Uncommon diversity in developmental mode and larval form in the genus <Emphasis Type="Italic">Macrophiothrix</Emphasis> (Echinodermata: Ophiuroidea)

Development mode in the ophiuroid genus Macrophiothrix includes an unusual diversity of planktonic larval forms and feeding types. The modes of development for seven congeners that coexist in coral reef habitats at Lizard Island, Australia were compared using larvae generated from crosses over several reproductive seasons from 1999 to 2003. Three species (Macrophiothrix koehleri Clark, Macrophiothrix longipeda Lamarck, Macrophiothrix lorioli Clark) develop from small eggs (<170 μm) into typical obligately feeding planktonic (planktotrophic) pluteus larvae with four larval arm pairs. The remaining four species develop from larger eggs (≥230 μm) into either facultatively-feeding or non-feeding (lecithotrophic) larval forms. The facultative planktotroph (Macrophiothrix rhabdota Clark) retains the ability to digest and benefit from food but does not require particulate food to complete metamorphosis. Among the lecithotrophic species, Macrophiothrix caenosa Hoggett retains the pluteus morphology with four pairs of larval arms, but is incapable of feeding, depending instead on maternal provisions for larval development. The remaining two lecithotrophs have simplified larval morphologies with only a single pair of full length (Macrophiothrix nereidina Lamarck) or highly reduced (Macrophiothrix belli Doderlein) larval arms and no functional mouth or gut. This genus includes the first example of facultative planktotrophy in ophiuroids, the first example in echinoderms of a complete pluteus morphology retained by a lecithotrophic larva, and three degrees of morphological simplification among lecithotrophic larval forms. Egg volume varies 20-fold among species and is related to variation in feeding mode, larval form, and development time, as predicted for the transition from planktotrophic to lecithotrophic development.     

Comparative sensitivity of various developmental stages of sea urchins to some chemicals

The sensitivity to some chemical agents was examined comparatively at sperm, fertilization, cleavage, blastula, gastrula, pluteus and metamorphosis stages of a sand dollar from Japanese waters (Peronella japonica) and a sea urchin from the Pacific coast of Australia (Heliocidaris erythrogramma). These agents included Cu sulphate, ABS and NH₃ chloride. Responses observed included departures from control rates of fertilization and developmental reduction at the attainment of first cleavage, gastrula, pluteus or metamorphosis. Developmental anomalies were noted at the fertilization, 2-cell, gastrula, pluteus and metamorphosis stages. Using minimum effective concentrations of the 3 chemicals at various developmental stages of P. japonica, it was found that sensitivity to chemicals varies from fertilization to metamorphosis. It seems that sperm activity is the most sensitive, and that fertilization and gastrulation are more sensitive than first cleavage, blastulation and pluteus formation. H. erythrogramma seems to show nearly the same responses to Cu, but is more sensitive at metamorphosis.Experiments conducted at the Seto Marine Biological Laboratory, Wakayama Prefecture, JapanExperiments conducted at School of Biological Sciences, the University of Sydney, Sydney, N.S.W., AustraliaContributions from the Seto Marine Biological Laboratory, No. 664     

Halogenated metabolites in two marine polychaetes and their planktotrophic and lecithotrophic larvae

