John S. Ryland

Physiology and Ecology of Marine Bryozoans

Publisher Summary This chapter reviews the physiology and ecology of marine bryozoans and presents the latest research in these fields to marine biologists generally rather than to bryozoologists particularly. Three classes are recognized: Phylactolaemata, Stenolaemata, and Gymnolaemata. The first of these is confined to fresh water; the second comprises those bryozoans with slender, tubular, calcified walls; the third is the dominant and most diverse marine class. A new bryozoan colony normally arises from a sexually produced larva. In stenolaemates the larva metamorphoses into the proancestrula, a hemispherical body that later elongates to form a tubular extension housing the polypide. Colony form in erect, branching species is known to be labile. Harmelin has studied the relationships between the branching pattern of the cyclostomes “Idmonea” atlantica and environmental factors. The chapter discusses the reproduction, larvae and metamorphosis of marine bryozoans. The larval organization completely breaks down during metamorphosis and, in Electra, a new structure, the ancestrula or primary zooid of the colony, arises in its place. During metamorphosis, which has been described by Atkins, there is a reversal of internal polarity: the distal end of the polypide and the blastogenic face of the ancestrula correspond to the posterior of the larva. The geographical distribution of marine bryozoans is discussed in the chapter.

Patterns of genetic subdivision in populations of a clonal cnidarian, Zoanthus coppingeri, from the Great Barrier Reef

Samples of an intertidal zoanthid, Zoanthus coppingeri, Haddon and Shackelton, 1891, were collected from three localities in the Great Barrier Reef region during 1992–1993, and subjected to allozyme electrophoretic analysis at seven polymorphic loci. The reduced ratio of observed to expected genotypic diversity indicated that populations were partly clonal, but they were not dominated by a few clones as occurs in some other cnidarians. Regular disturbance by wave action is postulated to prevent the formation of large stands of particular clones by clearing space and mixing genotypes over small scales. The sexual origin of clonal genotypes was confirmed by conformance to Hardy-Weinberg predictions of genotype frequencies at all but one locus. Values of the standardised genetic variance among populations, FST, were highly significant between localities and between replicate sites within localities separated by only 50 m. Strong genetic structure has not previously been described in a Great Barrier Reef invertebrate species, and is considered to be the consequence of stochastic changes in gene frequencies as a result of low levels of gene flow. High clonal longevity and low recruitment rates may maintain genetic differences over long periods. Similar effects may be seen in other Great Barrier Reef invertebrate species with comparable reproductive patterns.

Revision of methods for separating species of Protopalythoa (Hexacorallia : Zoanthidea) in the tropical West Pacific

The genus Protopalythoa Verrill, 1900 (family Sphenopidae) is defined and its status discussed. As with other zoanthid genera, species are difficult to separate by traditional methods. We have developed a rigorous approach to quantitative data derived from measurements and meristic characters. In support of genetic evidence, we have utilised the relationship between number of septa and polyp column diameter, and the capsule length of nematocysts (holotrichs and p-mastigophores), together with the presence or absence of basitrichs in the cnidom. Using this approach, the species Pr. mutuki (Haddon & Shackleton, 1891b) and Pr. heliodiscus, sp. nov. are separated, described and illustrated. Protopalythoa mutuki is interpreted as a normal micro-carnivore with autotrophic capability, while Pr. heliodiscus sp. nov. is an obligate autotroph with little ability to capture prey. The latter is the only zoanthid known to have vertical transmission of zooxanthellae.

High genetic variability and patchiness in a common Great Barrier Reef zoanthid (Palythoa caesia)

Allozyme electrophoretic analysis of seven polymorphic enzyme loci suggested that 1261 samples of Palythoa collected along 1765 km of the Great Barrier Reef during 1992–1993 were members of a single taxon, identified as Palythoa caesia Dana, 1846, with high external morphological variability and possibly encompassing several previously described species. Populations were slightly genetically differentiated (standardized genetic variance, FST=0.010, p<0.05), but there was no evidence of isolation by distance, or of the particular genetic distinction of geographic sets of reefs such as the Swains, as has been observed in other invertebrates. Differentiation was thought to be the result of random selection acting on patches of larvae, not the consequences of long-term reproductive isolation of populations.

Genetic evidence for cryptic speciation in the marine bryozoan Alcyonidium gelatinosum

The common subtidal marine bryozoan Alcyonidium gelatinosum (L.) is one of the most confusing species of a taxonomically very difficult genus. It shows massive variation for almost all morphological characters. A number of sympatric colonies collected off the coast of Lundy (Bristol Channel) could be clearly divided on morphological grounds into two quite distinct types. Using starch gel electrophoresis the products of a range of genetic enzyme loci were compared between the two morphotypes and also between these and other Bristol Channel populations of A. gelatinosum. Minimal differences were observed between one Lundy morphotype and the other allopatric populations, which it was consequently concluded were conspecific. The second morphotype showed considerable genetic differentiation [genetic identity (Nei, 1972) =0.417] from the other Lundy population, with no common alleles at some loci. The probability of obtaining the observed results by chance in a single population is significantly low (P<10−14). It is therefore concluded that the two morphotypes of A. gelatinosum from Lundy are not conspecific.

