How the pilidium larva feeds.

INTRODUCTION: The nemertean pilidium is a long-lived feeding larva unique to the life cycle of a single monophyletic group, the Pilidiophora, which is characterized by this innovation. That the pilidium feeds on small planktonic unicells seems clear; how it does so is unknown and not readily inferred, because it shares little morphological similarity with other planktotrophic larvae. RESULTS: Using high-speed video of trapped lab-reared pilidia of Micrura alaskensis, we documented a multi-stage feeding mechanism. First, the external ciliation of the pilidium creates a swimming and feeding current which carries suspended prey past the primary ciliated band spanning the posterior margins of the larval body. Next, the larva detects prey that pass within reach, then conducts rapid and coordinated deformations of the larval body to re-direct passing cells and surrounding water into a vestibular space between the lappets, isolated from external currents but not quite inside the larva. Once a prey cell is thus captured, internal ciliary bands arranged within this vestibule prevent prey escape. Finally, captured cells are transported by currents within a buccal funnel toward the stomach entrance. Remarkably, we observed that the prey of choice - various cultured cryptomonads - attempt to escape their fate. CONCLUSIONS: The feeding mechanism deployed by the pilidium larva coordinates local control of cilia-driven water transport with sensorimotor behavior, in a manner clearly distinct from any other well-studied larval feeding mechanisms. We hypothesize that the pilidium's feeding strategy may be adapted to counter escape responses such as those deployed by cryptomonads, and speculate that similar needs may underlie convergences among disparate planktotrophic larval forms.

Having cake and eating too: The benefits of an intermediate larval form in a brittle star Amphiodia sp. opaque (Ophiuroidea).

Most marine invertebrate larvae either obligately feed or depend on maternally provided reserves during planktonic development. A small number of species have the capacity to do both, in a mode of development known as facultative planktotrophy. We describe facultative feeding in a larva from the Oregon coast, and identify it as being an undescribed species in the genus Amphiodia, which we refer to as Amphiodia sp. opaque. We quantified the effects of food on larval and juvenile quality by culturing larvae, collected as embryos, with and without microalgal food at 15°C. The resulting juveniles were monitored under conditions of starvation. A cohort of juveniles of larvae caught as plankton was subjected to the same starvation treatment for comparison with our laboratory-reared larvae. We observed benefits to offspring that received food: larvae provided with microalgae developed more quickly and metamorphosed at higher rates. Furthermore, juveniles resulting from fed larvae were larger and were able to avoid starvation for longer after metamorphosis. Our results varied across two experimental years, suggesting that provisions provided by parents vary between populations and years. Juveniles from planktonic larvae exhibited sizes not statistically different from larvae cultured in the absence of food, but died from starvation more quickly.

Swimming Kinematics of Cyprids of the Barnacle Balanus glandula.

Larvae of barnacles typically pass through naupliar and cyprid planktonic stages before settlement and metamorphosis. As the final larval stage, cyprids swim much faster than nauplii and in turbulent fluid environments with high shears as they seek habitat. Cyprids swim with six pairs of reciprocating thoracic appendages and use two anterior antennules during settlement. Our understanding of how thoracic appendages generate movement is limited due to short stroke intervals (∼5 ms) that impede observations of the shape and trajectory of appendages. Here, we used high-speed videography to observe both free-swimming and tethered cyprids of the intertidal acorn barnacle Balanus glandula to produce a comprehensive description of thoracic appendage swimming kinematics. Cyprids used a drag-based method of swimming: their six pairs of thoracic appendages moved through metachronal power strokes and synchronous recovery strokes similar to the thoracopod motions in calanoid copepods during escape swimming. During the power stroke, plumose setae on each appendage pair spread laterally into a high surface area and high drag paddle composed of a meshwork of fused setules. This interconnected setal array collapsed into a low surface area and low drag shape during the recovery stroke. These effective swimming appendages allowed cyprids to move upward at an average speed of 1.4 cm/s (∼25 body lengths/s) with an average beat frequency of 16 beats/s, and reach an instantaneous velocity of up to 6 cm/s. Beat frequency of the thoracic appendages was significantly associated with speed, with higher beat frequencies indicating faster swimming speed. At their average speed, cyprids moved at the intermediate Reynolds number of ∼10, in which both viscous and inertial forces affected movement. Cyprids could alter swimming direction by sweeping the posterior-most appendage pair to one side and beating the remaining thoracic appendages synchronously through the power stroke with greater motion on the outside of their turn. These results greatly enhance our understanding both of cyprid motility and how small planktonic organisms can use swimming appendages with fused setule arrays to reach high swimming speeds and affect directional changes.