This study investigated the occurrence and ontogenetic changes of halogenated secondary metabolites in planktotrophic and lecithotrophic larvae and adults of two common, infaunal polychaetes, Streblospio benedicti (Spionidae) and Capitella sp. I (Capitellidae), with different life-history traits. S. benedicti contains at least 11 chlorinated and brominated hydrocarbons (alkyl halides) while Capitella sp. I contains 3 brominated aromatic compounds. These halogenated metabolites are potential defense compounds benefiting both larvae and adults. We hypothesized that: (1) planktotrophic larvae contain halogenated metabolites because they are not protected by adult defenses, (2) quantitative and qualitative variation of planktotrophic larval halogenated metabolites parallels that of adults, and (3) brooded lecithotrophic larvae initiate the production of halogenated metabolites only after metamorphosis. To address these hypotheses, volatile halogenated compounds from polychaete extracts were separated using capillary gas chromatography and identified and quantified using mass spectrometry with selected ion monitoring. All four life stages (pre- and post-release larvae, new recruits, adults) of both S. benedicti and Capitella sp. I contained the halogenated metabolites previously identified from adults. This is the first report of halocompounds identified and quantified in polychaete larvae. Allocation of potential defense compounds to offspring varied as a function of species, feeding type and developmental stage. Pre-release larvae of S. benedicti with planktotrophic development contained the lowest concentration of total halogenated metabolites (1.75 ± 0.65 ng mm⁻³), post-release and new recruits contained intermediate concentrations (8.29 ± 1.72 and 4.73 ± 2.63 ng mm⁻³, respectively), and planktotrophic adults contained significantly greater amounts (28.9 ± 9.7 ng mm⁻³). This pattern of increasing concentrations with increasing stage of development suggests synthesis of metabolites during development. Lecithotrophic S. benedicti post-release larvae contained the greatest concentrations of halometabolites (71.1 ± 10.6 ng mm⁻³) of all S. benedicti life stages and developmental types examined, while the amount was significantly lower in new recruits (34.0 ± 15.4 ng mm⁻³). This pattern is consistent with a previously proposed hypothesis suggesting a strategy of reducing potential autotoxicity during developmental transitions. Pre-release lecithotrophic larvae of Capitella sp. I contained the highest concentration of total halogenated metabolites (1150 ± 681 ng mm⁻³), whereas the adults contained significantly lower total amounts (126 ± 68 ng mm⁻³). All concentrations of these haloaromatics are above those known to deter predation in previously conducted laboratory and field trials. As a means of conferring higher larval survivorship, lecithotrophic females of both species examined may be expending more energy on chemical defenses than their planktotrophic counterparts by supplying their lecithotrophic embryos with more of these compounds, their precursors, or with energy for their synthesis. This strategy appears common among marine lecithotrophic larval forms. Received: 14 July 1999 / Accepted: 20 January 2000     

Embryonic,larval, and post-metamorphic development of the sea urchin <Emphasis Type="Italic">Cassidulus mitis</Emphasis> (Echinoidea; Cassiduloida): an endemic brooding species from Rio de Janeiro,Brazil

Cassiduloids are currently rare irregular echinoids with a highly conserved adult morphology. Aristotle’s lantern is present only during the post-metamorphic stage, and little is known about the early development of species in this group. Cassidulus mitis produces eggs of about 375 μm in diameter, lecithotrophic larvae with four reduced arms with skeletal fenestrated rods, cilia along the body surface, and a ciliated band on arms and lobes. Offspring is brooded among the female spines from embryo to settler’s stage. The echinopluteus larval stage is reached 6 days after fertilization, and the settler’s stage is formed at the age of 17 days. Aristotle’s lantern appears around the thirteenth day of development. The lantern is well developed and functional in settlers. It remains until at least 62 days after fertilization and can be used to acquire food from the environment. The early development of C. mitis is unusual concerning features of typical lecithotrophic larvae (such as reduced arms), but retains some features of planktotrophic larvae (such as skeletal rods and a ciliated band). Regarding egg size, early development in C. mitis seems to be transitioning from facultative lecithotrophic to typical obligate lecithotrophic pattern in echinoid larval evolution.     

Reproduction of sympatric populations ofHeliocidaris erythrogramma andH. tuberculata (Echinoidea) in New South Wales

