Functional morphology, ecology, & competition
   
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Research study 1
 

SCUBA divers are familiar with the flexibilility of the solitary tunicate Styela montereyensis, and it is not unusual to see them whipping back and forth parallel with the sea bottom in strong waves and currents.  An interesting aspect of their growth is that the inhalent siphon in individuals living in protected bays is more commonly oriented sideways or upwards (photo on Right, black and white arrows indicate inhalent or branchial siphon), whereas in individuals living in high-energy open-coast habitats it is oriented in a backwards direction (photo on Left). 

There are several functional implications of this.  First, when the current bends Styela over, water is pushed into the branchial chamber primarily by dynamic force; hence, is energy-saving (drawing, lower Right).  Second, because of the flexible stalk this occurs regardless of the direction of the current.  Finally, the flow through the branchial basket is reinforced by water being drawn out of the exhalent siphon by viscous entrainment.  Young & Braithwaite 1980 Biol Bull 159: 428.

NOTE these are good ideas, but the authors now need to measure flow rates through the branchial basket at different current velocities and directions to see if their
hypotheses are correct

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Research study 2
 

Observations of orientation of Styela montereyensis on vertical pilings in Neah Bay, Washington show that initially the juveniles have random orientation, but as they grow their stalks rotate so that, as adults, their inhalent (branchial) siphons are orientated into the waves 50% of the time. Young & Braithwaite 1980 Biol Bull 159: 428.

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Research study 3
  Although competitive interactions of tunicates, both intraspecific and interspecific, have been well-studied in tropical systems, little work appears to have been done in west-coast systems. Some colonial tunicates, such as Didemnum spp. are known to have defensive chemicals. In general, colonial tunicates are highly robust overgrowth competitors, and only a few species can hold their own against them. Some examples of stand-off competitive interactions are shown in the accompanying photos.
 

Colonial tunicate, possibly Didemnum/Trididemnum sp. maintains distance from the stinging cells of the cup coral Balanophyllia elegans 0.3X
photograph of Didemnum-type colonial tunicate unable to overgrow a sea anemone Urticina lofotensis
Didemnum/Trididemnum sp. grows over most organisms on this loose, gravelly bottom, but piles up against the column of an anemone Urticina lofotensis 0.2X
photograph of cup coral Balanophyllia elegans in stand-off competition with a colonial tunicate Didemnum sp.
Cup coral Balanophyllia elegans forces a stand-off in its competition for space with the colonial tunicate Didemnum sp. 1X
photograph of colorful morphos of colonial tunicates
There may be only 2-3 species here (the rest being colour morphs), but it would take an expert to figure it out. Only a few tubeworms Dodecaceria sp. seem able to grow through the mat of colonial tunicates 0.2X
photograph of stand-off competition between a colonial tunicate and a sponge
Two tunicates (on the Left) and two sponges (on the Right) compete for space. Based on the well-defined separation between the white tunicate, and yellow and orange sponges, a stand-off has been reached 0.2X
photograph of a sea anemone Anthopleura elegantissima surrounded by colonies of tunicate Didemnum carnulentum
Colonies of Didemnum carnulentum surround a sea anemone Anthopleura elegantissiima. Based on its content of toxic chemicals, Dedemnum is likely to be the ultimate stand-off winner (the anemone may be imprisoned) 0.3X
   
 
photo graph of tunicate fouling on a boat propellor courtesy Charley Waters and Washington Department of Fish & Wildlife  
Several species of tunicates fouling a boat propellor.
Included are many solitary Styela sp. and possibly 2 compound
forms (light and dark shades of orange). The Styela, although
in some cases almost completely overgrown, seem not to have
their siphons overgrown by the superior compound competitors.
Alternatively, if they have become so overgrown, they may have
died and disappeared 0.3X. Photograph courtesy Charley
Waters andWashington Department of Fish & Wildlife.
 
