What was the aim of this research?
In bryozoans, the individual animal (colony) consists of many tiny zooids (Fig. 1), each having a ring of tentacles to feed on plankton. Zooids are < 1 mm long, whereas colonies are much larger. Most bryozoans have zooids with a hard skeleton and can be preserved as fossils. The bryozoan fossil record began ~500 million years ago (mya) when complex animals first diversified, and so bryozoans are useful for evolutionary studies.
Evolution is highly dependent on random historical events — for example, a large asteroid hitting the earth 65 mya apparently hastened or even caused the extinction of dinosaurs. What would happen if we could start again 500 mya and let evolution proceed under a different set of historical circumstances? Would plants and animals look similar those we see today? Would humans evolve? What about other worlds — is it believable that the Na’vi people in the movie AVATAR could look so much like humans on Earth?
Bryozoans can partly answer these questions. From the fossil record, we know that ~85–90 mya, cheilostome bryozoans with a soft, unprotected frontal membrane (anascasns) evolved frontal spines for protection; later, the spines became tightly fused together to form first a loose cage (cribrimorphs) finally a very tight shield (ascophorans) (Fig. ２). Since the soft frontal membrane necessary for protrusion of the tentacles became covered, ascophorans evolved a special chamber (ascus) beneath the frontal shield to perform the function of the frontal membrane. Biologists once believed that changes so complex could have evolved only once. Our laboratory recently discovered that the same stages evolved independently again in a modern genus (Cauloramphus) only ~12 mya, which demonstrates that evolution can repeat complex outcomes regardless of historical conditions.
Fig. 1. Living bryozoan zooid (left).
Fig. 2. Evolution of bryozoans 85–90 mya, reconstructed from fossils (right0.
C. ordinarius....................C. cheliferoides...............C. peltatus........................C. ascofer
Fig. 3. Evolution (left to right) of zooids in the modern genus Cauloramphus, repeating steps that occurred in ancient evolution. (SEM images of dried modern speciemens; in scale, each panel is ~1 mm wide).
What equipment do we use in our research?
The largest instrument we use is a research ship, not only to collect bryozoans but to gather primary data on their habitat and distribution. The specimens shown in Fig. 3 were collected by trawling from the RF/V Sea Storm (Fig. 4) in the Aleutian Islands (Alaska, USA) at depths of 100–300 m and bottom temperatures of 4–5°C. Bryozoan zooids are small, and to observe them in as much detail as possible, we use scanning electron microscopy (SEM) (Fig. 5). In SEM, dried specimens are coated with a very thin layer of a heavy metal like gold and placed in a vacuum chamber in the microscope. The specimen is scanned back and forth with a narrow electron beam. Electrons bounce off the metal surface of the object and are picked up by a detector. The image of an object is captured on film or electronically. The images in Fig. 3 are SEM images.
Fig. 4. RF/V Sea Storm (left).
Fig. 5. Scanning electron microscope, Hokkaido University (right).
Fig. 6. Our lab’s DNA sequencing facility at Hokkaido University.
Fig. 7. Evolutionary relationships among the species in Fig. 3, reconstructed by molecular phylogenetic analysis of the mitchondrial COI gene.
At the finest level of analysis, we extract DNA from single bryozoan colonies and determine the nucleotide sequences of selected genes. Fig. 6 shows the automated DNA sequencing facility that our laboratory uses at Hokudai. DNA sequences can then be used to reconstruct the evolutionary relationships among species and determine how long ago evolution occurred. For example, Fig. 7 shows the evolutionary relationships among the Cauloramphus species illustrated in Fig. 3, based on a 658 bp fragment of the COI gene. The genetic distance between C. ordinarius and C. peltatus was 18%, indicating (based on the known rate of evolution for COI) that the evolutionary changes shown in Fig. 3 began ~12 mya.
Dick MH, Lidgard S, Gordon DP, Mawatari SF (2009) The origin of ascophoran bryozoans was historically contingent but likely. Proceeding of the Royal Society B 276: 3141–3148
Dick MH, Mawatari SF, Sanner J, Grischenko AV (2010) Cribrimorph and other Cauloramphus species (Bryozoa: Cheilostomata) from the Northwestern Pacific. Zoological Science: in press.
Dick, MH, Hirose M, Mawatari SF (in review) Molecular distance and morphological divergence in Cauloramphus (Cheilostomata: Calloporidae). Proceedings of the 15th International Bryozoology Association Conference, Kiel, Germany, 1-8 August 2010.