Reproduction ofHeliocidaris erythrogramma (Valenciennes) andH. tuberculata (Lamarck) was compared through examination of oogenesis, spermatogenesis and monthly measurement of the gonad index. These species occur sympatrically in the Sydney region. Their reproduction was examined at two sites near Botany Bay, New South Wales, Australia, from February 1989 through January 1990.H. erythrogramma produces buoyant, 450µm-diam eggs and the sperm have a head region 10µm in length. By contrast,H. tuberculata produces negatively-buoyant, 95µm-diam eggs and the sperm have a head region 4µm in length. Histochemical examination of the gonads revealed that periodic acid Schiff-positive (PAS +) material stored in the nutritive phagocytes appears to support vitellogenesis in both species. InH. tuberculata this material is utilized in the formation of PAS + yolk oligolecithal eggs, whereas inH. erythrogramma the PAS + material appears to be converted to lipid yolk in macrolecithal eggs.H. erythrogramma had a seasonal reproductive pattern with a 3 mo summer spawning period, whereas both populations ofH. tuberculata had a 9 mo breeding period characterized by the continual presence of nutrient reserves and vitellogenic oocytes which rapidly replaced spawned ova. Spawning ceased only for 3 mo over the summer. Due to the 9 mo spawning ofH. tuberculata it is not clear what factors serve to cue reproduction in this species.     

Diet,behaviour and reproduction of the whelk Buccinum undatum in the northern Gulf of St. Lawrence,eastern Canada

A laboratory experiment was used to examine whether a decrease in the width of sutures is the basis for shrinking of the test in echinoids under conditions of low food availability and whether the body condition of sea urchins is likely to be retained when shrinking occurs. This experiment was performed with H. erythrogramma specimens collected in January 1987 from Pt. Lillias in Corio Bay, Australia. Suture widths in the test and gonad volume were significantly less (relative to body volume) in Heliocidaris erythrogramma (Val.) in the low food treatment than the high food treatment after 4 mo. These results imply that sea urchins which shrink may have a poorer body condition and be less able to reproduce than sea urchins that do not shrink. A comparison of the expected to the observed changes in suture width showed that a decrease in suture width is involved in the shrinking of the test in H. erythrogramma and may be of sufficient magnitude to fully explain the observed reduction in test size. This suggests that the amount a sea urchin may shrink under low food conditions is constrained by the degree to which the sutures can be reduced in width.     

Fuels for development: evolution of maternal provisioning in asterinid sea stars

For marine invertebrates, larval developmental mode is inseparably linked to the nutritional content of the egg. Within the asterinid family of sea stars there have been multiple, independent, evolutionary transitions to lecithotrophic development from the ancestral, planktotrophic state. To investigate the evolution of maternal investment and development within the Asterinidae, we quantified individual lipid classes and total protein for eggs and larval stages of closely related species representing three developmental modes (planktotrophy, planktonic lecithotrophy and benthic lecithotrophy). Within species, maternal provisioning differed between females indicating that egg quality varied with parentage. Maternal investment was related to egg size but, after correcting for egg volume, we identified two major oogenic modifications associated with the evolution of lecithotrophic development: (1) a reduction in protein deposition that probably reflects the reduced structural requirements of nonfeeding larvae, (2) an increase in deposition of a single class of energetic lipid, triglyceride (TG). The exception was Parvulastra exigua, which has benthic, lecithotrophic development and lays eggs with a lipid to protein ratio close to that of planktotrophs. This oogenic strategy may provide P. exigua larvae with a protein “weight-belt” that assists in maintaining a benthic existence. Asterinids with planktotrophic development used a significant portion of egg TG to build a feeding bipinnaria larva. For Meridiastra mortenseni, female-specific differences in egg TG were still evident at the bipinnaria stage indicating that egg quality has flow-on effects for larval fitness. In lecithotrophic asterinids, TG reserves were not depleted in development to the larval stage whereas protein stores may help fuel early larval development. Available data indicate that there may be two evolutionarily stable egg lipid profiles for free-spawning, temperate echinoderms.     

Fitness benefits of size-dependent diet switching in a marine herbivore

Although diet is one of the most important parameters affecting the fitness of terrestrial and marine herbivores, host plant choice and subsequent fitness on that host are not always correlated. This study investigated the effect of diet on fitness of the sea urchin Holopneustes purpurascens, which show an ontogenetic change in host plant use subsequent to recruitment. To test whether fitness on host plant mirrored host plant choice, small and large individuals were collected from both hosts and fed either host plant (Ecklonia radiata or Delisea pulchra). Small urchins survived better than large individuals. Those fed E. radiata produced fewer test lesions, grew faster and were more fecund than those fed D. pulchra, irrespective of size. This pattern was enhanced when the host plant the urchin previously inhabited was assessed. Our results show that diet is driving the previously recorded ontogenetic change in host plant use associated with increasing size in H. purpurascens, where medium-sized individuals switch from D. pulchra to E. radiata.     

