X-ray diffraction study of the first larval shell of Ostrea edulis

X-ray powder diffraction was used to study the calcification of the first larval shell of Ostrea edulis (sampled in Limski kanal, Istria, Adriatic Sea in April 1986) from the trochophore stage to the veliger larvae (prodissoconch I), and development of the latter up to several days postfertilization (prodissoconch II). In the first stage, only the amorphous component is present (periostracum and organic matrix). The beginning of shell formation is manifested by the appearance of calcite (up to 1–4% of the total vol.) and then aragonite (2 to 7%). In a later stage of the veliger larvae the fraction of calcite decreases, as well as the fraction of the amorphous component, while the fraction of aragonite rapidly increases. In the prodissoconch II stage, aragonite is dominant, with a very small amount of amorphous component and traces of calcite. In contrast, the valves of the adult O. edulis are composed mainly of calcite, with traces of aragonite.     

Role of biochemical energy reserves in the metamorphosis and early juvenile development of the oyster Ostrea chilensis

Metamorphosis in the Chilean oyster Ostrea chilensis was complete 36 h after release of the larvae, when 100% of the individuals showed edge growth of the dissoconch. The size of the larval shell did not change during metamorphosis, although the total dry weight of the larva decreased considerably. During this period, when the gill ciliature was undeveloped and the oyster therefore unable to feed, energy demands were met by biochemical reserves retained from the larval phase. Proteins contributed the largest quantity of energy to the metamorphosing oyster, 69.3% of the total expended, whereas lipids supplied 24.3% and carbohydrates only 6.4%. The process of metamorphosis consumed 64.5% of the energy reserves held by the pediveliger at the time of release. When metamorphosis was complete, growth began and tissue reserves were replenished, protein and carbohydrate accumulating rapidly early in the juvenile stage. Received: 26 December 1997 / Accepted: 8 July 1998     

Biomineralization markers during early shell formation in the European abalone <Emphasis Type="Italic">Haliotis tuberculata</Emphasis>, Linnaeus

Larval shell formation was investigated in the European abalone Haliotis tuberculata. Stages of mineralization as well as enzymatic and endocrine biomarkers were monitored throughout larval development, from hatching to post-larval stages. Polarized light microscopy and infrared spectroscopy analyses revealed the presence of crystallized calcium carbonate arranged in aragonite polymorphs from the late trochophore stage. A correlation between the main steps of shell formation and enzymatic activities of alkaline phosphatase and carbonic anhydrase was seen. The variations of these biologic activities were related to the onset of mineralization, the rapid shell growth, and the switch from larval to juvenile shell following metamorphosis. Furthermore, a strong increase in the level of calcitonin gene-related molecules was measured in post-larvae, suggesting that endocrine control takes place after metamorphosis. The changes measured for the three biomineralization markers together with mineralogical analysis allowed us to correlate physiologic mechanisms with early steps of abalone shell formation.     

A morphological and structural study of the larval shell from the abalone <Emphasis Type="Italic">Haliotis tuberculata</Emphasis>

The larval shell of the marine gastropod Haliotis tuberculata was investigated by polarised light microscopy, scanning electron microscopy, Raman microspectroscopy and infra-red spectroscopy. Trochophore and veliger larval sections were used for histological examination of the growing shell and each larval stage was related to the shell development and the appearance of calcified formations. We determined the stage of initial calcification by specific staining combined with polarised light examination. The shell of 30-h-old pre-veliger larvae was found to be mineralized, confirming that calcification occurred before larval torsion. Using both infra-red and Raman spectroscopy, we showed that CaCO₃ deposition occurred at the pre-veliger stage and that the mineral phase initially deposited was essentially composed of aragonite.     