Reproduction in Zoanthidea (Anthozoa: Hexacorallia)

Summary The state of knowledge concerning the reproductive biology of zoanthids is reviewed. Colonies in the suborder Macrocnemina, with few exceptions, are gonochoric; those in Brachycnemina are probably basically hermaphroditic but accurate sampling in colonies that fragment is difficult and exceptions by species or populations occur. Two deepsea species of Epizoanthus breed continuously but temperate shallow water zoanthids, and those in the tropics outside ∼15° latitude are iteroparous with spawning in summer or when temperatures are rising. Oogenesis occurs over a few months but the spawning event may be very brief. Parazoanthus parasiticusin Bermuda spawns mainly on 1–2 nights in late August or September, apparently at full moon;Protopalythoa sp. 1, and probably Palythoa caesia in the Great Barrier Reef spawn 3–5 nights after full moon in November (early summer). These species with short, precisely defined spawning seasons may undergo a second, smaller-scale spawning event one month after the first....

Storage of exogenous sperm by the compound ascidianDiplosoma listerianum

Experimental evidence is presented for the storage of exogenous sperm by zooids of the simultaneously hermaphroditic didemnid ascidianDiplosoma listerianum. Cross-fertilized zygotes can be produced up to 1 mo after a ramet is placed in reproductive isolation. Previously published observations, considered in the light of the present data, suggest that the lumen of the ovary is the site of sperm storage. This is believed to be the first report of post-transmission storage of sperm by an ascidian. Existing records of storage of exogenous sperm in other taxa rarely involve aquatic animals in which spermatozoa are transmitted between mates singly through the surrounding water, as inD. listerianum. The potential significance of sperm storage in the mating systems of sessile aquatic invertebrates is discussed.

Semper's (zoanthid) larvae: pelagic life, parentage and other problems

Sempers larvae were obtained from <300 out of ∼1800 plankton tows taken in the worlds oceans (1964–1993). Zoanthellae (larvae of Sphenopidae) occurred at 217 stations and zoanthinae (larvae of Zoanthidae) at 86, the two larval types showing distributions clearly delimited by a minimum sea temperature (∼22 °C for zoanthellae, ∼18 °C for zoanthinae; a statistically significant difference, P<0.001). Length of formalin-fixed zoanthellae was ∼2–8.6 mm and of zoanthinae ∼1.5–5.9 mm. Endodermal zooxanthellae were present in 9/24 zoanthinae but in no zoanthellae (of 19). Three larvae contained an endo-commensal/parasitic amphipod. Septa were externally visible in larger zoanthinae and were counted in transverse sections of other larvae, a majority of which (both kinds) had 12 septa, the normal maximum. The pattern was brachycnemic in 40/43 larvae and anomalous (but non-macrocnemic) in three. If macrocnemic genera reproduce by Sempers larvae, they should have been represented in such a large sample. The distribution of adult Epizoanthus was examined: many species are deep sea (recorded down to ∼5000 m) but shallow-water species are relatively plentiful in, for example, the Adriatic and North Seas. No Sempers larva has ever been recorded from either. Some Parazoanthus species also occur in shallow water, especially associated with western Atlantic reef sponges. If they produce Sempers larvae, these have never been found. It is probable that macrocnemic zoanthids settle from planulae that do not develop into recognizable zoanthellae or zoanthinae.

Larval development of a tropical zoanthid (Protopalythoa sp.)

Summary For the first time the embryology and larval development, from spawning to settlement, are described for a zoanthid. The externally fertilized eggs of Protopalythoa sp. contain zooxanthellae and develop by radial cleavage to produce a hollow blastula. Gastrula formation takes place by a mechanism similar to invagination and proceeds by a flattening of the blastula followed by progressive concave/convex deformation.—After two weeks the larvae have developed to the stage of zoanthella, and are able to settle after 17 to 19 days.

Toxicity and palatability of some British ascidians

Bioassays were conducted to test crude extracts of nine temperate-water ascidians for bioactivity against a range of marine organisms. The extracts generally produced moderate to weak effects, with the exception of those from the ascidian Clavelina lepadiformis, which consistently produced high mortalities against invertebrates. Some of these were then tested in simple palatability assays for feeding-deterrent effects. In general, no strong effects were detected for assays with shore crabs (Carcinus maenas) and fishes (the goby Pomatoschistus minutus and the grey mullet Chelon labrosus). Feeding-deterrent effects were observed with Carcinus maenas for the extracts of Aplidium proliferum, Botryllus schlosseri and Dendrodoa grossularia. It is postulated that these ascidians are unpalatable, but no toxic effects are evident.