Feeding & growth
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  Research study 1
 

photograph of a sea pen Ptilosarcus gurneyi with feeding polyps retracted
Food of sea pens and sea pansies consists mainly of phytoplankton and detritus particles.  Interconnection of digestive cavities of the feeding polyps means that energy and nutrients gained by one polyp are shared with other polyps in the colony.

 

 

 

View of Ptilosarcus gurneyi with most of its
autozooids (feeding polyps) retracted 0.6X

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Research study 2
 

graph showing length of internal supporting rod in sea pens Ptilosarcus gurneyi with ageThe slender internal supporting rod of a sea pen is made of calcified protein.  In young colonies it can be seen as a whiteish-coloured line within the rachis.  To estimate age of a sea pen, the rod can be embedded in epoxy resin, hardened, sliced finely, and the annual growth rings counted.  Maximum age of sea pens in Puget Sound, Washington is about 15yr.  The author notes that a 24-cm tall sea pen has a supporting rod of 12cm. On the graph, then, a 30cm rod may be from a colony of 60cm height (equivalent to about 15yr of age). Birkeland 1974 Ecol Monogr 44: 211.

NOTE individual colonies of 1m in height, found in areas of Barkley Sound, British Columbia, may be in excess of 50yr of age

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Research study 3
 

graph showing feeding effectiveness of sea pens Ptilosarcus gurneyi in different current velocitiesphotograph of a juvenile sea pen Ptilosarcus gurneyi in a currentSea pens are classed as “passive” suspension-feeders.  As such, they must place their bodies directly into the current flow in order for the polyps to catch particles passing by, and there is no “hiding” from the currents even when the currents are strong.  Since one of the factors governing filtering rate is the volume of water processed, then the number of particles caught will increase with current velocity until drag forces begin to bend the sea pen’s body, thus decreasing the effective surface area of polyps presented to the flow (see graph on Right, plotted for a Ptilosarcus gurneyi colony of 8cm height).  At even higher current velocities, this bending combined with increasing inability of the polyps to catch particles racing by, causes capture effectiveness to drop off quickly.  Best 1988 Biol Bull 175: 332.  

NOTE  in contrast, “active” suspension-feeders are ones like bivalves and tunicates that pump water through a filter, or barnacles and porcelain crabs that rake particles from the water with fine-mesh sieves

Colony of about 10cm height being whipped around in the current 0.6X

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Research study 4
 

photograph of sea pansy Renilla amethystina (kollikeri)Sea pansies Renilla amethystina (kollikeri) in the laboratory have difficulty in capturing motile copepods even as small as 0.1mm in length.  Their natural diet seems to consist mainly of phytoplankton and equally small, suspended organic particles.  Use of a mucous net by Renilla to entrap particles has been described in an earlier publication, but this is thought to be unlikely in conditions of any degree of water turbulence. MacGinitie & MacGinitie 1968 Natural history of marine animals McGraw-Hill, NY; Kastendiek 1976 Biol Bull 151: 518; photo of R. amethystina courtesy Cabrillo Marine Aquarium, San Pedro, California.

NOTE  usually a quite colorful blue or mauve, albino colonies of R. kollikeri have been found in southern California.  Human 1973 Calif Fish Game 59: 89.

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Research study 5
 

photograph of colonies of soft coral Alcyonium sp courtesy McFadden 1986 J Exp Mar Biol Ecol 103: 1The soft coral Alcyonium sp.1 grows as small colonies in the low intertidal zone. Growth is by asexual reproduction (fission) producing aggregations of genetically identical daughter colonies. The colonies are able to creep around on the rocks, and a common configuration is for the small colonies to be evenly spaced out (photo on the Left). graph showing particle capture by different sized colonies of soft corals Alcyonium sp. in relation to current speedThe reason for the separation appears to be inter-polyp competition for food, so the smaller the colony size the better.

This is confirmed in laboratory tests at Friday Harbor Laboratories, Washington of feeding efficacy of different-sized colonies in relation to flow speed. The results show that per-polyp capture rate2 decreases as colony size increases (graph on Right).  The results suggest that this strategy may be an effective means of maximising nutrient intake per unit biomass of the colony. 

The author surmises that small outliers or "satellite" colonies act as “roughness elements” that reduce substratum-level current velocities, leading to turbulence eddies and increased food-particle dropout. This is tested by creating 3 different densities of colonies (single, 8mm-spaced, and 3.5mm-spaced), then measuring per-polyp food capture.  The histogram on the Left shows a representative sample3 of the data. Note that highest capture rates are realised by free-standing colonies and also that capture rates decline significantly with increasing density.  From these feeding data the author is able to predict4 the size at which a colony should undergo fission in a given current regime to maintain the most efficient ratio of polyps to biomass for most efficient feeding.  McFadden 1986 J Exp Mar Biol Ecol 103: 1.

NOTE1  colonies of this undescribed low-intertidal species, collected at Botanical Beach, British Columbia, are encrusting and small - usually no larger than 1.5cm diameter with about 100 feeding polyps.  The colonies are capable of slow locomotion, and can regulate their position with respect to neighbouring colonies, other organisms, and topography

NOTE2  the food particles are unhatched brine-shrimp Artemia cysts

NOTE3 author presents data for the 3 densities at 3 different current speeds. Shown here is a single set of data at a current speed of 0.22m . sec-1 as a representative set

NOTE4 could predictions also be made from inter-colony spacing distances in the field about current speed, wave direction, and so on?

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Research study 6
 

photograph showing sea whips Halipteris willemoesi from the Bering Seaphotogaph of cross-section of axial rod of a sea whip Halipteris willemoesiA study on Alaskan sea-whips Halipteris willemoesi by west-coast researchers uses counts of rings in cross sections of the axial rod to estimate growth rates and ages.  The rings appear as pairs of light and dark annuli (see photograph on Right)., assumed to represent summer and winter growth as seen in the trunks of trees.  Results show that the largest colonies (160cm axial-rod lengths) may be +40yr.  Average growth rates for small, medium, and large colonies are estimated to be 3.9, 6.1, and 3.6 cm . yr-1, respectively.  Based on the assumption of slowing growth with age, the authors estimate longevity of Halipteris to be about 50yr.  Wilson et al. 2002 Hydrobiologia 471: 133.

NOTE  the species lives in deep water and is not well studied.  The 12 specimens used in this study are from trawler by-catches in 2 locations

NOTE  other analyses show that the rod is composed of high-magnesium calcite

Cross-section of Halipteris of 167cm overall length.
Based on the authors calculations, there should be
about 37 double rings represented in this view

 

Sea whips Halipteris willemoesi arranged in groups
of small, medium, and large.
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