Genetic variation and covariation during larval and juvenile growth in Mercenaria mercenaria

Quantitative genetic variances and covariances were estimated for shell length of the hard clam Mercenaria mercenaria (L.) at three larval stages (prodissoconch I, 2 d and 10 d post-fertilization) in 1987 and in 1988 after ca. 9 mo of growth. At each sample interval additive genetic variance was a highly significant component of the total size variation. Narrow sense heritability estimates for shell length ranged between 0.58 (±0.10) for prodissoconch I and 1.08 (±0.29) for 2-d-old larvae. There was significant and positive genetic covariance in prodissoconch I and 2-d larval shell length which resulted in a highly significant genetic correlation (r _g=0.74) between these two traits. This covariance is not surprising since the prodissoconch I comprises the majority of the larval shell of a 2-d-old larvae. The genetic covariances between 2-d-old and 10-d-old larvae and between 10-d-old larvae and 9-mo-old juveniles were low and not significantly different from zero. These results indicate that there is substantial genetic variation for shell growth in M. mercenaria but this variation is not stable during development; the genetic variation in shell growth at one stage of development is not strongly related to the genetic variation in growth during other ontogenetic periods. In this study there were no evident constraints to natural selection for increased shell growth rate during development, which coupled with the high levels of genetic variation may suggest that in nature high rates of larval growth may not be normally subject to significant selective pressure.     

Calcification of the first larval shell of Tridacna squamosa (Tridacnidae: Bivalvia)

The transistional stage between the trochophore and the veliger larva of Tridacna squamosa Lamarck, including shell formation and initial organogenesis, is described. Calcification of the Prodissoconch I begins before formation of the organic component of the shell is completed, and proceeds in a pattern which reinforces the maximum stress areas of the shell, allowing the early onset of the larval escape behavior. Evidence is presented which suggests that the orientation of the mineral crystals in the larval shells is determined by an organic component of the shell and not by an active process of the clam during calcification.     

Differential timing of larval starvation effects on filtration rate and growth in juvenile Crepidula onyx

This study demonstrates that the timing of larval starvation did not only determine the larval quality (shell length, lipid content, and RNA:DNA ratio) and the juvenile performance (growth and filtration rates), but also determine how the latent effects of larval starvation were mediated in Crepidula onyx. The juveniles developed from larvae that had experienced starvation in the first two days of larval life had reduced growth and lower filtration rates than those developed from larvae that had not been starved. Lower filtration rates explained the observed latent effects of early larval starvation on reduced juvenile growth. Starvation late in larval life caused a reduction in shell length, lipid content, and RNA:DNA ratio of larvae at metamorphosis; juveniles developed from these larvae performed poorly in terms of growth in shell length and total organic carbon content because of “depletion of energy reserves” at metamorphosis. Results of this study indicate that even exposure to the same kind of larval stress (starvation) for the same period of time (2 days) can cause different juvenile responses through different mechanisms if larvae are exposed to the stress at different stages of the larval life.     

Effects of temporary starvation on larvae of the sea star Asterina miniata

Patchy food distribution may force temporary starvation conditions on planktonic larvae. This potential food limitation may affect survivorship, duration of larval period, and post-metamorphic succes. In this study, larvae of the asteroid Asterina miniata were subjected to temporary food deprivation of several durations and at different stages. Developmental effects were documented by quantification of larval stage, total length, time to metamorphosis, initial juvenile radius, range of settling times, and percent survival to metamorphosis. All starved treatments were significantly affected in settling time and most in percent survival. However, larvae starved later in development demonstrated tremendous tolerance of food deprivation (e.g. the total number of settlers in the treatment starved for 28 d was not significantly different from the fed control). Survival was lower in treatments starved earlier in development than those starved later. Food is apparently required until late in larval development to facilitate metamorphosis. The range of settling times was large; for example, the continuously-fed control treatment produced juveniles from Days 58 through 136. Temporary starvation had no effect on initial juvenile radius.     

