The arrangement of suckers on octopodid arms as a continuous character

9 Pages·1993·2.9 MB·English

by  Janet R Voight

MALACOLOGIA, 1993,35(2): 351-359
THE ARRANGEMENT OF SUCKERS ON OCTOPODID ARMS
AS A CONTINUOUS CHARACTER
Janet R. Voight
DepartmentofZoology, FieldMuseum ofNatural History,
Roosevelt Roadat Lake Shiore Drive, Chicago, Illinois 60605, U.S.A.
ABSTRACT
Studiesofoctopodidtaxonomyandclassificationhavecitedthenumberoflongitudinalsucker
rowsonoctopusarmsasifitwereapurelydichotomouscharacter.Thischaracter,however,has
been suspected to be continuously distributed and associated with increased sucker density
(Hoyle, 1886; Berry, 1914). This study tests that hypothesis by comparing the relationship
between the mean number of suckers per arm to mean arm length among octopodid genera
occurring above 500 m depth. Specimens of genera typified by a single sucker row but with
suckers arranged in a zigzag pattern are also included.
Mostspecimenswithtwosuckerrowsandwithsuckersarrangedinzigzagshavemoresuckers
atagiven arm length than dospecimenswith suckersarranged in asingle row, supporting the
hypothesis. Mostspecimenswith one suckerrow are separated from thosewithtwo rows bya
curve on the plot of the number of suckers versus arm length, although four specimens of
Pareledone spp., preserved with their arms straightened into a swimming position ratherthan
recurved, andtheholotypeofAphrodoctopusschultzeiareexceptional.Thenumberofsuckers
on the arms of these specimens predict that they will be arranged in one row. The zigzag
arrangement seen on the specimens may be due to preservation artifact in the case of the
specimensofPareledoneandinA.schultzeibythe6-8enlargedsuckersoneacharm.Variation
inthenumberofsuckerswithingroupsdefinedbythenumberofsuckerrowsisgreaterthanthat
betweengroups,suggestingthatthenumberofsuckerrowsisacontinuouscharacter. Evidence
provided here indicates thatA. schultzeishould be included among the species of Eledone.
Keywords: Octopodidae, sucker rows, classification, continuous character, Eledone, Aphro-
doctopus.
INTRODUCTION done sometimes carry suckers arranged in
double rows, orin azigzag pattern wherethe
Octopodid taxonomy and systematics is number of rows is arguable (Hoyle, 1904;
entering a dynamic period; preliminary at- Joubin, 1905, 1918; Gravely, 1908). Preser-
tempts to reconstruct evolutionary relation- vation maycontributetotheformation ofdou-
ships among members of the Octopoda ble sucker rows in these genera (Guérin,
(Voss, 1988; Voight, 1990) have lead to a re- 1908), but live animals also show sucker ar-
assessment of our assumptions about the rangements considered to be anomalous for
group(Voight, 1991, 1993;inpress).Onesuch their taxon (Chadwick, cited by Gravely,
assumption, expounded by Voss (1988), is 1908; Naef, 1923).
that the number of longitudinal sucker rows Whether the number of sucker rows on an
on the oral surface of the octopus arm is a octopusarm isavaluablecharacterforrecon-
dichotomous character that accurately re- structing phylogenies has been questioned
flects evolutionary relationships. (Owen, 1881; Hoyle, 1886; Berry, 1914; Naef,
Whether suckers on an octopus arm form 1923). Based on his discovery of only slight
one or two longitudinal rows has featured differences in the sucker musculature be-
prominently in diagnoses of octopod families tweenspecimensofOctopus, withtwosucker
(Rochebrune, 1884; Joubin, 1918), subfami- rows, and those of Eledone, with one sucker
lies (Voss, 1988), and genera (e.g. Robson, row, Guérin (1908) doubted that sucker ar-
1932; Roper & Mangold, 1991). Statements rangement was an adequate basis on which
such as in young Eledone "as suckers are to distinguish the genera. Berry (1914) sug-
added they never form two rows" (Hochberg gestedthatoctopussuckersareinherentlyor-
et al., 1992: 265; similarly, Rochebrune, ganized inasinglerowandthatonlybecause
1884) reflectthe degree to which the charac- ofcrowding are suckers displaced alternately
ter is thought to be dichotomous. Yet, the tothe side. Hefeltthatthis displacementcre-
arms of specimens of Eledone and Parele- ated the appearance of a double sucker row.