Growth and differentiation during delayed metamorphosis of feeding gastropod larvae: signatures of ancestry and innovation

Extent of larval growth among marine invertebrates has potentially profound implications for performance by benthic recruits because body size influences many biological processes. Among gastropods, feeding larvae often attain larger size at metamorphic competence than non-feeding larvae of basal gastropod clades. Delay of metamorphosis can further influence size at recruitment if larvae continue to grow during the delay. Some caenogastopod larvae grow during delayed metamorphosis, but opisthobranch larvae do not. Data on larval growth of neritimorph gastropods are needed to help determine which of these growth patterns for planktotrophic gastropod larvae is more derived. We cultured planktotrophic larvae from all three major gastropod clades with feeding larvae through delays of metamorphosis of 3–10 weeks. Larvae of the caenogastropod Euspira lewisii and the euthyneurans Haminoea vesicula (Opisthobranchia) and Siphonaria denticulata (Pulmonata) conformed to previously described growth patterns for their respective major clades. Furthermore, the caenogastropod continued to lengthen the prototroch (ciliary band for swimming and feeding) and to differentiate prospective post-metamorphic structures (gill filaments and radular teeth) during delayed metamorphosis. Larvae of the neritimorph Nerita atramentosa arrested shell growth during delayed metamorphosis but the radula continued to elongate, a pattern most similar to that of non-feeding larvae of Haliotis, a vetigastropod genus. Character mapping on a phylogenetic hypothesis suggests that large larval size and capacity for continued growth during delayed metamorphosis, as exhibited by some caenogastropods, is a derived innovation among feeding gastropod larvae. This novelty may have facilitated post-metamorphic evolution of predatory feeding using a long proboscis.     

Evidence for a water-borne cue inducing metamorphosis in the dorid nudibranch mollusc Adalaria proxima (Gastropoda: Nudibranchia)

The dorid nudibranch Adalaria proxima (Alder & Hancock) is a specialist predator of the cheilostome bryozoan Electra pilosa (L.). Natural induction of metamorphosis of the pelagic lecithotrophic larva of A. proxima was assessed in response to solutions from sonicated prey tissue and (live) E. pilosa-conditioned seawater (Electra-CSW). We exploited the tendency of larvae to become entrapped (rafted) at the air-water interface in cultures to examine whether larvae require direct contact with the live prey for metamorphosis to proceed. Larvae metamorphosed when rafted above colonies of live E. pilosa, above plankton mesh bags isolating live E. pilosa, and in choline chloride controls; there was no metamorphosis of larvae that were rafted in filtered seawater controls. Entrapped veliger shells remained rafted throughout the experimental period in all cases. No metamorphosis occurred in treatments containing either the supernatants or pelleted particulates obtained from sonicated colonies of E. pilosa. Both one-colony and three-colony Electra-CSW induced metamorphosis of larvae. These data are at variance with previous results in showing that direct contact with the live prey is not necessary for metamorphosis to proceed. Furthermore, the fact that competent larvae metamorphosed in response to Electra-CSW in the absence of any other cue strongly suggests that the inductive cue is water-borne.     

Feeding by larvae of two different developmental modes in Streblospio benedicti (Polychaeta: Spionidae)