Gametogenesis and larval development in Sabella spallanzanii (Polychaeta: Sabellidae) from the Mediterranean Sea

Sabella spallanzanii (Gmelin) is one of the best known and widely distributed Mediterranean polychaetes, but available data on its general biology has been inferred mostly from populations recently introduced to Australia. In the present paper, data on gametogenesis and larval development of a Mediterranean population from the Gulf of Taranto (Ionian Sea, Italy) are reported. Histological and electron microscopical analysis of gametogenesis showed that oogenesis, a long process beginning soon after spawning in February, is of extraovarian type. By contrast spermatogenesis is a very fast process lasting from September to December, when the coelomic cavity is completely packed with mature gametes and almost devoid of coelomocytes. During the period of spawning, some experiments of in vitro fertilisation were performed, always without success, while some of the collected females naturally spawned fertilised eggs (100%). Particularly interesting was the discovery of a small percentage of eggs which appeared fertilised but unspawned within the coelomic cavity of female. Investigation of the S. spallanzanii life cycle, from the naturally spawned fertilised eggs to embryonic and juvenile stages, was performed by daily microscopical observations and by SEM. Despite the presence of large eggs giving rise to a lecithotrophic larva, a long pelagic phase was observed, the longest found among Sabellidae. The larval development pattern is also described. Settlement began after about 2 weeks; metamorphosis occurred 10 d after settlement, when mucus was secreted abundantly and an external tube was formed. The long larval pelagic period and the development pattern, suggesting a high potential for dispersal, support both the introduction and invasive behaviour of the species in Australian waters. Received: 28 May 1999 / Accepted: 10 December 1999     

Metamorphosis and developmental evolution in <Emphasis Type="Italic">Ophionereis</Emphasis> (Echinodermata: Ophiuroidea)

The brittle star Ophionereis schayeri has abbreviated non-feeding development through a reduced ophiopluteus and a vitellaria larva. The metamorphic changes involved with development from a bilateral larva to a radial juvenile were examined in detail. The reduced ophiopluteus has a continuous ciliated band that breaks up into discontinuous ciliary ridges at the vitellaria stage. As the vitellaria develops, the juvenile rudiment forms in the mid-ventral region. The rudiment then undergoes a morphogenetic movement to the left of the larval anterior/posterior axis. This results in a dramatic transformation from bilateral to radial symmetry and is accompanied by development of juvenile structures and settlement, 6–7 days after fertilisation. Ophiuroid development through one larval stage, the ophiopluteus, is termed Type I; whereas development through two larval stages, the ophiopluteus and vitellaria, is termed Type II. We examined the evolutionary changes in the expression of Type II development in Ophionereis by comparing the ontogeny of six species with a range of larval forms. O. fasciata has the ancestral-like planktotrophic ophiopluteus. Vestigial pluteal structures in the larvae of O. schayeri, provide a link between ophiopluteal and vitellarial forms during evolution of non-feeding development. The diversity of larval forms in Ophionereis indicates that evolution of non-feeding development through a vitellaria (Type II) may have involved an increase in egg size, reduction of pluteal structures and shortening of the time to metamorphosis. Assessment of the phylogenetic significance of Types I and II development awaits additional comparative data on the metamorphic stages of other ophiuroid genera.Communicated by G.F. Humphrey, Sydney     

<Emphasis Type="Italic">Semibalanus</Emphasis><Emphasis Type="Italic">balanoides</Emphasis> in winter and spring: larval concentration,settlement, and substrate occupancy

