Reproduction

Ovicell of R. graeffei.

Bryozoans will reproduce both sexually (through spermcast mating) and asexually (Bishop & Pemberton 2006).  All cheilostomes are hermaphroditic and thus all fertile autozooids within a colony have both ovaries and testes, though their activity is syncopated within an individual (known as protandry) to discourage self-fertilization (Ryland 1970).  Ovaries tend to be in the distal part of the zooid, the oocytes contained within the peritoneum (from which they have differentiated) along the zooid wall while the testes are near the funiculus.  Gametes leave the body via terminal pores in the tentacles (as there are no gonoducts) and fertilization may be internal or external.  During external fertilization eggs move down a mesothelial ciliated groove and into an orifice at the base of the two dorsalmost tentacles called the coelomopore.  Sperm settle out of the water column and adhere to the ciliated tentacles of other zooids, which move the sperm through the coelomopore (Silén 1966). 

While the archetypal bryozoan most likely produced cyphonaute larvae, the Cheilostomata have evolved to brood their embryos in an external ovicell, extrazooidal incubation chambers, which are likely advantageous over a brooding method that took up space within the zooecium and cause the polypide to degenerate (Ryland 1970). However, embryos can be seen within the calcified structure of R. graeffei (see photo) which can give the colony an orangey-pink color.  

The most common strategy for gymnolaemates is to produce a small numberof large, macrolecithal (large yolked) oocytes which develop in the ovicells. An ovum will squeeze through the supraneural pore and into the ovicell, which is typically sealable with the operculum (Ryland 1970).  This type of brooding results in short-lived lecithotrophic larvae (Ostrovsky et al.2009)(see DEVELOPMENT AND SETTLEMENT).  

The reproductive season for R. graeffei is less likely to be affected by seasonality than for a temperate species.  However, there is no data available on the timing of sperm release, though this could be measured using settlement rate on plates monitored throughout the year as a proxy.  A study using this method was conducted for various latitudes for the bryozoan Bugula neritina, which showed a prolonged intensified settlement period (March through the end of December) in Hawaii (20^o N), though colonization persisted throughout the year (Ryland1970).  It is possible that R. graeffei follows a similar pattern on Heron Island, which is of comparable latitude (23^o S), with elevated recruitment during summer months.  Likely the patterns for individual species depends on temperature, day length, and phytoplankton availability (Ryland 1970).

Ovicell of R. graeffei (clear, protruding from branch) and embryos (orange balls). Photo: Bridget Bradshaw, Heron Island Reef, 2013.

Embryo Density

A brief study was conducted on Heron Island to investigate the density of embryos (in terms of abundance) as a function of colony size in Reteporella graeffei.  The hypothesis that the density of embryos would be greater in smaller colonies was based predominantly on the observation that in larger colonies, the color (which appeared to come largely from the embryos) was stronger along the skirting edge of the colony and absent near the anchor point.  Embryos were obvious and easy to count using a tally counter, particularly because all zooids faced outwards and only on one side of the colony. 

Embryo of R. graeffei under microscope (10x). Photo: Bridget Bradshaw, Heron Island Reef, 2013.

Two separate surveys were conducted, one counting total number of embryos within a colony and the other taking subsamples of branches (as the total counts were time consuming and inaccurate).  A total of 14 colonies were collected from the Northern reef crest on Heron Island (23° 26′ 31.2″ S, 151° 54′ 50.4″ E) over a period of two days (25^th and 26^th September, 2013) and were kept in open jars set within seatables when not being counted.  For both studies, the total area of the colony was estimated by being placed in a petri dish and the lid marked with a 1cm² grid.  Area was then estimated to the nearest 0.25cm².  Total embryo counts were made for five colonies ranging in size from 2.5 cm² to 8.5 cm² with the largest colony also having the highest embryo count of 2352.  For the remaining 9 colonies, the embryos on 3 branches chosen at random were counted. Ideally, subsamples would have been taken in cm² plots, however the embryos were not individually distinguishable beneath the petri dish lid.  Thus counts from each branch were averaged and compared to the total colony area. 

Branch of R. graeffei, embryos visible. Photo: Bridget Bradshaw, Heron Island Reef, 2013.

The ratio of embryo count to colony area revealed no obvious relationship and in linear regressions, colony area was very loosely related to embryo density for both studies (r²=0.06, total colony count; r²=0.24, subsample).  Because of the small sample size and non-ideal sampling method, it is possible that these results do not reflect what is happening within the population as a whole.  However, it is also possible that the sexual reproductive activity really is independent of colony size, and instead relieson other biotic factors, such as proximity to other colonies and nutrient availability to abiotic factors such as water temperature, turbidity, orgrowing space that may differ between microhabitats.  These variables could be assessed in a combination of field and laboratory work. Additionally, in order to see differences in energy allocation to sexual versus asexual reproduction, one could measure the rate of zooid formation on the edge of the colony.