Thursday, May 30, 2013

Two types of Aeolidia papillosa larvae

This is a picture of an adult nudibranch Aeolidia papillosa, found on the Charleston docks near OIMB. As with many other nudibranchs, A. papillosa package their eggs a few per egg capsule, the capsules embedded in a gelatinous ribbon, which are deposited as egg masses (pictured below). Early development is encapsulated, and embryos begin to move inside the capsule, before they hatch as shelled veligers (bottom picture).

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.

Reproduction in monogonont rotifers

This is a marine rotifer from the genus Synchaeta (according to a key in Wallace & Snell 2010). Rotifers are a phylum of small, mostly fresh-water invertebrates. This adult individual is 0.7 mm long. When I found it in the plankton sample, it was carrying two egg capsules attached to its foot (a little stalk at posterior end of the organism-see photo). While I watched it, one of the egg cases ruptured, releasing a fully formed and seemingly completely functional miniature (~ half a millimeter long) rotifer. The empty case remained attached to the foot of the adult, while the newly hatched rotifer (shown below) began swimming immediately.   

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, the vast majority of which are born post-embryonically. 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

Early development and spermatophore of the phoronid Phoronopsis harmeri

Phoronopsis harmeri is a member of Phoronida, a relatively small phylum of sessile, tube-building marine worms. Phoronids release sperm in packets called spermatophores, which probably drift in water currents before landing on the lophophore of another individual. In P. harmeri and some other species, these spermatophores are equipped with a mucoid spiral “sail” (Zimmer 1997). The spermatophore of P. harmeri, sail included, is pictured here. 

There is some controversy regarding the early cleavage pattern of phoronids. While some researchers have reported radial cleavage, others have reported spiral cleavage, biradial cleavage, and “derived spiral” cleavage (reviewed by Pennerstorfer and Scholtz, 2012). In spiral cleavage, the blastomeres of the early embryo divide along a plane that is oblique to the animal-vegetal (A/V) axis, and the cells are not stacked directly on top of each other but are offset from one another. In contrast, blastomeres undergoing typical radial cleavage divide either parallel or perpendicular to the A/V axis, and the cells stack directly on top of one another (Ruppert et al. 2004). Pennerstorfer and Scholtz (2012) quantified the number of embryos exhibiting spiral-like cleavage in Phoronis muelleri and the degree of tilt of the plane of division. They concluded that, beginning with the third cleavage, early cell division in P. muelleri tends to occur in a spiral pattern, although there is a high amount of variability among embryos of this species.

Pictured at left are examples of the 8-cell and 16-cell stage of P. harmeri, whose early development I observed while taking the Embryology course at OIMB. The sister cells in the 8-cell embryo (side view) appear to be nearly on top of one another, so the third cleavage (at least in some embryos) of this species is not obviously spiral. However, the four polar blastomeres in the 16-cell stage pictured here are noticeably offset from their four sisters, though not every embryo in our class culture looked like this at the 16-cell stage. At least some embryos in this culture exhibited alternating (dextral vs. sinistral) divisions (S. Maslakova, pers. comm.), as in typical spiralians. Is early development in P. harmeri also somewhat spiral?

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.

Saturday, May 25, 2013

Development of Trimusculus, a limpet-like marine snail

Trimusculus reticulatus, or the button-snail, is an unusual marine gastropod that looks like a limpet (i.e. a member of the Patellogastropoda) but is not. It is, in fact, a pulmonate gastropod (it has a lung) - a relative of the common garden snail. Pulmonate species are generally terrestrial or live in fresh water, but some are marine like T. reticulatus. The adults of T. reticulatus are found in the high intertidal underneath the overhangs of boulders or on the roofs of sea caves.

I collected the specimen pictured here from Strawberry Hill, Oregon. As shown, the shell of adult T. reticulatus is about the size of a penny.  The shell is shallow and uncoiled, like that of a true limpet, but unlike shells of most other gastropods.

T. reticulatus lays its eggs in petal-like gelatinous masses around itself (Johnson 1968). These “petals” were detached during collection. Each “petal” is about 5-10 mm in diameter and contains dozens of embryos; the picture has a 5 mm scale bar. The color is indicative of “petal’s” age.  The whitish masses contain embryos in earlier stages of development. In light brown masses the veliger larvae are about to hatch.  The “petals” pictured here were produced sequentially by a single individual.

This is an embryo from one of the whitish ”petals”.  Note the two polar bodies (at four o'clock) trapped inside the oval egg capsule.  Polar bodies mark the animal pole of the embryo.  This embryo is gastrulating (forming the primary gut). The indentation at the opposite (vegetal) pole is the blastopore – the opening of the primary gut, which will become the mouth of the larva.

