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Within‐colony variation in skeletalmineralogy of Adeonellopsis sp.(Cheilostomata: Bryozoa) from NewZealandKatherine E. Wejnert a & Abigail M. Smith ba Department of Marine Science , University of Otago , P. O. Box56, Dunedin, New Zealandb Department of Marine Science , University of Otago , P. O. Box56, Dunedin, New Zealand E-mail:Published online: 19 Feb 2010.

To cite this article: Katherine E. Wejnert & Abigail M. Smith (2008) Within‐colony variation inskeletal mineralogy of Adeonellopsis sp. (Cheilostomata: Bryozoa) from New Zealand, New ZealandJournal of Marine and Freshwater Research, 42:4, 389-395, DOI: 10.1080/00288330809509967

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New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42: 389-3950028-8330/08/4204-0389 © The Royal Society of New Zealand 2008

389

Short communication

Within-colony variation in skeletal mineralogy of Adeonellopsis sp.(Cheilostomata: Bryozoa) from New Zealand

KATHERINE E. WEJNERTABIGAIL M. SMITH

Department of Marine ScienceUniversity of OtagoP. O. Box 56Dunedin, New Zealandemail: abby.smith@otago.ac.nz

Abstract Three colonies of the erect, robustbranching, cheilostome bryozoan Adeonellopsissp. were collected from Doubtful Sound, NewZealand, to investigate within-colony variation incarbonate mineralogy. One-hundred-and-twenty-one sections from 9 branches were found to containmostly aragonite, with calcite ranging from 1.1 to7.3 wt% (mean ± SD = 2.4 ± 1.2 wt%, n = 120).The magnesium (Mg) content in calcite ranged from5.3 to 13.1 wt% magnesium carbonate (8.8 ± 1.0wt% MgCO3; n = 120). Calcite content decreasedwith increasing age proximally along the branch,indicating that secondary thickening is achievedwith aragonite. In contrast, an oscillating trend inMg content along branches suggests that seasonal orinterannual environmental parameters may influencethis geochemical parameter. Mineralogical variabilityhighlights the need for multiple samples from thesame colony to be measured when determiningquantitative carbonate mineralogy in bryozoans.Bimineralic bryozoans such as Adeonellopsissp. may have lower preservation potential thanmonomineralic bryozoans, and consequently maynot survive diagenesis to become fossils. Bimineralicspecies may also be more vulnerable to dissolutionpressure and possible ocean acidification thanmonomineralic calcitic taxa.

M08028; Online publication date 6 November 2008Received 11 June 2008; accepted 8 August 2008

Keywords carbonate mineralogy; bryozoans;aragonite; calcite

INTRODUCTION

Biomineralisation is an important marine processin seawater chemistry and the carbon cycle,preservation and fossilisation potential of skeletons,paleoenvironmental reconstruction, and evolutionarytrends through the Phanerozoic (e.g., Lowenstam &Wiener 1989). Some marine invertebrates are passivecalcifiers which reflect their environment, whereasothers exert considerable biological control (seeLowenstam & Wiener 1989) and variability in theirskeletal carbonate mineralogy. An understandingof the nature of this variability allows for clearerinterpretation of data ranging from stable isotopeanalysis of single specimens to patterns of evolutionover hundreds of millions of years.

Marine calcified bryozoans are active mineralisers,exhibiting a wide range of carbonate mineralogies(Smith et al. 1998, 2006). Although about two-thirdsof bryozoans are calcitic (with Mg content rangingfrom 0 to 13 wt% MgCO3), many are either entirelyaragonitic or bimineralic (Smith et al. 2006). Mostbryozoans with dual mineralogy are cheilostomes,and follow the classic pattern described by Ryland(1970), where aprimary calcitic skeleton is gradually"frosted" with secondary aragonite, such that olderparts of the colony have a greater proportion ofaragonite (Group B, Fig. 1). Genera in which at leastsome species exhibit this ontogenetic pattern includeArachnopusia, Caleschara, Calpensia, Celleporaria,Celleporina, Chapería, Hippomenella, Hippoporina,Metrarabdotos, Metroperiella, Odontionella,Pentapora, Schizomavella, Schizoporella,Steginoporella, and Tretosina (Smith et al. 2006).A few genera (e.g., Cellaria and Macropora) includespecies that precipitate both high-Mg calcite and low-Mg calcite as discrete minerals in the same skeleton(Group C, Fig. 1). A small group of bryozoans,including species from the genera Adeonellopsis,Gigantopora, Margaretta, Otionellina, Parasmittina,

