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Larval life

  Topics on reproduction include larval life considered here, and SPAWNING, BROODING, SETTLEMENT & METAMORPHOSIS, and EARLY JUVENILE considered elsewhere.
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Research study 1

histogram showing time of release of larvae of compound tunicate Distaplia occidentalisLight is an important cue for release and settlement of tadpole larvae.  In the colonial tunicate photograph of colonial tunicate Distaplia occidentalisDistaplia occidentalis larvae develop in special brood pouches, which are just the expanded distal portions of the oviducts.  Hundreds of larvae are released simultaneously from many zooids at dawn (see graph).  The tadpoles are at first photopositive and swim towards the surface, perhaps to ensure that their settlement will be in phytoplankton-rich shallow waters. A few moments later, they become photonegative and seek out shady spots on which to attach and metamorphose.  Watanabe & Lambert 1973 Biol Bull 144: 556.

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Research study 2

Tadpole larvae are often released by the parent when light appears after a period of darkness, such as at dawn. However, if local Didemnum species behave similarly to tropical members of the genus, the larvae will be released in late morning, swim for about 15min, and settle close to mid-day so they can avoid the harshest sunlit areas.  Olson 1985 Ecology 66: 30.


West-coast colonial tunicate Didemnum carnulentum
surrounding a sea anemone. Notice how the tunicates,
themselves known to be dominant space competitors, have
left room between themselves and the anemone 0.33X

photograph of sea anemone Anthopleura elegantissima surrounded by colonial tunicates Didemnum carnulentum
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Research study 3

Another environmental factor, gravity, may play a role in determining the final location of a settling site for some or most tadpole larvae.  In certain compound tunicates in Hawaii (Didemnum) the larvae are at first photopositive and swim to the sea surface.  Later, they become photonegative and seek out shady, dark areas of the sea bottom.  At about the same time, the larvae become geonegative which, even in the dark, leads to occupation of downward-facing surfaces.  Such locations not only provide protection from harmful sunlight (UV), but also minimise the risk of being overgrown by algae.  Hurlbut 1993 Mar Biol 115: 253.

NOTE  the study is included here only because several species of Didemnum inhabit west-coast shores and may have similar larval behaviour

NOTE  more commonly known as a negative geotaxis meaning movement opposite to the direction of gravity

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Research study 4
photograph of tunicates Corella inflata
A cluster of Corella inflata 0.6

Avoidance of light by the settling larvae is especially important graph showing survival of tunicates Corella inflata in light with and without UV componentin species such as Corella inflata that combine transparent tunics with high sensitivity to light.  In experiments where adults are kept in shallow trays and exposed to natural sunlight (see "sunlight plus UV" in graph) death occurs within a few days.  Even if UV-A and UV-B wavelengths are filtered out ("sunlight minus UV"), adults and juveniles still become severely damaged, and embryos fail to develop.  Bingham & Reitzel 2000 J Mar Biol Assn UK 80: 515.

after 5d in the dark individuals began dying (data not shown here) suggesting that conditions for survival in the laboratory are not good. This does not affect the validity of the other data, however


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Research study 5

Photonegative behaviour of tadpole larvae is common in many species of west-coast tunicates.  All of the following species, for example, exhibit the behaviour: Corella inflata, Ascidia paratropa, A. callosa, Styela gibbsii, Boltenia villosa,and Pyura haustor. A study at Friday Harbor Laboratories, Washington addresses the possible survival benefits of photonegative settling behaviour and, specifically, histograms showing survival of several west-coast tunicate species in shallow and deep waterasks the question: is the distribution of juvenile ascidians a product of larval settlement or of post-juvenile mortality?  The authors first culture eggs and larvae of the 6 species and settle the larvae onto small plastic or glass Petri dishes.  After keeping the juveniles for 1wk in the laboratory they place the dishes at 2 depths in the ocean (4.5m and 21m), half in shady and half in lighted conditions.  After 2wk, percentage survival of juveniles is assessed.  Overall, there is significantly higher survival of the juveniles in shady over lighted conditions, but the effect is highly species-specific (see histograms).  There seems also to be marginally better survival at deeper vs. shallow depths for some species but, overall, a depth effect is non-significant in the analyses.  Poor survival in lighted conditions in the shallow depth owes mainly to smothering by heavy growth of algae combined with subsequent grazing by herbivorous gastropods.  The data, at least for the shallow-treatments, support the idea that shaded substrata are refuges for juvenile ascidians.  The authors note that photonegative settling behaviour of the larvae will result in settlement at depth and in cryptic habitats.  They also comment that both behaviours equate to better juvenile survival, but recall that their data for depth effects do not support this observation.  Young & Chia 1984 Mar Biol 81: 61.

NOTE  another treatment included in the experiment is upward- vs. downward-facing dishes, but the upward-facing ones become heavily sedimented, killing most of the juveniles, and the results are not included here.  The study is conducted March-May

NOTE  the authors select a multi-factorial ANOVA design to analyse the data, but do not provide individual-species statistics

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Research study 6
  A tunicate larva, for example, of the drawing of tadpole larva of tunicate Distaplis occidentalis showing different spation motions involvedcompound ascidian Distaplia occidentalis (photo on Left) swims by flapping its broad tail back and forth, producing a wobbly, “tadpole”-like motion.  Because the tail flap in this species has stronger photograph of compound tunicate Distaplis occidentalisamplitude in the leftwards direction and actually beats obliquely from a leftward direction, it produces a yawing motion which, combined with a downward rolling motion caused by a more buoyant hind end, leads to an overall helical swimming trajectory in a clockwise direction.  Sloppy hydro-dynamics?  Or is some other function served, perhaps, as suggested by scientists who study helical swimming in ascidians, a way for the larva to get better directional sense to light with its single eye?  McHenry 2001 J Exp Biol 204: 2959; for a review of swimming mechanics of ascidian larvae see McHenry 2005 Can J Zool 83: 62.
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Research study 7
With its paddling muscular tail a tadpole larvae can swim quickly, but how do locomotory speeds match up to other west-coast invertebrate larvae that use ciliary beating, appendage thrashing, or body contractions to swim?  The locomotory rates shown on the Right are measured in the laboratory at 15°C. The authors note that larvae using muscles to paddle or thrash their way through the water are absolutely quickest (zoea and tadpole) or relatively quickest (relative to size, e.g., nauplius).  However, some ciliary swimmers, such as amphiblastula, are also comparatively quick.  Overall, though, ciliary swimmers do poorly, even if their small size is taken into account.  We conclude that muscles and appendages are good, not just for swimming, but for all other forms of locomotion.  This is demonstrated throughout evolution and manifested in the dominant life forms of arthropods and vertebrates.  Data are for lab tests averaged for several species, from a review by Chia et al. 1984 Can J Zool 62: 1205. table comparing locomotory speeds of various invertebrate larvae using different types of propusive mechanisms
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What is the function of the swimming in the larval examples given above?  Check out the ideas presented, then CLICK HERE for explanations.

The larvae locomote in order to eat. 

The larvae swim to find food.

Swimming is a means to chase down and capture prey.

The larvae swim to escape predators.

They swim to maintain position in the water column.

Tthe larvae swim to colonise new habitats.

The larvae swim to locate a spot to settle.

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