Graham Walker

Observations on the larval development of Sacculina carcini (Crustacea: Cirripedia: Rhizocephala)

The larval development of the parasitic barnacle, Sacculina carcini , has been closely followed. Size and volume measurements have been made for the four naupliar stages and cyprids of both male and female broods. Volume increases between naupliar stages are minimal (x 1.03–1.09) and in line with other barnacle species with lecithotrophic larvae. However, the large volume reduction between stage IV and cyprid for S. carcini , which is more acute for the female larvae, is a unique characteristic. The reduced setation formulae of the larval appendages of S. carcini and another rhizocephalan are compared to those of larvae, both lecithotrophic and planktotrophic, of a range of barnacles. Sex ratios and carapace length data for 184 S. carcini cypris populations are presented which reinforce the seasonal changes in sex ratios.

Further studies concerning the sex ratio of the larvae of the parasitic barnacle, Sacculina carcini Thompson

Abstract An experiment was carried out in the spring to test whether temperature and/or light affected the sex ratio of larval broods released by Sacculina carcini Thompson. Sacculinized crabs were maintained in the laboratory under continuous light and at different sea-water temperatures (18, 12, and 6°C). The externae of parasites maintained at 18 and 12°C released larvae regularly but those at 6°C did not. Released nauplii were cultured to cyprids and 150 from each population were measured (carapace length), sexed and the sex ratio calculated. During this spring experiment there was a change from males to females. The experiment was repeated in the autumn using animals maintained at 18°C only and under continuous light. The results showed a progressive change from females to males. The conclusion drawn is that temperature and/or light have no direct effect on the sex ratio of S. carcini larvae. A virgin externa was successfully inoculated by male cyprids in the laboratory and five larval broods subsequently released between July and October. The cyprids from each brood were scrutinized as above and the sex ratio compared with those derived from broods released by other externae maintained under identical conditions over the same time period. A progressive change from females to males was recorded over the period. The time taken for an externa to emerge after infecting crabs in the laboratory in May, June or July was variable. Six crabs were initially infected, but externae only emerged from three. After 5.5 months two virgin externae appeared on each of two crabs, but a period of 11.5 months elapsed before two externae emerged on the other crab and 14 months for a third externa to emerge on the same crab.

Settlement of Pomatoceros lamarkii (Serpulidae) larvae on biofilmed surfaces and the effect of aerial drying

The settlement responses of Pomatoceros lamarkii (Polychaeta: Serpulidae) larvae to biofilms of varying age on slate surfaces and to dried biofilms on slate surfaces were investigated in the laboratory. Settlement experiments were performed as multi-treatment, still water assays. Larvae did not settle on clean, non-biofilmed slates but settled on biofilms up to 28 days old. Settlement intensity was closely related to the bacterial density of a biofilm. Drying a biofilmed surface for 1-2 h at 20 degrees C to simulate a single tidal emersion completely negated the former inductive effect of the biofilm. Drying also negated the larval settlement-inducing effect that normally results from the presence of conspecific adults. The settlement inhibition lasted for approximately 5 days following a single drying event. Larvae settled readily on biofilms exposed to formalin and antibiotics. Treating biofilms with formalin or antibiotics before or after drying had no effect upon larval avoidance of dried biofilms. Freeze-drying a biofilm had the same effect as aerial drying. The biofilm drying effect could not be mimicked by exposing biofilms to hyper-saline seawater. The finding that P. lamarkii larvae do not settle on dried biofilms could have significance in explaining the natural distribution of this species in the intertidal.

Effect of Salinity on Development of Larvae of Heterosaccus Lunatus (Cirripedia: Rhizocephala)

Heterosaccus lunatus is a rhizocephalan barnacle which parasitizes the crab Charybdis callianassa in Moreton Bay, Queensland. Its free-swimming larvae are lecithotrophic and there are 4 naupliar stages before metamorphosis to the cypris stage. Sexes can be distinguished during this larval phase by size, the male larvae being larger at all stages. Experiments determined the effect of differing salinities on larval development of H. lunatus. A development index (D.I.) was calculated for each salinity tested and larval development was arbitrarily deemed successful if the D.I. was greater than 50%, i.e., > 50% of the nauplii became cyprids. Successful larval development occurred over the salinity range 24-40‰.