This is a hatched veliger larva.  Unlike the adult, the larva has a typical coiled shell – a vestige from the evolutionary past. One can also see the velum – the ciliated larval appendage used for swimming – pulled into the shell, the foot with an operculum, and the statocyst – a paired larval balance organ – that looks like a little marble inside a spherical capsule.

Johnson, K.J. (1968) Studies on the feeding, movement, and respiration of the pulmonate limpet Trimusculus (Gadinia) reticulatus (Sowerby) with notes on general morphology, egg masses, and veliger larva. Unpubl. student paper (abstract). Marine Science Center, Newport, Oregon

Wednesday, May 22, 2013

Unidentified wild-caught Müller’s larva

On April 17, 2013 I took a plankton tow off a dock in Charleston, OR. While sorting plankton I found a Müller’s larva (pictured here). The Müller’s larva is found in free-living marine flatworms from the order Polycladida (class Turbellaria), both in suborder Cotylea and Acotylea (Smith et al 2002). When a Müller’s larva hatches from the egg it usually has 8 lobes, though larvae of some local species have only 6 lobes (e.g. Pesudoceros canadensis). The entire larval body is covered in cilia, but the lobes usually bear longer cilia. Müller’s larvae may be either transparent or opaque and are often brown, mine was greenish. They normally have three eyes to begin with, two subepidermal and associated with the brain and a third, epidermal eye (Smith et al 2002). The two subepidermal eyes can be seen in this picture near the anterior end (upper right). The eye spots often increase in number after metamorphosis (Martín-Durán et al 2012). The larval lobes are reabsorbed during metamorphosis. 

My larva had at least 4 lobes, more likely 6 (two ventro-lateral, two dorsal-lateral, and two unpaired). One can see clearly two of the lobes in focus in these pictures. The lobes are often retracted or flattened when the larva is under the coverglass. Note the dark green branched gut inside the larva. Polyclads, in general, are characterized by a multilobed gut. Many Müller’s larvae apparently require food in order to reach metamorphosis but only a few species have been observed to feed in the laboratory on microscopic algae (Rawlinson 2010 and references therein).

Martín- Durán, J. M. and Egger, B. 2012. Developmental diversity in free-living flatworms. EvoDevo, 3:7.

Rawlinson, K. 2010. Embryonic and post-embryonic development of the polyclad flatworm Maritigrella crozieri; implications for the evolution of spiralian life history traits. Frontiers in Zoology 7: 12.

Smith NF, Johnson KB and Young, CY. 2002. Platyhelminthes. In: Atlas of Marine Invertebrate Larvae. Edited by C. M. Young. Academic Press. New York.

Bowling for Calliostoma

Calliostoma ligatum is a gastropod found on the rocky intertidal of the Pacific coast. Calliostoma are dioecous (i.e. have separate sexes) and, like most other archaeogastropods, free-spawn their gametes. Conveniently, C. ligatum can be easily induced to spawn in the lab if mildly harassed. 

The two individuals pictured here were collected intertidally at the South Cove of Cape Arago near Charleston, OR. I placed them out at room temperature in small bowls filled with sea water, set the bowls on my desk to allow the water to warm up, and flipped the snails upside-down. When they regained their footing (takes about a minute or two) I would flip them again. I repeated this for about fifty minutes (it usually only takes as little as half an hour) after which the snails began to spawn. The first picture shows a male actively releasing sperm, hence the cloudy water in the bowl.

This is a female which has recently released a mass of large 220 µm greenish eggs loosely connected by a jelly. She is flipped on her back, so one can see the bright orange foot rimmed with brown, the operculum, two cephalic tentacles and four pairs of epipodal tentacles. 

The third picture shows an egg two hours after fertilization. You can see here that C. ligatum eggs are surrounded by an egg envelope (a distinct membrane near the surface of the egg), and a thick bilayered jelly coat. Note the two tiny clear cells (at 6 o’clock) trapped inside the egg envelope. These are polar bodies, the tiny sister cells of the oocyte, which contain the DNA discarded during meiosis. As in many other marine invertebrates meiosis in Calliostoma is completed after fertilization. Presence of the polar bodies is a sure sign of fertilization. Note a single needle-like spermatozoan still trapped in the inner layer of the egg jelly (at about 1 o'clock). Clearly it did not make it!