Gorgonocephalus eucnemis (Echinodermata: Ophiuroidea) and Bursal Ventilation.

The basket star Gorgonocephalus eucnemis is an aerobic organism highly dependent on dissolved oxygen in surrounding waters. Previous observations on the anatomy of Gorgonocephalus state that five pairs of ossicles (the radial shields and genital plates) regulate the position of the roof of the body disc and are responsible for flushing seawater into and out of the bursae, though this seems never to have been empirically tested. In the current study, rates of bursal ventilation were investigated in response to an increase in the availability of food and, separately, exposure to hypoxic levels of dissolved oxygen. When fed with suspended krill particles, basket stars increased rates of bursal ventilation, ranging from 13% to 155%, resulting in a similar increase in volume of water moved in and out of bursae. This rate remained elevated for an average of 25 minutes after active feeding ended. Bursal ventilation rates also increased significantly (~60% average increase) when basket stars were exposed to hypoxic conditions (dissolved oxygen ≤ 3.5 mg O2 L-1 = 2.45 mL O2 L-1). Some specimens exhibited a loss of coordination in hypoxic conditions. All specimens recovered and resumed a normal rate of bursal ventilation when returned to normoxic conditions. Measurements show that dissolved oxygen levels decreased from outside to inside bursae and suggest that dissolved oxygen is absorbed in bursae during bursal ventilations. Increasing rates of bursal ventilation may help meet the increased oxygen demands when feeding and may help animals endure some exposures to hypoxia.

The bilaterally asymmetrical larval form of Stomopneustes variolaris (Lamarck).

This study describes the echinopluteus and juveniles of the Indo-Pacific echinoid Stomopneustes variolaris. Late 4-armed larvae had left postoral arms that were longer and more deeply red pigmented than the right arms. Two weeks into development, the sixth pair of arms, the posterolaterals, began to form as these larvae achieved an arbacioid form. The right posterolateral arm grew long, was heavily pigmented at the tip, and was oriented perpendicularly or obliquely to the main body axis. The left posterolateral arm was relatively short, with little pigment. Two of several hundred larvae examined showed different patterns. One, with a juvenile rudiment on the right side, had arms that were a mirror image of those of typical larvae. A second larva, without a rudiment, had equal postoral arms and long, deeply pigmented posterolateral arms. These patterns suggest a developmental link between the asymmetry of the larval arms and the formation of the juvenile rudiment. Adult Stomopneustes also often showed a fixed asymmetry, with the test higher and spines shorter on the side toward interambulacrum 3 and the test lower and spines longer on the opposite side (ambulacrum I). Cleared 1- and 2-day juveniles did not show any obvious asymmetry in the location of apical plates that form from the larval spicules, so there is no evidence for a morphological link between asymmetrical larvae and adults.

Permanently Fused Setules Create Unusual Folding Fans Used for Swimming in Cyprid Larvae of Barnacles.

