On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles

Abstract

many benthic marine invertebrates develop by means of free-livlng, dispersive larval stages. The presumed advantages of such larvae include the avoidance of competition for resources with adults, temporary reduct~on of benthic mortality while in the plankton, decreased likelihood of inbreeding in the next generation, and increased ability to withstand local extinction However, the direct~on of evolutionary change appears generally b~a s e d toward the loss of larvae in many clades, implying that larvae are somehow disadvantageous. Poss~ble disadvantages include dispersal away from favorable habitat, mismatches between larval and luvenile physiological tolerances, greater susceptib~lity to env~ronmental stresses, greater susceptibihty to predation. and vanous costs that may be associated with n~etanlorphosing in response to specific chemical cues and postponing n~etamorphosis in the absence of those cues. Understanding the forces responsible for the present distribution of larval and non-larval (aplanktonlc) development among benthic marine invertebrates, and the potential influence of human activities on the direct~on of future evolutionary change in 1-eproductlve patterns, will require a better understanding of the following issues. the role of macro-evolutionary forces in selecting for or against dispersive larvae, the relative tolerances of encapsulated embryos and free-living larvae to salinity, pollutant, and other environmental stresses; the degree to which egg masses, e g g capsules, and brood chambers protect developing embryos from environmental stresses; the relative magmtude of predation by planktonic and benthic predators on both larvae and early juveniles; the way In which larval and juvenile size affect vulnerability to predators; the extent to w h~c h encapsulation and brooding protect against predators; the amount of genetic change associated with loss of larvae from invertebrate life cycles and the time required to accomplish that change; and the degree to which continued input of larvae from other populations deters selection against dispersive larvae The prominence of larval development in modern life cycles may reflect difficulties In loslng larvae from llfe cycles more than selection for their retention.

Temporal window of receptivity and intraspecific variability in the responsiveness to metamorphosis-stimulating cues in the megalopa of a semi-terrestrial crab,Sesarma curacaoense

“…Since developmental delay confers energetic costs on competent larvae, often resulting in reduced fitness of early juveniles (see Pechenik, 1990Pechenik, , 1999Gebauer et al, 1999), phenotypes responding to lacking cues with a strong delay in metamorphosis should be favoured in habitats where suitable substrates for successful larval settlement and juvenile growth are rare or very patchy. On the other hand, physically suitable and densely populated habitats with high predation pressure (cf.…”

“…The occurrence of this response depends, first of all, on the perception of the cue; since this signal has mostly a chemical nature, a well developed larval chemoreceptive system is required (Crisp, 1974). Secondly, the larva must be capable of controlling the timing of metamorphic events, which implies phenotypic plasticity in the behavioural and developmental traits of competent larvae, but may produce also costs for later life-history stages (Pechenik, 1990(Pechenik, , 1999.…”

Nurse egg consumption and intracapsular development in the common whelk Buccinum undatum (Linnaeus 1758)

“…Embryos develop within the protective walls of a capsule that safeguards against factors such as physical stress, predation, infection and salinity changes (Thorson 1950;Pechenik 1983Pechenik , 1999Strathmann 1985;Rawlings 1995Rawlings , 1999. Periods of encapsulation vary; some species are released as veligers and undergo a planktonic stage before reaching adult life (mixed development), while others display direct development, hatching from capsules as crawling juveniles (Natarajan 1957;D'Asaro 1970;Pechenik 1979).…”

Thermal tolerance during early ontogeny in the common whelk Buccinum undatum (Linnaeus 1785): Bioenergetics, nurse egg partitioning and developmental success

“…Species undergoing non-planktonic development often develop within egg capsules which 77 protect against factors such as physical stress, predation, infection and salinity changes 78 (Pechenik, 1983(Pechenik, , 1999Rawlings, 1995Rawlings, , 1999Strathmann, 1985;Thorson, 1950). Within Thorson, 1950), but additional nutrition may also occasionally be 82 gained from intracapsular fluid (Bayne, 1968;Moran, 1999;Pechenik et al, 1984;83 Stöckmann-Bosbach, 1988) or capsule walls (Ojeda and Chaparro, 2004).…”