Planktotrophy versus lecithotrophy

Many marine invertebrates undergo indirect development, a kind of life history that includes a larval stage distinct from the adult. Two types of larval development are distinguished based on the source of larval nutrition.  Lecithotrophy, meaning “feeding on yolk”, refers to development with a non-feeding larva, which depends on the egg’s yolk reserve supplied by the mother. Planktotrophy, meaning “feeding on plankton” refers to development via a larva that must feed in the plankton in order to develop to metamorphosis. 

Species with planktotrophic development produce many small energy-poor eggs with adequate nutrient reserves for the development of a feeding larva. These larvae must begin to feed immediately upon acquisition of feeding structures since they rapidly deplete their relatively insignificant yolk stores. Lecithotrophic species, on the other hand, produce fewer but larger eggs. These large yolky eggs develop into non-feeding larvae which usually lack feeding structures (e.g. mouth, gut, ciliary bands for capture of food particles). Lecithotrophic larvae spend comparatively less time in the plankton, and begin to feed after metamorphosis. Here, I pictured eggs of two sea star species with contrasting development. The opaque orange egg on the left (~1.2 mm in diameter) belongs to Mediaster aequalis, a lecithotroph, while the relatively transparent oocyte on the right (~150 µm) belongs to Pisaster ochraceus, a planktotroph. 
This picture depicts a 3-week old pluteus larva of Dendraster excentricus (sand dollar) which develops from a 125 µm egg and exemplifies planktotrophic development. Planktotrophic larvae, like this pluteus, have a functional gut, and one can often see food in their stomachs. Note several green single-celled algae (Dunaliella) inside the larval stomach. Feeding larvae typically spend weeks to months in the plankton before metamorphosis. This larva has begun to form the juvenile rudiment - an unpaired sack-like structure visible between two of the larval arms on the left side of the image. This is also the left side of the larva, which is viewed from the dorsal side (anterior at upper left). With plenty of food Dendraster plutei may reach metamorphopsis after 3-4 weeks at ambient sea temperature.

The last picture depicts a wild-caught lecithotrophic doliolaria larva of Cucumaria miniata a local species of sea cucumber, that develops from large yolky eggs (~ 500 µm). Its planktonic development lasts only about two weeks and the larvae lack the capacity (and the need) to feed.

Monday, May 20, 2013

Early cleavage in echinoderms

Early development in echinoderm embryos follows a regular and predictable pattern. The first and second cleavages are equal and meridional (cut along the animal-vegetal axis of the egg). The third cleavage is equal and equatorial, i.e. perpendicular to the plane of the first two divisions, resulting in two tiers of 4 equal-sized cells. The nature of the fourth cleavage (from 8 to 16 cells) differs between echinoids and asteroids.

The first picture shows a side view of a 16-cell stage of the sea star Pisaster ochraceus (animal pole, marked by the two polar bodies, is at about two o’clock). As you can see here, all cells (12 of 16 are visible in this focal plane) are equal in size.

The second picture shows a side view of a 16-celled embryo of the sand dollar Dendraster excentricus (animal pole up). As in most other echinoids, the four cells of the animal pole tier divided equally and meridionally, producing a tier of eight equal-sized cells (called mesomeres). Four mesomeres are clearly visible in this focal plane. The blastomeres of the vegetal tier divided equatorially and unequally producing four smaller cells at the vegetal pole, called micromeres (two are visible), and four larger cells called macromeres (two are in focus). The micromeres of echinoids give rise to the larval skeletogenic cells. In comparison, asteroids, have no micromeres and no larval skeletal spicules (see a post by Nick Hayman).

Other differences in early development of the two classes are also apparent here. In the asteroids, developing oocytes in the adult ovary are arrested at prophase I. Meiosis is normally completed after fertilization (which traps the polar bodies inside the fertilization envelope). This is why you can see the polar bodies in the top picture. On the other hand, in echinoids oocytes complete meiosis in the ovary (prior to fertilization), thus no polar bodies can be seen inside the fertilization envelope. 

Friday, May 17, 2013

Development of the nudibranch Janolus fuscus

Nudibranchs are arguably some of the most flamboyant animals of the Pacific coast. Janolus fuscus (shown here), a sub-tidal species that is commonly found on the floating docks in Oregon, is no exception. Surprisingly, it’s brilliant and delicate morphology is a beautiful display of camouflage. Janolus fuscus is remarkably difficult to spot when it is on its primary food source, the bryzoan Bugula pacifica. Its distinctly lined cerata break up the nudibranch's outline, making the body less conspicuous. Its translucent flesh allows the nudibranch to blend in. The two characteristics seem gaudy until the nudibranch is viewed in it’s native habitat, where they act as a veil. Often, the egg masses of J. fuscus are more visible than the adults.