Many crustacean swimming appendages carry arrays of plumose setae-exoskeletal, feather-like structures of long bristles (setae) with short branches (setules) distributed along two sides. Although closely spaced, setae are not physically interconnected. Setal arrays function during swimming as drag-based leaky paddles that push the organism through water. Barnacle cyprids, the final, non-feeding larval stage, swim with six pairs of legs (thoracopods) that open and close setal arrays in alternating high-drag power strokes and low-drag recovery strokes. While studying cyprid swimming, we found that their thoracopods contained setae permanently cross-linked by fused setules. These cuticular connections would seem highly unlikely because setae are individually produced exoskeletal secretions, and the connections imply unknown processes for the production or modification of crustacean setae. We describe the morphology and function of plumose setae on cyprids of Balanus glandula and other species across the clade Cirripedia. Setules from adjacent plumose setae are seamlessly joined at their tips and occur in three distinct linkage patterns. Thoracopods lack muscles to open and close the array; interconnected setae are instead pulled apart, producing a paddle-like fan with high drag when appendages spread laterally during power strokes. Setules are spring-like, passively closing setae into tight bundles with low drag during recovery strokes. The linked setules occur in the three main clades of the Cirripedia. This cuticular arrangement is effective in swimming, may eliminate the need for muscles to close the setal array, and may represent a unique swimming structure within the Crustacea.

Heat-shock response of the upper intertidal barnacle Balanus glandula: thermal stress and acclimation.

In the intertidal zone in the Pacific Northwest, body temperatures of sessile marine organisms can reach 35 degrees C for an extended time during low tide, resulting in potential physiological stress. We used immunochemical assays to examine the effects of thermal stress on endogenous Hsp70 levels in the intertidal barnacle Balanus glandula. After thermal stress, endogenous Hsp70 levels did not increase above control levels in B. glandula exposed to 20 and 28 degrees C. In a separate experiment, endogenous Hsp70 levels were higher than control levels when B. glandula was exposed to 34 degrees C for 8.5 h. Although an induced heat-shock response was observed, levels of conjugated ubiquitin failed to indicate irreversible protein damage at temperatures up to 34 degrees C. With metabolic labeling, we examined temperature acclimation and thermally induced heat-shock proteins in B. glandula. An induced heat-shock response of proteins in the 70-kDa region (Hsp70) occurred in B. glandula above 23 degrees C. This heat-shock response was similar in molting and non-molting barnacles. Acclimation of B. glandula to relatively higher temperatures resulted in higher levels of protein synthesis in the 70-kDa region and lack of an upward shift in the induction temperature for heat-shock proteins. Our results suggest that B. glandula may be well adapted to life in the high intertidal zone but may lack the plasticity to acclimate to higher temperatures.

Offspring size effects mediate competitive interactions in a colonial marine invertebrate.

Over the past 30 years, numerous attempts to understand the relationship between offspring size and fitness have been made, and it has become clear that this critical relationship is strongly affected by environmental heterogeneity. For marine invertebrates, there has been a long-standing interest in the evolution of offspring size, but there have been very few empirical and theoretical examinations of post-metamorphic offspring size effects, and almost none have considered the effect of environmental heterogeneity on the offspring size/fitness relationship. We investigated the post-metamorphic effects of offspring size in the field for the colonial marine invertebrate Botrylloides violaceus. We also examined how the relationship between offspring size and performance was affected by three different types of intraspecific competition. We found strong and persistent effects of offspring size on survival and growth, but these effects depended on the level and type of intraspecific competition. Generally, competition strengthened the advantages of increasing maternal investment. Interestingly, we found that offspring size determined the outcome of competitive interaction: juveniles that had more maternal investment were more likely to encroach on another juvenile's territory. This suggests that mothers have the previously unrecognized potential to influence the outcome of competitive interactions in benthic marine invertebrates. We created a simple optimality model, which utilized the data generated from our field experiments, and found that increasing intraspecific competition resulted in an increase in predicted optimal size. Our results suggest that the relationship between offspring size and fitness is highly variable in the marine environment and strongly dependent on the density of conspecifics.

Macroevolutionary consequences of developmental mode in temnopleurid echinoids from the Tertiary of southern Australia.

