Monday, June 17, 2013
O. lurida were harvested for a burgeoning West Coast market beginning in the late 19th century, but commercial stocks were depleted by the early 20th century. Habitat degradation and, in some cases, predation by introduced species and disease further contributed to the decline of this estuarine species (Polson et al. 2009). Many estuarine scientists are interested in restoring the species along the West Coast, as oyster beds provide habitat for a variety of other species and help stabilize the estuarine shoreline (e.g. Jackson et al. 2001, Ruesink et al. 2005).
Jackson, J. B. C., Kirby, M. X., Berger, W. H., Bjorndal, K. A., Botsford, L. W., Bourque, B. J., Bradbury, R. H., Cooke, R., Erlandson, J., Estes, J.A., Hughes, T. P., Kidwell, S., Lange, C. B., Lenihan, H. S., Pandolfi, J. M., Peterson, C. H., Steneck, R. S., Tegner, M. J., and Warner, R. R. 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293:629-638.
Polson, M. and Zacherl, D. 2009. Geographic distribution and intertidal population status for the Olympia oyster. Journal of Shellfish Res 28: 51-58.
Ruesink, J. L., Lenihan, H.S., Trimble, A.C., Heiman, K.W., Micheli, F., Byers, J.E., and Kay, M.C. 2005. Introduction of non-native oysters: Ecosystem effects and restoration implications. Ann Rev Ecol Evol Syst 36:643-689.
Strathmann, M.F., Kabat, A.R. and O’Foighil, D. 1987. Phylum Mollusca, Class Bivalvia. In Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast: Data and Methods for the Study of Eggs, Embryos, and Larvae. Strathmann, M. F. University of Washington Press. Seattle and London.
Friday, June 14, 2013
This is a recently hatched juvenile of the ctenophore (comb jelly) Beroe sp. At the aboral pole (up, opposite the mouth) there is a dome made of cilia - the equivalent of the statocyst capsule. Inside this dome you will note an aggregate of small marbles - that is the statolith. This aboral sense organ detects gravity and controls the movement of comb rows (ctenes) and the ctenophore’s orientation. The statolith rests on four tufts of support cilia, connected via ciliary grooves to the ctene rows. Tilting changes the gravitational pressure of the statolith on the support cilia, which ultimately controls the beating rate of of the ctenes. Differentially beating ctenes on one side allows the animal to turn and return to vertical position (Hyman 1940).
Here is another kind of balance organ in an ascidian tadpole larva. This tadpole was released by the colonial ascidian Distaplia occidentalis. In ascidian tadpoles the balance organ is called a statocyte, and occupies the bottom of a sensory vesicle, which also contains a light sensing organ - the ocellus or eye (Cloney et al. 2001). The statocyte contains a single melanin granule, the statolith. Both the statolith and the ocellus are visible on this picture. The ocellus is the black crescent shape, while the statolith is the black round shape underneath. These two organs are involved in the perception of environmental cues that drive ascidian tadpole behavior (Zega et al. 2006).
Wednesday, June 12, 2013
Sunday, June 9, 2013
This H. nudus zoea hatched six days ago to swim and feed in the plankton and go through several larval molts before metamorphosing into a benthic juvenile. Two compound eyes look out with a multi-faceted view of its surroundings, the mosaic of images from many ommatidia, or eye-lets. A distinctive dorsal spine projects from the top of its carapace, and a rostral spine is located anterior to the eyes. These spines may discourage predation. Note a thin-walled sac at the base of the dorsal spine, the zoea's beating heart. The segmented abdomen ends in forked telson.
At nine days old, this crab zoea is close to undergoing a molt. The process of molting, called ecdysis, is characteristic of a large clade of animals (including arthropods and nematodes) known as the Ecdysozoa. A hard exoskeleton covering the body of these animals must be shed to accommodate growth. During successive molts of zoea, the abdomen adds new segments, and pleopods bud from them. Setae develop on the telson and the maxillipeds, the thoracic appendages present in newly hatched zoea. Hemigrapsus passes through five zoeal stages before becoming a megalopa. This stage resembles the adult form with stalked eyes and five pairs of pereopods (walking legs). The next ecdysis will take the megalopa to the juvenile stage and a benthic existence for the rest of its life.