Measurement of the Adhesion of the Podia in the Asteroid Asterias Rubens (Echinodermata)

The adhesion of single podia of Asterias rubens (Echinodermata) was tested under different conditions in order to determine those factors which have an effect on the adhesive forces. In common with many other marine organisms, the adhesion of the podia is sensitive to surface properties of the substratum. The effect of immersion and emersion on tenacity (force per unit area) has been tested. Working with the asteroids completely immersed in sea-water appears to be the best technique to measure the adhesive forces of the podia. The mean tenacity of the podia of A. rubens on glass underwater is 1·98×10 5 Nm −2 . The measurement of the adhesion strength of marine invertebrates and of its variation under different conditions may give clues to how marine bioadhesives function. Adhesive forces have usually been measured in invertebrates using either permanent or transitory adhesion, but only rarely recorded in animals using temporary adhesion (for review see Walker, 1987). Of all macrobenthic organisms, echinoderms have exploited temporary adhesion most efficiently. In echinoderms, adhesive systems are usually associated with the podia and are involved in locomotion, attachment, feeding, or burrowing (Flammang, 1996). The paucity of information regarding the adhesive strength of echinoderm podia is due possibly to the fact that these animals possess a multitude of podia that are not all attached at the same time, making it difficult to evaluate the exact number of podia involved in adhesion at any precise instant in time. For example, maximum detachment forces involving many podia have been measured for several asteroid species. Feder (1955) measured up to 4 kg (39·64 N) in Pisaster ochraceus , Lavoie (1956) over 3 kg (29·43 N) in Asterias forbesi , and Christensen (1957) 5 kg (49·05 N) in Evasterias troscheli. Unfortunately, the number of podia adhering to the substratum was not estimated in any of these studies. It is not possible, therefore, to calculate tenacity (adhesive force per unit of surface area) which makes comparisons impossible either between these different asteroids or with other marine invertebrates (see Walker, 1987). Tenacity has been considered in only one study (Paine, 1926) where the mean adhesive force using single podia of the asteroid A. vulgaris was 17·2 g (0·17 N), giving, when divided by a mean measurement of the surface area of the podial discs, a tenacity of 1·25×10 5 N m 2 .

The effect of salinity on the development of Loxothylacus panopaei larvae (Crustacea: Cirripedia: Rhizocephala)

The rhizocephalan barnacleLoxothylacus panopaei, which parasitizes the mud crabRhithropanopeus harrisii, releases its larvae as nauplii. The nauplii develop through four stages to the cypris stage; both larval forms are lecithotrophs. Parasitized crabs were acclimated in 10‰, 15‰, and 20‰ water during August and the development of releasedL. panopaei larvae was monitored in a range of salinities. After 48–60 h, the nauplii and cyprids in each experimental salinity were counted and the numbers used to calculate a developmental index. The range of salinities in which successful development to cyprids occurred (developmental index >70%) could be shifted as a consequence of the acclimating salinity.L. panopaei larvae can develop successfully over a 10–30‰ salinity range, indicating that settlement onto host crabs and virgin externae is likely to take place in nature within this range.

Patterns of Biofilm Succession on a Sheltered Rocky Shore in Hong Kong

Successional patterns are dependent on the nature of the substratum, water flow, concentrations of organics as well as the availability of bacteria, algal spores and invertebrate larvae in the coastal environment. Bacteria play an especially important role in biofilm formation as they are generally the earliest colonizers. In the present study, both winter and summer biofilm succession patterns were examined on glass coverslips inverted on experimental racks attached at two tidal levels on a sheltered shore in Hong Kong. In the succession, bacteria were followed by diatoms and cyanobacteria. Encrusting algae appeared in the late stages of the experiment (day 80 in summer and day 60 in winter). Colonization by bacteria was much slower in summer and their density remained low throughout the experimental period. The first appearance of diatoms and cyanobacteria, however, was more rapid in the summer. Bacteria and diatoms on the low-shore surfaces also had a faster succession rate than on the high-shore surfaces, suggesting that desiccation/aerial temperature are the causal factors for such differences.