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390 New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42

Aragonite Calcite

20 40 60Wt% calcite

80

High MgCalcite

IntermediateMg Calcite

¡Low M g' Calcite

100

Fig. 1 Modes of bimineral cal-cification in bryozoans. Group A:Mainly aragonitic, with up to 10%calcite. Group B: Ontogeneticallybimineralic, with primary calciteand secondary aragonite. GroupC: Dual calcitic, with high-Mgcalcite and low-Mg calcite asdiscrete minerals. (Based on datafrom Smith et al. 2006.)

Pentapora, and Schizomavella (Group A, Fig. 1), ischaracterised by skeletal aragonite to which 0 to 10wt% intermediate-Mg to high-Mg calcite is added(Smith et al. 2006). It is unclear whether this kindof bimineral skeleton is ontogenetic or otherwiseachieved.

The erect, robust branching bryozoan Adeonel-lopsis is a large, conspicuous subtidal genus foundthroughout New Zealand (Nelson et al. 1988).Initially, all New Zealand members of the Adeonidae(Gymnolaemata: Cheilostomata: Ascophorina) wereclassified as A. yarraensis, but it now appears thatat least five morphologically similar Adeonellopsisspecies occur in New Zealand and Australian waters(Lidgard & Buckley 1994). The erect rigid robustbranching species of Adeonellopsis used in this study(Fig. 2) is probably morphospecies B of Lidgard &Buckley (1994).

Adeonellopsis sp. skeletons are formed mainly ofaragonite, with 0.0 to 11.3 wt% calcite (Smith et al.1998, 2006). The small amount of calcite found inthis species is intermediate- to high-Mg calcite (4 to9 wt% MgCO3) (Smith et al. 1998, 2006). Averageannual growth rate along a branch is 7 mm/yr, asdetermined by mark-and-recapture of Adeonellopsissp. (Smith et al. 2001). Large colonies, 20-30 cmhigh, may be some 20–40 years old (Smith et al.2001).

In Doubtful Sound, a temperate fiord on the westcoast of South Island, New Zealand, conspicuouspurplish-black colonies of Adeonellopsis grow inexposed areas on the near-vertical rock walls closeto the entrance of the fiord. In some areas, such as

around Bauza Island, colonies may be found in waterdepths of 10 to 25 m at population densities of about1 colony/m2 (Smith et al. 2001) Estimated carbonateproduction by these bryozoans is 24 g CaCO3/m

2/yr (Smith et al. 2001). Longevity, size, SCUBA-accessibility, and ease of subsampling mean thatAdeonellopsis sp. in Doubtful Sound is an excellentcandidate for investigation of mixed mineralogy inmainly-aragonite bryozoans. An understanding ofthe patterns of biomineral distribution within theskeleton will help to elucidate the mechanism andecological role of bimineral precipitation in thisgroup.