Saturday, June 8, 2013
This is a picture of the 5-day old planula larva of P. flavicirrata. As in other hydrozoans and scyphozoans, the planulae of P. flavicirrata are lecithotrophic - they are non-feeding, rather they depend on yolk reserves to reach metamorphosis. Planulae are ovoid in shape and uniformly ciliated. Many hydrozoan planulae develop muscles only after metamorphosis (S. Maslakova, pers. com.), but in my culture, planulae of P. flavicirrata were clearly contractile!
Believe it or not, this is the same individual as shown above. It has used its muscles to elongate. Hydrozoan planulae have two types of epithelial muscle cells that are separated by mesoglea (Gröger 2001).
A close up view of the same individual shows the two cell layers: the outer ectodermis and the inner endodermis, separated by a thin extracellular layer of mesoglea. Myoepithelial cells in ectodermis and endodermis have muscle fibers (not detectable without special staining) that run along the mesoglea.
Gröger, H and V. Schmidt. 2001. Larval Development in Cnidaria: A Connection to Bilateria? gensis 29 (3):110-114.
Mills, C.E. and J.T. Rees. 2007. Key to the Hydromedusae. In "The Light and Smith Manual Intertidal Invertebrates from Central California to Oregon". Edited by J.T. Carlton. University of California press. Los Angeles. Pp. 137-150.
Wrobel, David and Claudia Mills. 1998. Pacific Coast Pelagic Invertebrates A Guide to the Common Gelatinous Animals. Monterey Bay Aquarium and Sea Challengers: California.
Thursday, June 6, 2013
Cloney, R. A. (1978). Ascidian metamorphosis: review and analysis. In: Settlement and metamorphosis of marine invertebrate larvae. Chia, F.-S. and Rice, M.E. (eds). Elsevier. New York. pp. 255-282.
Flores, A. R. and Faulkes, Z. (2008). Texture preferences of ascidian tadpole larvae during settlement. Marine and Freshwater Behaviour and Physiology. 41(3): 155-159.
Sasakura, Y., Mita, K., Ogura, Y., and Horie, T. (2012). Ascidians as excellent chordate models for studying the development of the nervous system during embryogenesis and metamorphosis. Development, Growth, and Differentiation. 54(3): 420-437.
This is Clytia gregaria, formerly known as Phialidium gregarium. The instructor of our Embryology class collected many adult individuals of this hydromedusa from the plankton off F-dock in the Charleston Marina Complex. I like cnidarians and I wanted to follow the development in this species.
This is a gastrula stage. It is 24 hours old. The circle of cells inside the blastocoel is the developing gut of the embryo, otherwise known as the archenteron. The archenteron opens to the outside via the blastopore which will later develop into the mouth. Cells clustered at one end of the archenteron are the mesoderm cells which will form the muscles and coelomic sacs in the larva.
This is a young actinotroch larva that is ready to feed on microscopic algae. It has a complete gut with mouth and anus. The mouth opens under the anterior hood and leads into a spacious vestibule, which leads into the gut. Note a thickened region of epidermis directly overlaying the vestibule - this is the apical sense organ. Sandwiched between the apical sense organ and the vestibule is a thin-walled sac - this is the anterior coelomic cavity, the protocoel. The gut proper has two distinct compartments - the stomach, which occupies most of the actinotroch’s body, and the short hindgut that opens via an anus at the posterior end. The actinotroch will later develop a crown of tentacles posterior to the mouth which will assist in capturing food.
Here you can see how we collect sperm from a spawning male of the sand dollar. Injection of ~1 ml of 0.5M KCl into the body cavity of the sand dollar induces spawning by causing strong contractions of the gonad.