The cypris larvae of the parasitic barnacle Heterosaccus lunatus (Crustacea, Cirripedia, Rhizocephala): some laboratory observations

Heterosaccus lunatus parasitizes the portunid crab, Charybdis callianassa in Moreton Bay, Australia. With the host crabs maintained at 22.5 degrees C this sacculinid rhizocephalan released larval broods every 6-7 days. During July-August 1996 and particularly August 1999 such broods showed the change-over from male only larvae in the early broods to females only in the later broods. As the host crabs were maintained under similar aquarium conditions in both years it is concluded that the light/dark cycle is the principal cue triggering this larval sex reversal. Oogenesis in the parasite externa is somehow controlled to produce two different sized ova - male larvae develop from large ova and females from small ova. A working hypothesis outlining how sex is probably determined for the larvae of sacculinids is erected. H. lunatus is considered the ideal sacculinid for the further experimental work necessary to verify the proposed sex-determining mechanism and its control processes. Measurements of the maximum swimming speeds of H. lunatus male and female cyprids showed the larger males to be the faster in absolute terms (27.95 compared with 17.60 mm s(-1), respectively), however, the calculated relative speeds were almost identical at approximately 90 body lengths s(-1). Settlement experiments confirmed that female H. lunatus cyprids settle only on the gills of C. callianassa; these cyprids needed to be at least 2 days old before they were able to settle.

DISTRIBUTION, ADULT MORPHOLOGY, AND LARVAL DEVELOPMENT OF SACCULINA SINENSIS (CIRRIPEDIA: RHIZOCEPHALA: KENTROGONIDA) IN HONG KONG COASTAL WATERS

Abstract Sacculina sinensis (Cirripedia: Rhizocephala) is a parasitic barnacle first reported in Hong Kong by Boschma (1933) to infect the intertidal crab, Leptodius exaratus. Since then, no studies have been conducted to investigate its distribution and morphology, which are important in providing background information for further taxonomic and ecological studies. In the present study, S. sinensis has shown to have a limited distribution in Hong Kong, being confined to only four out of the nine sheltered boulder shores sampled and with a mean infestation rate of 11.3 percent. The localized distribution of S. sinensis in Hong Kong is probably due to its short larval development times (four naupliar stages and one cypris stage, completed in about six days) and hence limited larval dispersal distance. The externa of S. sinensis is ovoid in shape with two distinct shoulders, and the outer cuticle surface is spinous. From histological studies, the male receptacles are located posteriorly in the visceral mass, and each male receptacle is globular in shape. The receptacle duct is folded and connected to the receptacle via a small tube with a thick cuticle lining. The larval morphology of S. sinensis was investigated using a scanning electron microscope. The appendages bear setulate setae in all naupliar stages. In the cyprid, segment III of the antennule consists of the attachment disc in both sexes, and the male has a large posterior aesthetasc which is absent in females. On segment IV, male cyprids have a large terminal aesthetasc, a subterminal aesthetasc, and three terminal setae. Segment IV of female cyprids has a subterminal aesthetasc which is smaller than that on the male cyprids. The larval sex ratio showed seasonal variation with 100 percent males in summer and 80 percent females in winter. Such seasonal variation in sex ratio would allow the female cyprids to infest the host crabs in winter, producing virgin externae by summer for the male cyprids to settle, and allow the externae to mature.

SOME OBSERVATIONS ON THE EPIZOIC BARNACLE OCTOLASMIS ANGULATA WITHIN THE BRANCHIAL CHAMBERS OF AN AUSTRALIAN SWIMMING CRAB

Abstract An epizoic stalked barnacle, Octolasmis angulata, was identified within the branchial chambers of Charybdis callianassa, a swimming crab from Moreton Bay, Queensland, Australia, making this crab a new host for O. angulata. In the present study fifty-two crabs, 30–49 mm carapace width, were dissected, and thirty-three were found to have the epizoite. The number and position of the O. angulata within the branchial chambers were noted. Octolasmis angulata is principally found attached to the cuticle of the anterior chamber wall in the epibranchial space, although attachment to the gills does occur. Charybdis callianassa is also parasitized by the sacculinid barnacle Heterosaccus lunatus, and one such parasitized crab contained eighty-seven O. angulata, the highest number recorded in the present study. The factors governing O. angulata distribution within the branchial chambers of C. callianassa are discussed.