351
352 VOIGHT
The biological significance of this character Adam (1941), Taki (1961) and Kubodera &
had yetto be evaluated despitethis alternate Okutani (1986); seven points for Eledone
hypothesis. were from Massy (1916), Rees (1956) and
This paperteststhe hypothesisthatsucker Adam (1951, 1984). Three specimens of E.
crowding is associated with the formation of cirrhosa and data from the type of P. turqueti
doublesuckerrowsbyexaminingtherelation- (Joubin, 1905), all with suckers in a zigzag
ship between the number of suckers on an arrangement, were included. Only taxa with
arm and arm length among octopodid genera mean depth distributions above 500 m were
typically occurring above 500 m depth. Spec- included to avoid the effects of decreased
imens of taxa characterized by one sucker sucker size associated with increased depth
row that have suckers in a zigzag arrange- distribution (Voight, in press).
ment are predicted to showthe same pattern Suckerswere counted as described byToll
as taxa with two sucker rows. The phyloge- (1988), using a combination of macroscopic
netic significance of sucker arrangement is and microscopictechniques. Suckerson right
assessed. arms l-IVwerecounted; leftarmswereused if
the right were damaged. Only normal arms
were used for data analysis; injured arms or
MATERIALS AND METHODS those with incomplete regeneration were ex-
cluded. Hectocotylized arms of males (one of
Totestthe hypothesisthatthe formation of the third pair of arms specialized for sper-
double sucker rows is associated with sucker matophore transfer) were considered sepa-
crowding, the number of suckers on octopus rately from normal arms.
arms with one sucker row was compared to The analysis requires that each datum be
thatwithtwosuckerrowsasafunctionofarm independent, that is, free of any correlations
length. The hypothesis predicts that more orassociation with otherdata in the analysis.
suckers will occupy arms with two rows than Becauseall non-hectocotylized armsofan in-
with one row at the same arm length. Speci- dividual specimen are subjectto identical ge-
mens oftaxatypified byone rowwith suckers neticand environmental variablesorcontrols,
arranged in a zigzag pattern will reflect the they are not independent. Statistical tests of
pattern shown by specimens with two sucker theworking null hypothesis, thateach normal
rows. arm of an individual specimen has the same
Specimens included in this analysis (n = number of suckers, were prohibited by the
142) were from the California Academy of small sample size within an individual, inevi-
Sciences, San Francisco; Field Museum of table errors in counting, and errors in regen-
Natural History, Chicago; Rosenstiel School eration that may have failed to restore all
of Marine and Atmospheric Science, Univer- suckers. This hypothesis was rejected if the
sity of Miami; the United States National Mu- numberofsuckers on different arm pairs var-
seum, Washington, D.C.; and University of iedconsistentlyinallavailablespecimensofa
California Museum of Paleontology, Berke- given species.
ley. Octopuses with suckers arranged in a Only male specimens of Eledone caparti
double row were represented by specimens were available, and only in this species was
of Octopus, Hapalochlaena and Macrotrito- the null hypothesis rejected, as indicated by
pus and the type specimen of Macrochlaena Adam (1950). Typical of Eledone, these
(Robson, 1926). DatafromToll (1988)forCis- males have sucker-derived modifications at
topus, Pteroctopus, Robsonella and Scaeur- the arm tips (Haas, 1989: Fig. 2). When the
gus and from Roper & Mangold (1991) for number of modifications and suckers were
Aphrodoctopusincreasedthe numberofgen- summed, the result was virtually invariant
erawith two sucker rows included. Datafrom within an individual (Table 1). Because within
Toll (1988) also increased the data available individual specimens of all other species ex-
for species of Octopus. amined, the number of suckers was essen-
Representing octopuses with suckers ar- tially equal among the arm pairs, data taken
ranged inasinglerowweretypicalspecimens from only one or two arms were considered
of the genera Eledone, Pareledone, Vossele- representative and were included.
done, and Tetracheledone. To ensure com- Despite the anomalous pattern seen on
pleteandunbiasedrepresentationofthetaxa, arms of E. caparti, sucker counts of males
eight data points for Pareledone were taken with heteromorphic arm tips were repre-
from reports of Joubin (1905), Berry (1917), sented in the analysis by mean sucker num-
SUCKER ARRANGEMENT ON OCTOPODID ARMS 353
TABLE 1 Sucker counts, heteromorphic arm tip counts and arm lengths for normal
arms and hectocotylized arms (R3) of males of Eledonecaparti.