Females of the spionid polychaete Streblospio benedicti (Webster) produce either small eggs (60–70 μm diameter) and planktotrophic larvae, or large eggs (100–200 μm) and lecithotrophic larvae that reportedly do not feed. This intraspecific polymorphism, a form of poecilogony, is potentially useful in studies of larval ecology and evolution, but necessary data on larval form and function are lacking. This study describes the morphology and nutritional biology of larvae obtained from Atlantic (South Carolina) and Pacific (California and Washington) populations from 2003 to 2005. The two types of larvae produced by Atlantic S. benedicti differed greatly in length (229±22 μm SD for planktotrophs vs. 638±40 μm for lecithotrophs) and chaetiger number (2–5 vs. 10–11) at release from the female’s brood pouch. Planktotrophic larvae bore long provisional chaetae on their first chaetiger; provisional chaetae were absent in lecithotrophic larvae. Larvae from Pacific populations were all of the lecithotrophic form, and were similar to their Atlantic counterparts in all respects. High-speed video microscopy revealed that both types of larvae used opposed bands of cilia to capture suspended particles and transport them to the mouth, where they were often ingested. Lecithotrophic larvae reared with suspended phytoplankton (Rhodomonas sp., 10⁴ cells ml⁻¹) for 2 days grew significantly faster than sibling larvae reared without added food, indicating that these larvae can digest and assimilate ingested food. Larvae of S. benedicti that develop from large eggs are thus facultative planktotrophs instead of obligately non-feeding lecithotrophs, a result that affects the interpretation of comparative studies of the ecology and evolution of larvae in S. benedicti and certain other marine invertebrates.     

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).     

Settlement of larval blacklip abalone, Haliotis rubra, in response to green and red macroalgae

Surfaces from the habitat of adult Haliotis rubra were tested as inducers of larval settlement to determine the cues that larvae may respond to in the field. Settlement was high on the green algal species Ulva australis and Ulva compressa (Chlorophyta), the articulated coralline algae Amphiroa anceps and Corallina officinalis, and encrusting coralline algae (Rhodophyta). Biofilmed abiotic surfaces such as rocks, sand and shells did not induce settlement. Ulvella lens was also included as a control. Treatment of U. australis, A. anceps and C. officinalis with antibiotics to reduce bacterial films on the surface did not reduce the settlement response of H. rubra larvae. Similarly, treatment of these species and encrusting coralline algae with germanium dioxide to reduce diatom growth did not significantly reduce larval settlement. These results suggest that macroalgae, particularly green algal species, may play an important role in the recruitment of H. rubra larvae in the field and can be used to induce larval settlement in hatchery culture.     

Molecular and morphological evidence of a single species, Boccardia proboscidea (Polychaeta: Spionidae), with multiple development modes

Most species of benthic marine invertebrates have a single mode of larval development. Poecilogonous species are those that produce more than one type of larval offspring. Reports of variable development within one species, especially in combination with widely differing ecological habitat, are frequently attributed to cryptic species. The spionid polychaete Boccardia proboscidea Hartman, 1940 exhibits development that varies both within a single brood and among broods produced by different females. Some females have planktotrophic development and produce many small larvae with a 2 week planktonic period before metamorphosis. Other females produce broods containing both planktotrophic larvae as well as nurse-egg-ingesting (adelphophagic) offspring that hatch as juveniles. Molecular analysis (RAPD-PCR) showed that a significant proportion of genetic variance is attributable to geographic origin, and not to developmental type. Adults of both developmental types showed no consistent differences in taxonomically important features (e.g. type and arrangement of chaetae, modified fifth setiger, caruncle, branchiae, pygidium) when examined with SEM. These data support the hypothesis that developmental variability in this species is a case of poecilogony, and is not attributable to cryptic species. Received: 21 April 1998 / Accepted: 20 April 1999     

Reproductive energetics of two species of dorid nudibranchs with planktotrophic and lecithotrophic larval strategies