If the specimen has ripe gonads, after a few moments gametes (sperm or eggs) begin to emerge from the five small openings called gonopores on the aboral (opposite side from the mouth) surface. Sperm is cream colored, and eggs are pink. To collect eggs, we inverted a spawning female (oral end up) over a beaker full of filtered seawater. The eggs collect at the bottom of the beaker.
There are several ways to get fertilizable oocytes from sea stars. One way is to inject the body cavity with 1-methyl adenine (1-MA) solution (about 1 ml of 100 μM 1-MA in distilled water per 100 ml of body volume). 1-MA, also known as maturation-inducing substance, stimulates spawning, ovulation (release of oocytes from follicles), and oocyte maturation (completion of meiosis) in sea stars. The advantage of this method is that the adult remains intact. The disadvantage is that it will have completely spawned out, and won’t be useful for future embryological experiments for at least a few months. In order to use the same individual several times, one can take advantage of the sea star’s regenerative abilities. Instead of injecting 1-MA into the body of the intact individual, dissection of just a piece of ovary or testis is sufficient. For example, Patiria miniata, the bat star, tolerates biopsy very well, then promptly heals, so one can re-use the same individual many times. One can use a biopsy tool to cut a small window at the base of an arm, and pull a small piece of gonad out. However, some species, like Pisaster ochraceus do not heal well after biopsy, and heal much better if an entire ray is severed to remove gonads (apparently, the cutting of the radial nerve stimulates regeneration).
As shown here, one can use a razor blade to cut the ray and reveal paired gonads within the body cavity. Ovaries in P. ochraceus are pink or orange-ish, while testes are creamy white. Some oocytes thus dissected complete meiosis spontaneously and can be fertilized, however, most need some help in the form of 1-MA. Incubation of dissected ovaries with 1 μM of 1-MA in filtered sea water for about an hour stimulates ovulation and oocyte maturation.
Tuesday, June 4, 2013
The many small bumps (clearly visible in the bottom picture) cover the juvenile rudiment and develop into adult spines. The larger bumps of the juvenile rudiment are the developing rays of the seastar. Once the juvenile rudiment and the brachiolar arms are formed (which may be as early as 9-10 days) the larva is capable of metamorphosing. M. aequalis will only settle and metamorphose on a suitable substratum, such as a tube of the annelid Phyllochaetopterus. If no suitable substratum is available, larvae can delay metamorphosis for as long as 14 months (Birkeland et al. 1971).
Birkeland, C., Chia, F-S., Strathmann, R.R. 1971. Development, substratum selection, delay of metamorphosis and growth in the seastar, Mediaster aequalis stimpson. Biological Bulletin. 141:1, 99-108.
Saturday, June 1, 2013
The greenish ~ 265 micron eggs are loosely connected by jelly when released, are very delicate despite the protection of the chorion, and easily rupture when handled. Most of the eggs in our culture disintegrated after the first few days, but a few developed into planktonic planuliform larvae, one of which is depicted here - a week and two days after fertilization. Note the two large reddish eyes and conspicuous lipid droplets. Most hoplonemertean larvae have several pairs of relatively smaller eyes (e.g. see blog posts by Jenna Valley and Kirstin Meyer). The larva depicted here is 460 micron long.
Thursday, May 30, 2013
Many marine invertebrates are characterized by a particular type of development e.g. either lecithotrophic or planktotrophic (see Jon Gienger's blog post, Planktotrophy versus lecithotrophy). Interestingly, Aeolidia papillosa veligers hatching from the same egg capsule can be polytypic: some released as yolk-laden lecithotrophic larvae, and others as yolk-free planktotrophic larvae (Williams, 1980). Williams (1980) also noted that larvae that hatched without yolk reserves were, paradoxically, larger than those released with yolk reserves, although both types of larvae developed from uniformly small eggs. A simple explanation might be that because these two types of larvae develop within the same egg capsule, it is possible that the yolk-laden (slower developing) larvae are prematurely released from the egg capsule by their yolk-free (faster-developing) siblings. However, yolk-laden larvae hatched from egg capsules that did not contain any yolk-free larvae. What’s more, smaller larvae were apparently less likely to feed on unicellular algae (e.g. Chlorella, Dunaliella) than their larger siblings. Both yolk-free and yolk-laden veligers were present in the egg masses I looked at, in addition to yolk-laden trochophores, indicating that larvae were still developing.
