessary utilization of an energy resource (i. e. the soma) caused byaninability to meet the energy demands of reproduction through feeding alone.Thiswas notthe case in Oanuricata individualswhich exhibited a much smaller maximum body size andwere able to feed at a sufficiently rapid rate to maintain reproduction.In the latter case, thelonger reproductive period served to maximise the total reproductive output.Several differentmeasures of"Reproductive Effort" (RE)were calculated.Thesegenerally indicated that the REofOmuricatawas considerably greater than that ofA. proxima.Althoughsuch differences have been used in the literature to classify therespective costs ofdifferent larval types or"reproductive strategies", the variability of theRE's obtained fromthe differentmeasures used here has led to the suggestion that thegeneral lackof association between REand reproductive strategy whichhas been reportedelsewhere may(partially) be attributable to the differentmeasures ofREemployed indifferentstudies.Studies of the embryonic and larvalperiod showed that the egg-to-juvenile period ofO. muricata was approximately 50% longer than that ofA. proxima.This difference wasprimarily attributable to the extended pelagic development ofO. muricatalarvae. Estimatesof the degree ofdispersal, and hence gene-flow, between populations of these species weretested by investigating the biochemical genetics of such populations. No data were availablefor O. muricata,but A. proxima populations proved to be more genetically heterogeneousthan had been expected. It is therefore concluded that actual pelagic dispersal may beconsiderably abbreviated over that expected on the basis oflarvalculturedata alone.Amodel is developed to explain the possible consequences ofdifferentegg-to-juvenile periods (whichaccrue from different larval types) onboththe ecology of thebenthic adult, and on overall energy partitioning to reproduction.However,although(probable) proximate causes and effects of the different reproductive traits exhibited byA. proxima and Oanuricataare shown, it has notbeen possible to determine the exactselective pressures which caused A. proxima to diverge from the ancestral "O. muricata"stock through the evolution of a pelagic lecithotrophic larva.