This study measured the progression from pelagic larvae to juvenile barnacles, and examined whether recruitment of barnacles, Semibalanus balanoides Linnaeus, at two intertidal sites in contrasting hydrodynamic regimes was determined by pre-settlement or post-settlement processes. The two sites were 1.5 km apart in the vicinity of Woods Hole, Mass., USA. Quantitative plankton samples were taken twice weekly from December 1997 to May 1998 at a nearby site as an estimate of nearshore larval abundance. The presence of S. balanoides nauplii was noted, and cyprids were enumerated and measured. Larval settlement at the two sites [Gansett Point, Buzzards Bay (GP) and Little Harbor, Vineyard Sound (LH)] was estimated from examination of replicate settlement plates exposed for 2 or 3 days throughout the settlement season, and from replicate plots on marked rock quadrats at each site. On both plates and rocks settled cyprids and metamorphs were enumerated. Space occupancy on unmanipulated rock quadrats by all stages from cyprids to adult barnacles was also examined. Settlement occurred from 2 January to 20 May, and major settlement peaks coincided with peaks in pelagic cyprid concentration at LH, but not at GP. Space occupied by juvenile barnacles was close to zero up until late February despite substantial settlement prior to that. At LH, juvenile barnacle cover was zero at the end of the observations; all settlement failed. Almost 100% of settled cyprids failed to metamorphose within 2 days from late January to late March. Then the proportion metamorphosing increased sharply coinciding with a sudden increase of 3°C in water temperature. Observed site differences in space occupancy by juvenile barnacles suggest that while cyprid supply is a necessary condition for barnacle settlement, other factors affecting metamorphosis of settled cyprids and early juvenile mortality determine recruitment.     

Precipitation of aragonite by calcitic bivalves in Mg-enriched marine waters

To understand the relative importance of biological versus physicochemical control over biomineralization, we have tested if the chemical composition of the medium (i.e., the Mg/Ca ratio) can change the mineralogy of mollusk shells. The shells of mollusks are made of calcite and/or aragonite, which are by far the most common CaCO₃ polymorphs. Several species of bivalves with predominantly calcitic shells have been cultivated in artificial seawater with a Mg/Ca molar ratio within the range of 8.3–9.2, well above the present value for seawater (5.2). Four out of six species used (the scallop Chlamys varia, the oyster Ostrea edulis, the saddle oyster Anomia ephippium and the mussel Mytilus edulis) survived long enough to secrete significant amounts of calcium carbonate. The deposits (sometimes extensive) formed on the interior shell surfaces were predominantly aragonitic. Three individuals of C. varia also increased their length by adding new shell at the margin. Contrary to the internal shell deposits, these margins were high-Mg calcite. This implies that the marginal mantle is able to exert a more strict control on the secreted mineral phase than the mantle facing the internal shell surface. This is the first report on an in vivo experimentally forced switch in bivalve shell mineralogy, from calcite to aragonite due to a change in water chemistry.     

Gill morphogenesis in the oyster Ostrea chilensis

Despite the importance of the gills in the acquisition of food by suspension-feeding bivalve mollusks, there is almost no information on gill organogenesis. By means of a series of stereoscan electron micrographs, this paper describes gill development in the Chilean oyster, Ostrea chilensis, from the brooded larval stages to 1-month-old spat. A single gill rudiment was observed on each side of the mantle at a shell length of 320 μm, and the rudiments increased in number and size until the end of the brooding period. During metamorphosis the gill filaments increased in number from 5 or 6 to between 7 and 9. The loss of the velum and the absence of functional gill filaments during metamorphosis are consistent with previous observations of weight loss during this critical period of the life history, because the newly settled juvenile lacks the ability to remove particles from suspension. The end of metamorphosis (100% of spat with dissoconch edge) was reached 36 h after larval settlement, when the gill filaments began to grow cilia, which increased in density and differentiated as the spat developed and acquired the capability of suspension-feeding, accounting for the increase in body weight previously recorded during this stage. The larval rudiments gave rise to the inner demibranchs. The outer demibranchs were observed 10 days after settlement, located between the inner demibranch and the mantle. In 1-month-old spat, the gill did not show differentiation between primary and secondary filaments, indicating that the heterorhabdic condition characteristic of adult oysters had yet to be attained. Received: 11 December 1998 / Accepted: 21 August 2000     

Reduced planktotrophy in larvae of <Emphasis Type="Italic">Clypeaster</Emphasis><Emphasis Type="Italic">rosaceus</Emphasis> (Echinodermata,Echiniodea)