One possible explanation for this polytypic development may be bet-hedging (varying strategy to increase the overall chances of offspring survival and success). Lecithotrophic larvae are expected to survive to metamorphosis better than the planktotrophic under conditions of scarce food, whereas planktotrophic larvae may be more successful when phytoplakton is abundant. Producing both types of larvae may be advantageous when phytoplankton has spatially and temporally patchy distribution (Williams, 1980).
Williams, L.G. (1980). Development and feeding of the larvae of the nudibranch gastropods Hermissenda crassicornis and Aeolidia papillosa. Malacologia 20:99–116.
Interestingly, rotifers can reproduce both sexually and asexually. One class, Bdelloidea, appears to lack males altogether (Wallace & Snell 1991). Sexes are separate in the Class Monogononta, to which the marine genus Synchaeta belongs. For the majority of the year, females produce offspring asexually by generating diploid embryos that develop without fertilization into females. Under certain conditions though, females produce haploid eggs. As in certain social insects (e.g. bees), if these eggs remain unfertilized, they develop into haploid males. If fertilized, diploid eggs can remain in a diapause (resting state) for up to 20 years (Fradkin 2007), and eventually develop into females who feed and grow before becoming sexually mature (Ricci & Melone 1998). These resting eggs allow the population to outlive adverse conditions (e.g. desiccation, freezing etc.). Haploid males are sexually mature at birth, do not feed and live a short life (usually about half as long as females) with the apparent sole purpose of fertilizing females via hypodermic insemination (Ricci & Melone 1998). Another fascinating fact is that most rotifers have constant cell numbers as adults (~ 1000) (Fradkin 2007). In other words, no cell divisions take place after embryogenesis is completed, and growth is accomplished solely by enlarging existing cells. In contrast, humans start out as one cell, but end up with trillions in the adult body, . Additionally, in some asexually reproducing rotifer genera, generational clones have progressively shorter life spans (i.e. daughters live a shorter life than their mothers, grand-daughters live even shorter, and so on), which eventually leads to extinction of the line (see King 1969 for a review).
A dearth of information about development, distribution, and ecology in NE Pacific rotifers leave members of this phylum prime candidates for future research.
Fradkin, S.C. (2007). Rotifera. Light's manual; intertidal invertebrates of the central California coast. 4th ed. pp 280-282. J.T. Carlton (ed.). University of California Press.
King, C.E. (1969). Experimental studies of aging in rotifers. Experimental Gerontology 4: 69-79.
Wallace, R.L., Snell T.W. (2010). Rotifera. In: Ecology and classification of North American freshwater invertebrates. 1st ed. pp 173-235. J.H. Throp and A.P. Covich, eds. Academic Press.
Ricci, C., Melone, G. (1998). Dwarf males in monogonont rotifers. Aquatic Ecology 32: 361-365.
Wednesday, May 29, 2013
Pennerstorfer. M. and Scholtz, G. 2012. Early cleavage in Phoronis muelleri (Phoronida) displays spiral features.Evolution and Development. 14(6): 484-500.
Rupert, E., Fox, R., and Barnes, R. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. 7 ed. Brooks/Cole.
Zimmer, R. 1997. Phoronids, Brachiopods, and Bryozoans: The Lophophorates. In Embryology: Constructing the Organism. Gilbert, S. and Raunio, A. Eds. Sinauer Associates, Inc. Sunderland, MA.