Nudibranchs are simultaneous hermaphrodites, so a mature adult can mate with any other mature adult of the species. Nudibranchs deposit eggs in characteristic gelatinous masses. An egg mass of J. fuscus depicted here is a cylindrical, jelly-filled cord with egg capsules
arranged in a single row, resembling white beads on a string.

As you can see, each capsule contains numerous embryos. The embryos undergo spiral cleavage and develop into trochophore larvae at around the forth day after egg deposition.

Two days later, the trochophores develop into veligers, with a shell and velum. Veligers hatch after about two weeks of development, and spend about six weeks in the plankton feeding on unicellular algae. Then the larvae settle on a bryzoan (preferably Bugula pacifica) and undergo meta-morphosis to become a juvenile (e.g. loose the shell and become elongated). Unlike the larvae, the juveniles feed on bryzoan lophophores. The juveniles grow into mature adults, all the while camouflaged by the bryzoan upon which they depend.

M. Wolf. 2012. The reproductive ecology of a northeastern Pacific nudibranch, Janolus fuscus, with an examination of its endoparasitic copepod, Ismaila belciki, Biol. Bull. 222: 137–149.

Tuesday, May 14, 2013

Egg capsule and larva of the intertidal snail Littorina scutulata

Pictured here is the adult, egg case, and veliger larva of Littorina scutulata. L. scutulata is a small (up to 1.5 cm shell height) gastropod, common in the mid- to high-intertidal of the west coast  of North America (Kozloff and Price 1996, Reid 2007).  The shape of the penis, tentacle pattern, and characteristics of the egg capsule distinguish L. scutulata from its co-occurring sister species, L. plena.

L. scutulata females typically lay transparent egg capsules shaped like a stack of two unequally sized disks. The egg capsules of L. plena females look similar, except that the upper and lower disk are equal in diameter (Hohenlohe 2002). The upper picture shows a side view of the L. scutulata egg capsule, in which the distinctive shape is evident. The lower picture focuses on the embryos, each of which develops in its own membrane (Buckland-Nicks et al. 1972).

The veligers hatch from the egg capsule about a week after egg laying, and measure about 160 micrometers long. They are competent to settle after spending four weeks in the plankton and reaching about 300-360 micrometers in length. The veliger shown here is about 210 microns long. One can see the velum and its two ciliary bands (the more prominent and more anterior prototroch, and the less prominent metatroch). Note also the larva’s paired eyes and statocysts. The statocyst is a balance organ that looks like a round vesicle with a marble inside. 

Reid, D. 2007. Littorina. In: Intertidal Invertebrates from Central California to Oregon. Carlton, James T. ed. University of California Press. Berkeley and Los Angeles, California. 2007.

Kozloff, E. N. and Price, L. H. 1996. Phylum Mollusca: Class Gastropoda. In: Marine Invertebrates of the Pacific Northwest. Kozloff, E.N. ed. University of Washington Press. Seattle and London.

Buckland-Nicks, J., Chia, F-S, and Behrens, S. 1972. Oviposition and development of two intertidal snails, Littorina sitkana and Littorina scutulata. Canadian Journal of Zoology. 51: 359-365

Hohenlohe, P. A. 2002. Life history of Littorina scutulata and Littorina plena, sibling gastropod species with planktonic larvae. Invertebrate Biology. 12 (1): 25-37.

Filopodia of mesenchymal cells in gastrulating echinoderms

These pictures show gastrulating echinoderm embryos.  I was fascinated by the role filopodial processes (small-thread-like-extensions) of mesenchymal cells play during echinoderm gastrulation.

The first two pictures show a 30-hour old gastrula of the sand dollar Dendraster excentricus. During echinoid gastrulation the first cells to enter the blastocoel (the spatial gel-filled cavity inside the embryo) are called primary mesenchyme cells. Then, the archenteron (the primary gut) buckles in (invaginates). Through a series of cell rearrangements called convergence and extension, the archenteron (a large tube visible here inside the embryo) elongates and extends towards the roof of the blastocoel. In the meantime the primary mesenchyme cells form two groups and begin to secrete calcareous larval spicules (two tri-radiate spicules are visible here on either side of the archenteron). Approximately two thirds of the way up, a second group of cells ingress into the blastocoel from the roof of the archenteron.  This is where the filopodia come in.

This image is a close up view of the tip of the archenteron, showing a secondary mesenchyme cell with a long conical projection reaching toward the roof of the blastocoel. These processes attach to the roof of the blastocoel and aid in the final extension of the archenteron.