Phylogenetic analyses have demonstrated that nonfeeding larvae have evolved from feeding larvae many times among marine invertebrates. In light of this observation, it is surprising that an intermediate strategy, a larva that can feed but is provisioned with enough energy to metamorphose without acquiring exogenous food (i.e., facultative planktotrophy), is rare. A hypothesis for the lack of facultative planktotrophic species among marine invertebrates is that the transition from feeding to nonfeeding is rapid due to this intermediate stage being evolutionarily unstable. Evidence that would support this hypothesis is if species with facultative planktotrophy have reduced food assimilation when compared with obligate planktotrophs. We studied a species with facultative planktotrophic larvae, Clypeaster rosaceus, that is very near the boundary between facultative and obligatory planktotrophy, to answer two questions: (1) does feeding during the larval stage result in energy gains in larval or juvenile stages and (2) if not, are larvae capable of assimilating exogenous food at all. Our measurements of energetics in larval and juvenile stages show that C. rosaceus larvae accumulate very little if any energy when fed, but stable isotope data indicate that larvae are able to assimilate some food. Our results are consistent with similar studies on facultative planktotrophic larvae suggesting poor food assimilation and rapid loss of larval feeding after a population evolves the ability to reach metamorphosis without feeding (lecithotrophy).     

Planktotrophic larval development is associated with a restricted geographic range in Lasaea,a genus of brooding,hermaphroditic bivalves

Members of the intertidal, near-cosmopolitan mollusc genus Lasaea brood their young either to a planktotrophic veliger or crawl-away juvenile stage of development. Developmental mode can be reliably inferred from brood masses and from prodissoconch structure. I have conducted a global developmental survey of this genus based mainly on examination of hundreds of museum lots. With one exception, Lasaea species with a planktotrophic larval development were restricted to the western Pacific. Congeners that lack planktotrophic larvae were found on all continents apart from Antarctica, and also on a large number of oceanic islands. These results indicate that (1) Lasaea species releasing crawl-away juveniles have a markedly greater collective geographic range than congeners with planktotrophic larvae; (2) pelagic larvae are not necessary for long-distance dispersal in this genus; (3) rafting has played a key role in the evolutionary success of the genus Lasaea; (4) cross-fertilizing Lasaea species with feeding larvae are less successful in utilizing chance rafting events to colonize new areas than are congeners lacking these traits.     

Elevated CO2 alters larval proteome and its phosphorylation status in the commercial oyster, Crassostrea hongkongensis

Ocean acidification (OA) is beginning to have noticeable negative impact on calcification rate, shell structure and physiological energy budgeting of several marine organisms; these alter the growth of many economically important shellfish including oysters. Early life stages of oysters may be particularly vulnerable to OA-driven low pH conditions because their shell is made up of the highly soluble form of calcium carbonate (CaCO₃) mineral, aragonite. Our long-term CO₂ perturbation experiment showed that larval shell growth rate of the oyster species Crassostrea hongkongensis was significantly reduced at pH < 7.9 compared to the control (8.2). To gain new insights into the underlying mechanisms of low-pH-induced delays in larval growth, we have examined the effect of pH on the protein expression pattern, including protein phosphorylation status at the pediveliger larval stage. Using two-dimensional electrophoresis and mass spectrometry, we demonstrated that the larval proteome was significantly altered by the two low pH treatments (7.9 and 7.6) compared to the control pH (8.2). Generally, the number of expressed proteins and their phosphorylation level decreased with low pH. Proteins involved in larval energy metabolism and calcification appeared to be down-regulated in response to low pH, whereas cell motility and production of cytoskeletal proteins were increased. This study on larval growth coupled with proteome change is the first step toward the search for novel Protein Expression Signatures indicative of low pH, which may help in understanding the mechanisms involved in low pH tolerance.     