Onchidoris muricata (Müller) and Adalaria proxima (Alder and Hancock) are sympatric, potentially competing species of dorid nudibranchs, which preferentially graze the cheilostome polyzoan Electra pilosa (L.). O. muricata is small and lays small eggs which hatch as poorly-developed planktotrophic veliger larvae. A. proxima is larger and reproduces by means of larger eggs which hatch, as well-developed lecithotrophic larvae, that can metamorphose within approximately 24 h of release. A. proxima larvae can feed in the plankton, but do not require extrinsic nutrition to undergo complete development. Both species spawn in February–april, and have a strictly annual life-cycle. Comparisons of the calorific content of spawn have shown that A. proxima apportions a greater number of calories to reproduction, but that O. muricata makes a greater relative effort. A. proxima shown considerable individual variability in reproductive effort, which fails to correlate with, body size or rate of spawning. A more deterministic situation applies to O. muricata, because body size and fecundity follow an allometric relationship. It appears that there is a threshold of absolute energy required to support the lecithotrophic larval strategy in nudibranchs, and that this is not attained by the smaller species, O. muricata. A. proxima thus appears to have both reproductive strategies open to it, and to have adopted lecithotrophy in order to offset the unpredictability of energy available for reproduction.Communicated by J. M. Mauchline, Oban     

Poecilogony in an estuarine opisthobranch: planktotrophy, lecithotrophy, and mixed clutches in a population of the ascoglossan Alderia modesta

A San Diego population of the opisthobranch mollusc Alderia modesta (Lovén, 1844) exhibits poecilogony, the presence of two development modes within a single species. In spring, half of the adults spawned masses containing ∼300 eggs with a mean diameter of 68 μm. After 3 d, these egg masses hatched planktotrophic veligers with a maximum shell dimension of 116 μm. The remaining adults spawned masses containing ∼30 eggs with a mean diameter of 105 μm. These egg masses hatched after 5 to 6 d, releasing lecithotrophic larvae with a maximum shell dimension of 186 μm. About 1% of field-collected adults produced mixed clutches containing a continuum of larval sizes, spanning the size extremes of planktotrophy and lecithotrophy and hatching larvae with a mean maximum shell dimension of 152 μm. Adults producing planktotrophic and lecithotrophic larvae were interfertile, and no hybrid breakdown was observed through the F3 generation. When starved, adults which previously produced only lecithotrophic larvae switched to producing planktotrophic larvae or mixed clutches with both planktotrophic and lecithotrophic larvae. Sequence-polymorphisms from a portion of the mitochondrial cytochrome c oxidase I gene support the conclusion that the two reproductive morphs represent a single species. Most of the lecithotrophic larvae and a small percentage of the larvae from mixed clutches were metamorphically competent within 3 d of hatching. A. modesta is the only molluscan species as yet known to have both planktotrophic and pelagic lecithotrophic development within a single natural population. Received: 14 August 1997 / Accepted: 11 April 1998     

Fertilization in a suite of coastal marine invertebrates from SE Australia is robust to near-future ocean warming and acidification

Climate change driven ocean acidification and hypercapnia may have a negative impact on fertilization in marine organisms because of the narcotic effect these stressors exert on sperm. In contrast, warmer, less viscous water may have a positive influence on sperm swimming speed and so ocean warming may enhance fertilization. To address questions on future vulnerabilities we examined the interactive effects of near-future ocean warming and ocean acidification/hypercapnia on fertilization in intertidal and shallow subtidal echinoids (Heliocidaris erythrogramma, H. tuberculata, Tripneustes gratilla, Centrostephanus rodgersii), an asteroid (Patiriella regularis) and an abalone (Haliotis coccoradiata). Batches of eggs from multiple females were fertilized by sperm from multiple males in all combinations of three temperature and three \textpH/P_\textCO2 {\text{pH}}/P_{{{\text{CO}}_{2} }} treatments. Experiments were placed in the setting of projected near-future conditions for southeast Australia, an ocean change hot spot. There was no significant effect of warming and acidification on the percentage of fertilization. These results indicate that fertilization in these species is robust to temperature and \textpH/P_\textCO2 {\text{pH}}/P_{{{\text{CO}}_{2} }} fluctuation. This may reflect adaptation to the marked fluctuation in temperature and pH that characterises their shallow water coastal habitats. Efforts to identify potential impacts of ocean change to the life histories of coastal marine invertebrates are best to focus on more vulnerable embryonic and larval stages because of their long time in the water column where seawater chemistry and temperature have a major impact on development.     