This picture shows a 62 hour old gastrula of a sea star Pisaster ochraceous. Asteroids lack the primary mesenchyme cells, so the mesenchyme cells that ingress at the tip of the archenteron are the only mesenchyme they have. Note the lack of primary mesenchyme cells in the blastocoel and the numerous branched filopodia on the mesenchyme cells at the tip of the archenteron.  

The epitokes of Autolytus: swarming to mate

On April 24th, the spring 2013 Embryology class collected invertebrates in the large boat basin in Charleston. We scooped up rapidly swimming polychaete worms from the water's surface. These members of the Autolytus genus from the Syllidae family are benthic dwellers (benthos is Greek for “depths of the sea”). To reproduce, they undergo an incredible reproductive transformation known as epitoky (epitokos, Greek for “fruitful”). Depending on the species, either the entire worm or its posterior end develops chaetae for swimming and large eyes, and the gut is degenerated as well. This epitoke then swims up to swarm and mate. In cases where only a portion of the worm is transformed, the rest of the worm resumes its normal benthic life, but the epitokes usually die after spawning. For the subfamily Autolynae, the female epitokes are called sacconereis. We most often see bright orange epitokes, though the picture at left shows a brilliant blue-green sacconereis about 30 mm long.

Swarming in epitokes may be triggered by such factors as rising water temperatures and the lunar phase. The male circles the female, wrapping her in a mucous trail of sperm. The eggs are fertilized as they emerge, and either undergo planktonic development or, as is the case in the species shown here, collect in several brood sacs on the ventral side of the sacconereis. Even after larvae hatch from the brood sack, they hang on to the female. Can you see their eyes staring out?

This larva from one of the brood sacks of the above individual is a 500 µm long nectochaete, with chaetae bristling from three segments like legs on a cartoon stick figure. The apical tuft at its anterior end contains the sensory cells that may be involved in detecting the chemical cues during larval settlement. Once out of the brood sack, the larva may propel itself using several transverse bands of cilia. Three of the ciliary bands are visible here - one on its head, one just anterior to the first pair of chaetae, and one at the posterior end. The chaetae may also aid in locomotion. Four large ocelli (eye spots) are arrayed anterior to its large muscular pharynx, the clear oval shape anterior to the gut. The gut itself is filled with greenish yolk droplets packed into the egg by the mother. There will be no need for this nectochaete to feed during its brief life in the plankton. It will live on yolk reserves until settling out to begin its benthic existence.

Dorresteijn A & Westheide W. (2010). Reproductive strategies and developmental patterns in annelids. Dordrecht London: Springer.

Rouse G & Pleijel F. (2001). Polychaetes. Oxford New York: Oxford University Press.

Rouse G & Pleijel F. (2006). Reproductive biology and phylogeny of Annelida. Enfield, NH: Science Publishers.

Thursday, May 2, 2013

Birefringent structures in marine invertebrates

If one treats light as a transverse wave, as per Maxwell’s equations, a beam of unpolarized light can be thought of as a collection of these waves oscillating in all planes (Bennett, 1950). A polarizer placed over this ray would then block the transmission of all but one plane of oscillation. Normally a second polarizer, called an analyzer, placed perpendicular to the first would then remove that last plane of light, allowing no transmission. However, if that light is first passed through an anisotropic medium, that is one that is highly ordered, it causes light to refract in a secondary, angled path in addition to the one that will be cancelled out by the analyzer. This double refraction is called birefringence (Inuoe, 1970). This effect can be seen in the luminescence of various calcareous structures of marine invertebrates.

The image at left is a four-week old pluteus larva of the sand dollar, Dendraster excentricus, that I cultured in the lab. The larval spicules of the pluteus, which are made primarily of calcite, an anisotropic material, are illuminated. The larva’s left postoral arm can be seen in the plane of focus, one of its six arms at this stage of development.
Here, the two rings of calcite larval ossicles of a doliolaria, the non-feeding planktonic larva of a sea cucumber, can be seen to be birefringent. These ossicles will eventually be embedded in the epidermis of the adult holothuroid, toughening the exterior of the animal.

This picture shows the tri-radiate calcite spicules of the sea sponge, Leucilla nuttingi. These three-pointed rods are imbedded in the organism’s mesohyl, and are characteristic of sponges in the class Calcarea. A section of the sponge’s exterior was excised and then allowed to dissolve in a solution of 8% bleach, leaving only its calcareous skeleton.

Bennett, HS. Methods Applicable to the Study of Both Fresh and Fixed Material, in McClung’s Handbook of Microscopial Technique, 3rd ed., Paul Hoeber, New York, 1950. pp. 591-677

Inoué, Shinya. An Introduction to Biological Polarization Microscopy. Program in Biophysical Cytology. Woods Hole, MA 1970.