Organogenesis and histogenesis in the planktotrophic veliger of Doridella steinbergae (Opisthobranchia: Nudibranchia)

Development of the planktotrophic veliger of the dorid nudibranch Doridella steinbergae (Lance) was studied by histological examination of 4, arbitrarily defined larval stages. Following an embryonic period of 7 1/2 to 8 days (12° to 15°C), the newly hatched veligers possess a functional digestive tract, a pair of nephrocysts, a secondary kidney, a pair of cerebral ganglia, a larval shell consisting of a two-thirds whorl, and the metapodial component of the foot. Development during Stage I mainly involves growth of the larval shell and the visceral organs. Stage II is marked by the retraction of the mantle fold from the shell aperture and the appearance of the eyespots, gonadal rudiment, larval heart, and the optic, pedal, and pleural ganglia. At Stage III the radular sac rudiment evaginates from the esophageal wall, the buccal ganglia differentiate, and the propodial rudiment begins to develop on the ventral surface of the metapodium. Stage IV veligers, which are competent to metamorphose, possess 6 pairs of radular teeth, lipid deposits in the left digestive gland, rudiments of the adult kidney and the oral lip glands, an hypertrophied mantle fold, a propodium, and densely packed cilia over the entire ventral surface of the foot. The length of the obligatory larval period, from hatching of the veliger until the attainment of metamorphic competence, is 25 to 26 days under laboratory culture conditions and the larval shell grows from 142 to 168 m in length. The sequence of morphogenetic events and the structure of the competent veliger of D. steinbergae is compared to that of other opisthobranch veligers. It is suggested that the relatively small maximal shell size attained by D. steinbergae results from precocious retraction of the mantle fold. It is further suggested that interspecific differences in the kinds of structures that develop during the veliger phase of opisthobranchs may relate to variations in the requirements of the juvenile phase. The functional adaptations of the gut of planktotrophic veligers are discussed and compared to those of lecithotrophic veligers.     

Physiological and biochemical changes during lecithotrophic larval development and early juvenile growth in the northern stone crab,Lithodes maja (Decapoda: Anomura)

Larvae of the northern stone crab, Lithodes maja L., were reared in the laboratory from hatching to the second crab stage. complete larval development (at constant 9°C) lasted about 7 wk, invariably consisting of three pelagic zoeal stages and a semibenthic Megalopa; only two zoeal stages have been described in the literature. All larval stages are lecithotrophic. First feeding was consistently observed only after metamorphosis, in the first juvenile crab stage. In short intervals (every 1 to 5 d), developmental changes in biomass, B (expressed as: dry weight, W; carbon, C; nitrogen, N; hydrogen, H) and oxygen consumption (respiration, R) were measured in larvae and early juveniles; additionally, protein and carbohydrates were measured, but only in the zoeal stages and early Megalopa. Unusually high C contents (varying between 56 and 61% of W in eggs and freshly hatched Zoea I larvae from 12 different females) and high C:N weight ratios (8 to 11) indicate enhanced initial lipid stores, which are utilized as the major metabolic substrate during both embryonic and lecithotrophic larval development. Predominant degradation of lipids is shown indirectly; the C:N ratio decreased significantly, from 10 (at hatching) to 6 (at metamorphosis), while larval protein decreased only little, from ca. 55% of W (at hatching) to 48% (in the Megalopa). From hatching to metamorphosis, about 27% of the initially present W, 48% of C, 18% of N, and 52% of H were lost. This decrease in larval biomass can be described as an exponential function of development time. The major part of these losses were associated with metabolic energy requirements, while exuvial losses were comparably small. In each of the zoeal stages, only about 1 to 2% of late premoult (LPM) B was shed with the exuvia. The Megalopa, which produces a much thicker, calcified exoskeleton, lost 20% of LPM W, but only 5 to 8% of organic constituents (C, N, H). Much higher exuvial losses were measured in the Crab I stage (51% in W, 21% in C, 5% in N, and 7% in H). Maximum respiration was found in the actively swimming zoeal stages, a minimum in the predominantly benthic, mostly inactive Megalopa. The Crab I stage exhibits also a sluggish behaviour and low R, in spite of beginning food uptake and growth. Immediately after metamorphosis, the juvenile crab gained rapidly in W, in particular in its C fraction. A transitorily steep increase in the C:N ratio indicates a replenishment of partially depleted lipid stores, but also a rapid initial increase of inorganic C in the heavily calcified exoskeleton. Instantaneous rates of growth, assimilation, and net growth efficiency (K ₂) were high during the initial (postmoult) phase in the first juvenile crab stage (C-specific growth rate: 6% d^-1; K ₂:70%), but decreased towards zero values during laterstages of the moulting cycle; metabolism remained practically constant during the Crab I stage. Entirely lecithotrophic larval development from hatching to metamorphosis in L. maja is considered an adaptation to seasonally short and limited planktonic food production in subarctic regions of the northern Atlantic.     

