Planktonic marine invertebrate embryos and larvae lead transitory lives of great risk and grave uncertainty. Estimation of the magnitude of risk and mortality, however presents a formidable challenge. Three methods are currently in use to estimate levels of mortality among populations of marine invertebrate larvae: 1) theoretical models where rates of instantaneous mortality are correlated with other life history parameters, 2) estimation of mortality based on laboratory observations of predator-prey interactions and 3) analysis of relationships among gamete production, larval populations, and densities of post-larvae in the field. Theoretical exercises indicate that rates of larval mortality are correlated with duration of the planktonic period, and that differences in mortality rates may be offset by differences in egg size and fecundity. Results from laboratory investigations suggest that mortality rates change with larval size or age. Attempts to monitor cohorts of larvae in the plankton offe...

Natural mortality of marine invertebrate larvae

We summarize what is known of the biophysical interactions that control vertical migration and dispersal of decapod larvae, asking the following main questions: How common is vertical migration in decapod crustacean larvae? What is the vertical extent of the migrations? What are the behavioural mechanisms that control vertical migrations? How does vertical migration interact with the physics of the ocean to control the dispersal of larvae? These questions are analysed by first giving a synopsis of the physical processes that are believed to significantly affect horizontal transport, and then by describing migration patterns according to taxon, to ecological category based on the habitat of adults and larvae, and to stage within the larval series. Some kind of vertical migration has been found in larval stages of virtually all species that have been investigated, irrespective of taxonomic or ecological category. Most vertical migration schedules have a cyclic nature that is related to a major environmental cyclic factor. Tidal (ebb or flood) migration and daily (nocturnal and twilight) migration are the two types of cyclic migration that have been identified. In general, all species show some type of daily migration, with nocturnal migration being the most common, whereas tidal migrations have only been identified in species that use estuaries during part of their life cycle. Moreover, there are several examples indicating that the phasing and extent of migration both change throughout ontogeny. Reported ranges of vertical displacement vary between a few metres in estuaries and several tens of metres (sometimes more than 100 m) in shelf and oceanic waters. Vertical movements are controlled by behavioural responses to the main factors of the marine environment. The most important factors in this respect are light, pressure and gravity, but salinity, temperature, turbulence, current and other factors, also influence behaviour. Many of these factors change cyclically, and the larvae respond with cyclic behaviours. The type of response may be endogenous and regulated by an internal clock, as in the case of some tidally synchronised migrations, but in most cases it is a direct response to a change in an environmental variable, as in diel migration. The reaction of the larvae to exogenous cues depends both on the rate of change of the variable and on the absolute amount of change. A series of dispersal types, involving different spatial and temporal scales, have been identified in decapod larvae: retention of the larval series within estuaries; export from estuarine habitats, dispersal over the shelf, and reinvasion of estuaries by the last stage; hatching in shelf waters and immigration to estuaries by late larvae or postlarvae; complete development on the shelf; and hatching in shelf waters, long-range dispersal in the ocean, and return to the shelf by late stages. In all of these cases, vertical migration behaviour and changes of behaviour during the course of larval development have been related to particular physical processes, resulting in conceptual mechanisms that explain dispersal and recruitment. Most decapod larvae are capable of crossing the vertical temperature differences normally found across thermoclines in natural systems. This ability may have significant consequences for horizontal transport within shelf waters, because amplitude and phase differences of the tidal currents across the thermocline may be reflected in different trajectories of the migrating larvae.

Interactions between behaviour and physical forcing in the control of horizontal transport of decapod crustacean larvae.