Taxonomic revision and cladistic analysis of a morphological dataset for Australian Tertiary temnopleurids resolve the phylogeny of the group and allow the testing of a series of hypotheses about the evolution of larval development and consequences of changes in development. Australian Tertiary temnopleurids encompass all three major developmental types found in marine invertebrates (planktotrophy, lecithotrophy, and brooding). Planktotrophy is plesiomorphic for this clade, and nonplanktotrophic larval development evolved independently at least three times during the Tertiary. The change to a nonplanktotrophic mode of larval development is unidirectional with no evidence of reversal. In addition, there is no evidence of an ordered transformation series from planktotrophy through planktonic lecithotrophy to brooding. In common with previous studies of other invertebrate groups, analysis of the raw data suggests that nonplanktotrophic taxa within this clade have significantly shorter species longevities, more restricted geographic ranges and higher speciation rates than taxa with planktotrophic development. However, analysis using phylogenetically independent contrasts is unable to confirm that the stratigraphic and geographic patterns are unbiased by the phylogenetic relationships of the included taxa.

Larval development and metamorphosis of the deep-sea cidaroid urchin Cidaris blakei.

Cidaroids, one of the two major sister clades of sea urchins, first appeared during the lower Permian (ca. 270 mya) and are considered to represent the primitive form of all living echinoids. This study of Cidaris blakei, a deep-sea cidaroid urchin with planktotrophic larvae, provides a description of development from fertilization through early juvenile stages and is the first report of a deep-sea urchin reared through metamorphosis. C. blakei resembles other cidaroids in its lack of a cohesive hyaline layer, the absence of an amniotic invagination for juvenile rudiment formation, and the presence of spines with a single morphotype at metamorphosis. C. blakei differed from other cidaroids in the presence of an apical tuft, the extent of fenestration of postoral skeletal rods, the shape of juvenile spines, and an extended (14-day) lecithotrophic stage prior to development of a complete gut. The development of C. blakei, 120 days from fertilization to metamorphosis, was protracted relative to that of shallow-water cidaroids. Preliminary work on temperature tolerances suggests that C. blakei larvae would be unable to survive the warmer temperatures higher in the water column and are therefore unable to vertically migrate.

Low rates of bindin codon evolution in lecithotrophic Heliocidaris sea urchins.

Life-history variables including egg size affect the evolutionary response to sexual selection in broadcast-spawning sea urchins and other marine animals. Such responses include high or low rates of codon evolution at gamete recognition loci that encode sperm- and egg-surface peptides. Strong positive selection on such loci affects intraspecific mating success and interspecific reproductive divergence (and may play a role in speciation). Here, we analyze adaptive codon evolution in the sperm acrosomal protein bindin from a brooding sea urchin (Heliocidaris bajulus, with large eggs and nonfeeding or lecithotrophic larval development) and compare our results to previously published data for two closely related congeners. Purifying selection and low relative rates of bindin nonsynonymous substitution in H. bajulus were significantly different from selectively neutral bindin evolution in H. erythrogramma despite similar large egg size in those two species, but were similar to the background rate of nonsynonymous bindin substitution for other closely related sea urchins (including H. tuberculata, all with small egg size and feeding planktonic larval development). Bindin evolution is not driven by egg size variation among Heliocidaris species, but may be more consistent with an alternative mechanism based on the effects of high or low spatial density of conspecific mates.

Dispersal of deep-sea larvae from the intra-American seas: simulations of trajectories using ocean models.