Identification of settling and early postlarval stages of mussels (Mytilus spp.) from the Pacific coast of North America, using prodissoconch morphology and genomic DNA

 Detailed inventories of the benthos and field studies of the settlement and recruitment processes of marine benthic invertebrates require accurate identification of newly settled larvae and early juvenile stages. We provide morphological criteria, visible under a good quality dissecting stereomicroscope, by which to discriminate between species of the settling larval and early postlarval stages (∼250 to 700 μm shell length) of mussels of the genus Mytilus on the west coast of Vancouver Island and Southern California. Compared to the bay mussel (M. trossulus), the sea mussel (M. californianus) has: (i) a shallower and flatter umbo, the latter corresponding to a significantly less pronounced prodissoconch I (PI) curvature and (ii) a greater PI length; as well as (iii) a wider separation between the provincular lateral teeth (PLT). The PLT distance is a new term denoting the separation between the midpoint of two reddish pigment spots of the provinculum (larval hinge apparatus) region of settling larvae and early postlarvae of Mytilus spp. from the East Pacific Coast. These spots mark the larger provincular lateral teeth, situated at either end of the provinculum. We confirmed the validity of morphological criteria by comparing PCR products of genomic DNA of provisionally identified postlarvae. Furthermore, measurements of PI lengths and PLT distance from well-preserved postlarvae of sea mussels (M. californianus) and of bay mussels (M. galloprovincialis) from Southern California indicate that the PI morphology and morphometry, and PLT distance criterion apply for that region as well. The criteria presented here can also apply to the advanced (competent) veliger stages, as the latter may settle (i.e. become the “settling” stage) upon encountering a suitable substrate. Our present and previously published work provide economical and effective identification methods that can be used to discriminate among early life history stages (∼250 μm to 5.0 mm shell length) of Mytilus spp. along the west coast of North America. Received: 10 November 1999 / Accepted: 6 September 2000     

The functional morphology and development of the alimentary tract of larval and juvenile barnacles (Cirripedia: Thoracica)

The alimentary tract of the nauplius larva of Balanus spp. consists of cuticle-lined foregut and hindgut, with intervening endodermal midgut constricted into anterior and posterior regions. The anterior midgut cells in the region of the constriction (constriction cells) secrete proteins (probably digestive enzymes). The remaining anterior midgut cells, often containing lipid droplets, form the absorptive region of the tract. Glycoprotein globules and lipid droplets within anterior midgut cells are the remants of the yolk in a pre-hatched larva, this yolk additionally supporting the larva through the non-feeding first nauplius stage. Nauplius Stages II to VI are actively feeding planktonic stages which increase in size and build up lipid reserves. These accumulated reserves support the non-feeding cyprid, first through its planktonic life and then through settlement and subsequent metamorphosis to the juvenile barnacle. Juvenile barnacles start to feed between 2 and 5 days after metamorphosis.