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Research study 4
 

photograph of tunicate Ciona intestinalishistogram showing change in species richness after experimental removal of tunicates Ciona intestinalisIn areas of San Francisco Bay the non-native tunicate Ciona intestinalis lives in dense aggregations.  The question arises as to whether its aggressive colonising tendencies have negative effects on local species diversity.  This is tested by deploying 120 fouling panels (PVC) of different sizes divided into 3 treatments: 1) experimental removal, where new recruits of C. intestinalis are removed on a weekly basis, involving taking the panels out of water for a short period, 2) unmanipulated control, where panels are left in the water continuously, and 3) manipulated control, where panels are taken out of the water for the same length of time as in the experimental removal, but without removing tunicates. 

After 4mo, counts of species on the plates shows that absence of C. intestinalis produces communities with significantly higher species richness than the controls (see histogram). Species types vary as well, with certain species being found with Ciona and others being absent.  Predominant taxa are bryozoans (35% of all species found) and tunicates (33%).  Interestingly, of 40 species found, 32 are either nonindigenous or of uncertain indigenous status.  Larger size of settling panel also significantly increases species richness.  The authors conclude that the non-native C. intestinalis acts to depress local species diversity, while at the same time fundamentally altering the dynamics of community assembly.  The mechanisms involved appear to combine high recruitment leading to successful preemptive occupation of space by Ciona, and reduced recruitment of other species, possibly by chemical interference with settling larvae?. Blum 2007 J Exp Mar Biol Ecol 342: 5.

NOTE  sizes of the panels range in 4 increments from 49cm2 to 1177cm2

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Research study 5
 

graph showing effect of settlr size on resultant colony size in a colonial tunicate Botrylloides violaceusgraph comparing colony growth in the colonial tunicate Botrylloides violaceus in the presence and absence of previously established coloniesWhat kinds of intraspecific competitive interactions do juvenile colonial tunicates have and what effect does differing maternal investment have on the outcome of these interactions?  This is investigated by researchers at the Oregon Institute of Marine Biology using offspring of the encrusting colonial tunicate Botrylloides violaceus.  Colonies are collected from the field and larvae produced by them are settled either singly or multiply in flat dishes.  Larval size1 and subsequent settler size is measured and used as an assessment of parental investment. The dishes are placed out in the sea in various arrangements to determine the relationships between offspring size and survival, and offspring size and growth, and the effect of competition on these relationships over periods of up to 5wk.  Combinations include solitary settlers, and different-sized settlers growing in proximity either with contact between them, or with no contact. 

Results show, as expected, that settler size strongly and positively affects resultant colony size (see graph2 on Left).  Note that the data points are relatively “tight” after 1wk; by 5wk there is considerably more scatter (data not shown).  Settler density also significantly affects the growth form of the colony, with high-density colonies having zooids 20% smaller than low-density colonies.  When larvae of different sizes are settled together, leading to contact interactions, the larger settlers are significantly more likely than smaller ones to be aggressively dominant.  The result of such competition is not overgrowth, as expected; rather, colonies originating from smaller settlers tend to grow away from colonies originating from larger settlers and are comparatively thinner in size.  Finally, if larvae are settled onto areas containing established competitors, their survival is less than if they are settled onto areas lacking established competitors (63% vs. 93%) and their growth is significantly impeded (see graph on Right).  Overall, then, offspring size determines the outcome of competitive interactions.  The authors remark that their study shows for the first time in a benthic marine invertebrate that mothers3 have the potential to influence the outcome of competitive interactions involving their offspring.  Marshall et al. 2006 Ecology 87: 214.

photograph of colonial tunicate Botrylloides violaceusNOTE1  measured as the length of the body minus tail (termed “head” length by the authors).  Settler size is measured as the total area of the branchial basket from above.  Colony size is estimated as colony area as determined from photographs

NOTE2  the researchers replicate their experiments in 2 different areas.  Results from one replicate are presented here

NOTE3  the authors use the generic term “mother” for convenience to refer to the parental zooid producing the particular larvae being referred to; recall that tunicates are hermaphroditic


The burgundy-coloured colony at 6 o'clock may be Botrylloides
violaceus
:; identity of the other colonies is unknown 0.5X

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