1. The first zoeal stage of Callinectes sapidus shows negative geotaxis unaffected by salinity changes of 5‰; high barokinesis at pressure increments above 1 atm; an increase in swimming rate with a salinity increase; and maintenance of swimming rate as temperature drops.2. Stage IV larvae show both positive and negative geotaxis. As salinity drops, positive geotaxis prevails; as it increases negative geotaxis prevails. Stage IV larvae show a tendency to reduce swimming rate as pressure increases, as temperature drops, and as they become acclimated to higher salinities.3. Stage VII larvae show positive geotaxis and reduced swimming rate in response to increased pressure, reduced temperature, and as they are acclimated to increased salinity.4. Between hatching and the seventh (terminal) zoeal stage, passive sinking rate increases 3.2-fold, while swimming rate increases 4.4-fold.5. These responses to environmental stimuli produce a pattern of early stages moving to surface waters and later stages to deeper ...

The behavioral basis of larval recruitment in the crab callinectes sapidus rathbun: a laboratory investigation of ontogenetic changes in geotaxis and barokinesis

1. Experiments were conducted to determine the effect of salinity on phototaxis and geotaxis by Stages I and IV zoeae of the crab, Rhithropanopeus harrisii.2. Larvae were exposed to sudden salinity changes and stimulated with various intensities of 500-nm light in the horizontal plane. Although the pattern of phototaxis of larvae exposed to 40‰ was unchanged from that at 20‰ (acclimation salinity), the level of positive phototaxis to higher intensities was significantly greater and the level of negative phototaxis to low intensities significantly lower at 40‰. Exposure to low salinity sea water (5‰) generally reverses the sign of phototaxis, since a significantly higher level of negative phototaxis and lower level of positive phototaxis occurs at light intensities above 10-2 Wm-2.3. The minimum amount of salinity decrease from the acclimation salinity that induces a reversal in phototactic sign from positive to negative phototaxis at 0.19 Wm-2 ranges from 1 to 2‰, and appears to be independent of acclimat...

The effect of salinity upon phototaxis and geotaxis in a larval crustacean.

We conducted in situ observations and experiments to evaluate predation on inverte- brate larvae in near-natural plankton assemblages captured in large-volume in situ corrals. In these captured assemblages, we placed known numbers of marked larvae and determined their fate after 24 h. Recovery of marked larvae averaged 99% (±0.25%, SE), enabling a thorough and direct deter- mination of predators and predation rates. The highest predation rate observed was on bivalve veligers by the heterotrophic dinoflagellate Noctiluca scintillans (7% lost in 24 h). Gastropod veligers experienced no predation and echinoplutei experienced only rare predation. While N. scintillans preyed on bivalve veligers in some runs, most runs yielded little or no predation. These observations suggest that larvae can encounter relatively safe assemblages. To investigate low predation, corral experiments were conducted which presented prey at near-natural and unnaturally high densities and in the presence or absence of natural background plankton. Predation observed at high prey densities decreased or disappeared at near-natural prey densities. This suggests that low encounter rates may explain some of the low predation. Predation rates also decreased in the presence of nat- ural background plankton. Background plankton may occupy the predator's time and decrease opportunities for encounters with larvae, obscure larvae from detection or capture, or serve as sub- stitute food. Since predation was usually low or absent in diverse corral assemblages, we recommend confirming natural relationships and predation rates for suspected predator-prey combinations before making assumptions about predator effects. It may frequently be the case that planktonic larvae suffer little or no predation by planktonic predators.

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Low rates of predation on planktonic marine invertebrate larvae

1. Experiments were conducted to determine the phototaxis responses of each larval stage of two species of Xanthid crabs and to assess the influence of light on depth regulation.2. Two primary orientations in response to light are described.3. Secondary orientation in response to light results in positive phototaxis in all four zoea stages and the megalopa of each species.4. In response to changes in light intensity, the first zoea stage of P. herbstii demonstrated high photokinesis. Although some increase in swimming rate in response to increased light intensity was noted in larvae of L. floridanus, particularly in the first zoea stage, the differences were not statistically significant. It is suggested that larvae of L. floridanus may swim at their capacity at low light intensities, in contrast to larvae of P. herbstii.5. Light response dominates that of gravity in the first zoea stage; in the megalopa, gravity response dominates that of light in a majority of individuals.6. Light aimed down the axis of...

Stephen D. Sulkin

Papers

The influence of light in the depth regulation of crab larvae