MATERIALS AND METHODS

Three colonies (with diam. greater than 20 cm) ofAdeonellopsis sp. were collected in 12 m waterdepth just off Bauza Island (45°18 S, 166°55 E) inDoubtful Sound in January 1998 (see Smith et al.2001 for location map). The colonies were killed inethanol and rinsed clean of sediment and epifauna.The three longest branches (57 to 77 mm) with anidentifiable live growing tip from each colony werechosen, with the remainder of the colony archivedin the Department of Marine Science, University ofOtago. Organic material was removed by immersionin 10% bleach for 6 h. After being rinsed in distilledwater and dried, each branch was manually pickedclean of epizoa under a binocular microscope, thenphotographed. Branches were trimmed of sidebranches, retaining the longest possible continuous

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Wejnert & Smith—Minerology of Adeonellopsis 391

Fig. 2 Adeonellopsis sp. from Doubtful Sound, southwestern New Zealand. A, Living colony in situ on the wall ofBauza Island at 12 m water depth (scale = 5 cm); B, bleached and dried colony (scale = 1 cm); C, scanning electronmicrograph of growing tip showing zooid characteristics (scale = 1 mm).

branch. Each was cut (orthogonal to growth direction)with a scalpel into 10 to 15 sections 5 mm in length(numbered from the growing tip), and each sectionwas weighed to the nearest mg.

Approximately 0.1 g of well-ground samplepowder was placed in a clean mortar with 0.01 ganalytical grade NaCl as an internal standard, anda small amount of 95% ethanol. The solution wasmixed with a pestle until even in texture and colour,then a portion was smeared on a glass slide untilthere was even coverage over an area of about 20mm × 20 mm. The slide was gently tapped to removeany air bubbles, and air dried.

A Philips PW1050 X-ray diffractometer (XRD)with a copper (Cu) target x-ray source scannedeach sample between 26 and 33 20. There were50 counts per degree, and the count time was 1 s.Calcite peak position was corrected based on theinternal standard halite peak, then Chave's (1952)method of determining Mg content (in wt% MgCO3)was applied (y = 30x - 882). The peak height ratiocalibration curve of Gray & Smith (2004) wasused to determine the proportions of calcite andaragonite: Wt% calcite = 80.4 (PR)2 - 180.9(PR) +101.2, where PR (peak height ratio) is the sum ofthe maximum peak heights measured from the base

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392 New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42

20 •

calc

iteU

l

c

§ 1 0 -

3 5 "

0 •

Biomineral carbonates

ÈWiJ Adeonellopsis

Bryozoans

20 40 60 80 100

Wt% calcite

Fig. 3 Carbonate mineralogy of segments of Adeonellop-sis sp. from Doubtful Sound, southwestern New Zealand,compared to mineralogical space occupied by all bryo-zoans (dark box) and that of all carbonate biomineralisers(Smith et al. 2006).

line for aragonite dA1 and d^ divided by the sum of § ̂aragonite peak heights and the peak height of calcite •< §"dC. Peak height ratios were used rather than peak ^ Iintensity because diffractograms showed sharp clear •§ Cpeaks indicating a weU-defined crystallinity. ,2 ̂

RESULTS

A total of 121 sections from nine branches ofAdeonellopsis sp. were analysed using XRD (Table1). The sections weighed from 7 to 194 mg (mean ±SD = 61 ± 35 mg, n = 121). One XRD profile (C1B2)exhibited indistinct peaks and was discarded. Mgcontent ranged from 5.3 to 10.7 wt% MgCO3 (8.7 ±0.9 wt%; n = 119). One outlier with an exceptionallyhigh Mg content (C3C7) was discarded. Calcitecontent ranged from 1.1 to 7.3 wt% calcite (2.3 ±1.1 wt%, n = 117). Three outliers were discarded(C1A10, C1C12, C1C13). There appeared to beno systematic relationship between Mg contentand wt% calcite (Fig. 3). The mineralogical spaceoccupied by Adeonellopsis sp. is the range of Mgcontent (7.8 wt%) times the range of calcite content(6.2 wt%) (Fig. 3) and thus 48.4 wt%2 or 3% of thespace occupied by the phylum Bryozoa. Mg contentdid not exhibit any along-branch trend (Fig. 4A).Proportional calcite content, however, did appearto decrease along the branch, at least initially (Fig.4B).