Reducing per capita food supply alters urchin condition and habitat

When food supply declines or population density increases, the per capita food availability is reduced causing a decline in condition of consumers. Many consumers alter their feeding behaviour and ultimately the surrounding community (e.g. overgrazing and formation of urchin barrens). This study tested the hypothesis that sea urchin populations are of greater density and poorer condition in barrens (little food) than forest habitat (lots of food). We then tested the hypothesis that a decrease in per capita food supply to urchins has a negative effect not only on their condition but also on their surrounding habitat. We experimentally assessed the effect of limited food supply and increased density of a subtidal Australian sea urchin (Heliocidaris erythrogramma) on their condition (i.e. gonad index) and surrounding benthic habitat (i.e. turf-forming algae). Our results show that a reduction in food supply led to poorer consumer condition and greater herbivory on surrounding local habitat. We provide evidence that per capita food reduction is one of the necessary conditions for the over-consumption by urchins (i.e. urchin barrens), a proposed but previously untested mechanism.     

Some observations on gametogenesis,larval development and substratum selection of the sea pen Ptilosarcus guerneyi

Ptilosarcus (Leioptilus) guerneyi (Gray) maintained in the laboratory, were observed to spawn in late March, 1972. Gametes, developed in the leaf proper, discharged through the mouths of feeding polyps and were fertilized externally in the sea water. The sea pen's eggs are 500 to 600 in diameter; a large female is capable of producing over 200,000 eggs in one season. A pear-shaped and free-swimming planula larva developed 4 days after fertilization, at a temperature of 12 °C. The larvae were ready to settle and metamorphose when 7 days old if favorable substratum was available, but would remain as planulae for at least 30 days if kept in glass dishes only. The 30-day-old larvae would metamorphose if a suitable substratum (coarse sand, for example) was presented. The larvae do not feed and, hence, development is lecithotrophic. Studies of histogenesis showed that metamorphosis greatly enhanced the rate of cellular differentiation. The high fecundity, lecithotrophic development, and the ability of substratum selection by the larvae explain the success of this species in maintaining a high-density population in many areas of sandy substratum in the shallow waters of Puget Sound (USA), despite the fact that it is preyed upon by 7 species of predators.     

Reproductive cycle of Centrostephanus rodgersii (Echinoidea), with recommendations for the establishment of a sea urchin fishery in New South Wales

The reproductive cycle of the sea urchin Centrostephanus rodgersii (Agassiz) was investigated in two populations, at Clovelly and Little Bay, in Sydney, New South Wales, Australia. C. rodgersii were collected at monthly intervals from February 1992 through January 1993. The reproductive cycle was determined by histological examination of oogenesis and spermatogenesis, monthly measurements of gonad index (GI), and induction of spawning by KCl injection. C. rodgersii has an annual reproductive cycle that was highly synchronous in both populations. From February to June, gametogenesis was accompanied by a decline in the amount of nutritive tissue in the gonads. The urchins were mature from June to September, with peak spawning between July and August, as indicated by a significant drop in GI. The breeding season of C. rodgersii therefore coincides with the lowest sea temperatures and the shortest days of the year. The gonads returned to the recovering condition within a month of spawning, with a substantial thickening of the nutritive layer along the gonad wall, and the GI returned to near pre-spawning levels. As a result, the spent phase was rarely found in C. rodgersii. With the exception of a significant decrease in the GI following spawning of urchins from the Clovelly population, the GI measurements did not show any distinct pattern through time. Specimens spawned in response to KCl injection from mid-May to early October, with the maximum response in July. Although all individuals sampled were at a similar stage of maturity at any one time, inter-site differences were seen with all of the methods used. Gonad indices from Little Bay were consistently higher and less variable than those from Clovelly for most of the year. The Little Bay population could also be induced to spawn for a longer period of time than could the Clovelly population. The breeding season of the Little Bay population appears to be longer than that of the Clovelly population. The relationship between size and sexual maturity was also examined. All C. rodgersii with a test diameter of >60 mm could be induced to spawn and produced viable gametes. C. rodgersii has been nominated for commercial exploitation in New South Wales, and the results of this investigation are used to make recommendations on the timing and size limits for a fishery utilising this species.