Using data on ocean circulation with a Lagrangian larval transport model, we modeled the potential dispersal distances for seven species of bathyal invertebrates whose durations of larval life have been estimated from laboratory rearing, MOCNESS plankton sampling, spawning times, and recruitment. Species associated with methane seeps in the Gulf of Mexico and/or Barbados included the bivalve "Bathymodiolus" childressi, the gastropod Bathynerita naticoidea, the siboglinid polychaete tube worm Lamellibrachia luymesi, and the asteroid Sclerasterias tanneri. Non-seep species included the echinoids Cidaris blakei and Stylocidaris lineata from sedimented slopes in the Bahamas and the wood-dwelling sipunculan Phascolosoma turnerae, found in Barbados, the Bahamas, and the Gulf of Mexico. Durations of the planktonic larval stages ranged from 3 weeks in lecithotrophic tubeworms to more than 2 years in planktotrophic starfish. Planktotrophic sipunculan larvae from the northern Gulf of Mexico were capable of reaching the mid-Atlantic off Newfoundland, a distance of more than 3000 km, during a 7- to 14-month drifting period, but the proportion retained in the Gulf of Mexico varied significantly among years. Larvae drifting in the upper water column often had longer median dispersal distances than larvae drifting for the same amount of time below the permanent thermocline, although the shapes of the distance-frequency curves varied with depth only in the species with the longest larval trajectories. Even species drifting for >2 years did not cross the ocean in the North Atlantic Drift.

Morphological evolution of newly metamorphosed sea urchins--a phylogenetic and functional analysis.

Newly metamorphosed juvenile sea urchins are highly variable across taxa. This contribution documents and illustrates structural, functional, and phylogenetic variation among newly metamorphosed juvenile sea urchins for 31 species from 12 ordinal or familial lineages. The classic juvenile with five primary podia, 20 interambulacral spines, and variable numbers of juvenile spines is found commonly among new metamorphs across lineages, but there are many examples, which depart from this pattern and most likely reflect adaptation to settlement habitats. At metamorphosis juveniles can have 5-25 functional podia. They can have 0-65 spines, 0 or 5 sphaeridia (balance organs). They may have zero or up to eight pedicellariae. While competent larvae that delay metamorphosis may continue to develop juvenile structures, variation across species is much greater than within species and there are strong phylogenetic and functional differences among juveniles. Heterochronic changes in expression of these structures can account for differences among taxa. Based on this sample, juvenile characters such as spines, podia, and larval pedicellariae are expressed in ways that suggest they are developmental modules whose expression can be readily changed relative to one another and to the time of metamorphosis.

Linking stages of life history: How larval quality translates into juvenile performance for an intertidal barnacle (Balanus glandula).

Many marine invertebrates with complex life cycles produce planktonic larvae that experience environmental conditions different from those encountered by adults. Factors such as temperature and food, known to impact the larval period, can also affect larval size and consequently the size of newly settled juveniles. After documenting natural variation in the size of cyprids (the final larval stage) of the barnacle Balanus glandula, we experimentally manipulated temperature and food given to larvae to produce cyprids of differing sizes but within the size range of cyprids found in the field. In a set of trials in which larvae of B. glandula were raised on full or reduced rations in the laboratory and subsequently outplanted into the field as newly metamorphosed juveniles, we explored the effects of larval nutrition and size on juvenile performance. Larvae that received full rations throughout their feeding period produced larger cyprids (with more lipid and protein). These larger cyprids grew faster as juveniles and sometimes survived better in the field than juveniles from larvae that had their food ration reduced in the last feeding instar. For naturally settling barnacles brought into the laboratory within 2 days of settlement and fed, we found that initial juvenile size was a good predictor of juvenile size even after 2 weeks of growth. By manipulating food given to juveniles that were derived from larvae fed either full or reduced rations, we found that larval nutritional effects persisted in juveniles for 2-3 times the period that larvae experienced altered food rations.

Larval Form and Metamorphosis of a "Primitive" Sea Urchin, Eucidaris thouarsi (Echinodermata: Echinoidea: Cidaroida), with Implications for Developmental and Phylogenetic Studies.