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10 20 30 40 50 60

Distance from growing tip (mm)

70 80

DISCUSSION

If variation in mineralogical content were entirelyontogenetic, we would expect to see a similar trendalong all branches with distance from growingtip. Although Mg content did not exhibit such atrend, calcite content did. When the three outlierswere discarded (Fig. 4B; the calcite contents ofthese outliers suggest perhaps contamination byinternally lodged particles such as Foraminifera),the relationship between distance along branch andcalcite content is relatively good (Fig. 4B). Youngzooids appear to be composed of proportionallymore calcite than older zooids.

The decrease in relative calcite content withage is consistent with the notion of "sacrificial"calcite, where an external layer of calcite is subjectto abrasion and bioerosion, protecting stronger andmore energetically expensive aragonite (Gray &Smith 2004). In bryozoans, however, unlike molluscs,the skeleton is secreted beneath soft tissues and isnot in direct contact with sea water. In this instance,we saw no evidence of abrasion and/or bioerosionin older zooids. It is more likely that secondarythickening of the skeleton with age adds aragonite

(Ryland 1970; Cheetham 1986), and Adeonellopsiscolonies are thicker and more robust at the base(Smith et al. 2001).

A number of bimineralic cheilostomate bryozoansadopt this strategy. Typically the skeletons areformed of calcitic basal and vertical walls (Taylor etal. 2008). The inner layer of the frontal shield, too, iscalcite, but the external layer is aragonitic (Taylor etal. 2008). Older zooids thus have increasingly thickeraragonitic layers on the frontal shield (Carson 1978;Taylor et al. 2008). Orificial rims, teeth, condyles,ascopore plates, and ovicells are also usually calcitic,whereas avicularia may be calcitic or aragonitic(Taylor et al. 2008).

If, in contrast, variation were to be environmental(as, for example, with stable isotope composition;Smith & Key 2004), we might see oscillations inmineralogy along branches (see Fig. 4A). The samplesize required for XRD, 5 mm of branch length,is almost a full year's growth so seasonal signalsdo not register clearly. Nevertheless, interannualvariation could be detected on the scale sampledhere (see Smith & Key 2004). Although there waslittle similarity among branches (owing perhaps tointerbranch variation in growth rates), the oscillations

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Wejnert& Smith— Mineralogy of Adeonellopsis 39.5

were similar in scale to those observed in oxygenisotopes in the same species (Smith & Key 2004).With increased resolution it could be possible toalign the branches so that internannual and seasonalvariations are detectable (see, e.g., Lombardi et al.2008).

Even with the three outliers discarded, the rangeof Mg content in these colonies, from 7 to 11 wt%MgCO3, is greater than that found by Smith & Key(2004) (almost no variation along a single branch)and higher than that reported from the literature bySmith et al. (2006) (4 to 9 wt% MgCO3, but fewsamples were taken from the same branch). Thisvariability highlights the need for multiple samplesfrom multiple branches from multiple colonies to bemeasured when determining carbonate mineralogyin the bryozoans.

Bimineralic bryozoans have lower preservationpotential than entirely calcitic bryozoans, asaragonite is often dissolved or remineralised duringdiagenesis, weakening the skeletal structure andpossibly removing important diagnostic featuresfor taxonomic identification (see, e.g., Buge 1957;Taylor et al. 2008). Loss of aragonitic avicularia orfrontal shield material may result in misidentificationor sufficient weakening of the colony structure toallow for its destruction. The greater solubility ofaragonite, compared to calcite, also suggests thatbimineralic bryozoans, suchas Adeonellopsis, are atgreater risk from global surface ocean acidification(see, e.g., Orr etal. 2005) thanmonomineralic calcitetaxa.

ACKNOWLEDGMENTS

We thank Brian Stewart, Damián Walls, and Liz Girvanfor their assistance. The departments of Marine Scienceand Geology at University of Otago provided support andresources. We thank Marcus Key of Dickinson College,Paul Taylor of Natural History Museum, and an anonymousreviewer for useful comments on the manuscript.

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