The order Cidaroida (Echinodermata, Echinoidea) is universally recognized as an ancient (∼230 mya) lineage and is thought to be the sister group to the more modern euechinoids. The present study on Eucidaris thouarsi corroborates earlier findings that cidaroids have a characteristic larval form that is different from that of euechinoids and gives the first detailed description of juvenile rudiment formation and metamorphosis in a cidaroid. Larvae of E. thouarsi lack an amniotic invagination (vestibule), have many (∼-20) juvenile spines on the larval epidermis and do not histolyze the entire larval epidermis at metamorphosis. Consequently, metamorphosis of cidaroid larvae is simple when compared to that of euechinoids. In larvae of E. thouarsi, epithelial cells appear to grow over the epidermis that becomes radial nerve tissue, but this process is not visible externally and may occur by a different mechanism than that reported for euechinoids. Typical development and metamorphosis of the class Echinoidea is usually represented by the euechinoids of the family Echinidae. The present study shows that feeding larvae of echinoids have greater variability than previously recognized in developmental patterns and processes, including differences in the fates of larval epidermal tissues and the timing of production of adult spines. The growth of podia exposed on the left side of the larval body is strikingly similar between cidaroid and asteroid larvae and is an example of probable convergence of characters among the echinoderms. The absence of a vestibule in cidaroids also raises uncertainties about the homology of this structure across the phylum Echinodermata.

DEVELOPMENTAL MODE AND SPECIES GEOGRAPHIC RANGE IN REGULAR SEA URCHINS (ECHINODERMATA: ECHINOIDEA).

Among marine benthic organisms, the ability to disperse, primarily during the larval stage, is widely thought to influence the extent of species geographic range. Because related species often differ in their modes of larval development (pelagic, feeding larvae; pelagic, nonfeeding larvae; or brooded development), and these can have dramatically different planktonic intervals, the mode of development may influence geographic range. A global survey of 215 regular echinoids shows that species with pelagic, feeding larvae have significantly larger ranges than those with pelagic, nonfeeding larvae, but there is no difference in ranges between species with pelagic, nonfeeding larvae and those with brooded development. These patterns are maintained within the Cidaroida and the Temnopleuroida, which account for the great majority of species with pelagic, nonfeeding development and brooded development. This limited effect of developmental mode on geographic range is found among species occurring predominantly in waters shallower than 100 m. For species occurring deeper than 100 m, there is no significant difference in geographic range related to type of development. The relationship between developmental mode and species range was examined more closely for circa 30 species for which the developmental period was known from laboratory observations. Adjusting the developmental times to a common temperature, 20°C, using realistic values for Q10 from 2.0 to 3.6, showed a highly significant, negative correlation between egg volume and developmental time, indicating the potential for developmental mode to influence the planktonic interval. However, there was no relationship between time in the plankton, estimated from unadjusted developmental times, and extent of species geographic range. These results suggest that developmental mode may influence extent of species geographic ranges indirectly through the consequences of dispersal for gene flow or recovery from disturbance.

EFFECTS OF EGG SIZE ON POSTLARVAL PERFORMANCE: EXPERIMENTAL EVIDENCE FROM A SEA URCHIN.

Many life-history and developmental studies of marine invertebrates assume that eggs of species with nonfeeding larvae are large because they provide materials for rapid development. Using the sea urchin Heliocidaris erythrogramma which has 400 µm eggs and nonfeeding larvae, we removed an acellular, lipid-rich component from the blastula equivalent to ca. 40% of the egg volume and ca. 50% of the organic mass. Experimentally manipulated, reduced-lipid larvae survived well, developed in the usual time (3.5 d), and high percentages of the original numbers metamorphosed into anatomically normal juveniles. Control juveniles were larger at metamorphosis, grew more, and survived longer than siblings that lacked this lipid-rich material. Moderate increases in egg size in species with nonfeeding larvae may enhance postlarval performance significantly and therefore may reflect selection on early juvenile traits. The contrasts of our results and comparable experiments with feeding larvae suggests that egg size may play fundamentally different roles in species with feeding and nonfeeding larvae. The accommodation of materials reserved for the juvenile stage should be considered among hypotheses on evolutionary modification of developmental patterns.