Specimen ARM Suckers Modif.
354 VOIGHT
300-
240
200
150-
D
100
60 120 180 240 300 340
Mean Arm Length
FIG. 1. Plottedforthenormal ofeachspecimenarethemeannumberofsuckersversusthemeanarm
length. Upper case letters represent specimens with a double sucker row: A, Octopus bimaculatus; B, O.
briareus;C, Cistopusindicus; E, O. selene; F, O. fitchi;G, O. chierchiae, O.peniciliferana O. stitiochru-s; H,
O. hubbsorumand Hapalochlaenaspp., I, O. digueti; L, O. californicus; N, Macrotritopusdefilippi/horhdus;
O, O. macropus/ornatus; P, Pteroctópus tetracirrhus; Q, Octopus (Macrochlaena) winckworthi; R,
sone"ll'a;fontanianus;S, Scaeurgusunicirrhus/patagiatus; U, O. bimaculoides;V, O. vulgaris;X, O. f/7osus;Y,
O. ?Aphrodoctopusschultzei. Lowercase letters representspecimens oftaxawith a single sucker
pr,owP:.ap,oEllyemdoornpehac;apar,rtEi;cc,irPrahroesal;edso,nPe.ch(aMrecgoatil;elee,dToenter)acsteineolie;dot,neP.sptiunriqcuiertriu;s;vg,,VE.osgsaeulcehdao;nme,cEh.armrousac;hax,taP;.
adelieana, P. auroraeP. harrissoniand P. nigra(onespecimeneach);y, E. massyae.Thecurve, whichwas
fitted by eye, generally separates specimens with a single sucker row (below) from those with two sucker
rows and suckers in a zigzag arrangement (above). The points within circles represent specimens of
Pareledonewith suckers in zigzags below the curve.
SUCKER ARRANGEMENT ON OCTOPODID ARMS 355
of specimens of Eledone can carry at least ers arranged in zigzags despite being few in
135suckers;specimensof Octopuscan have number. These specimens may violate the
up to 300 suckers on an arm. The number of pattern because their arms were preserved
suckers on an arm of E. cirrhosa and E. mo- straight, in a swimming position, as recom-
schata approaches that of some specimens mended by Roper & Sweeney (1983). The
with two sucker rows. The numberofsuckers arms of comparable specimens that are re-
on the arms ofthetype of P. turqueti(Joubin, curved in preservation carry a single sucker
1905)cannotbedistinguishedfromthatofoc- row.
topuses of equal size with two sucker rows. In fixation, unrestrained arms recoil, appar-
Although most octopuses with one sucker ently due to contraction of the web. On a re-
rowareseparatedfrom thosewith twosucker curved arm, theoral, suckered surfaceonthe
rows by a very narrow margin (Fig. 1), within outercurveofthearm isintension;theaboral
each group the average number of suckers surface, forming the inner curve, is com-
borneon an arm ofagiven lengthvaries con- pressed. Artificially straightened arms are
siderably. Atarm lengths near200 mm, spec- subject to different forces, which may invali-
imens with one sucker row average from 46 date comparisons between straight and re-
(P. senoi)to 112 (E. moschata) suckerson an coiled arms. When straight arms are flexed
arm, specimens with two sucker rows aver- aborally, the space between the suckers in-
age from 135 (in Cistopus indicus) to 247 (in creases and their arrangement can approach
Macrotritopusspp.) suckers on an arm. Liter- a single row.
ature-based and specimen-based data report That a curve rather than a line separates
a comparable number of suckers on arms of mosttaxawithonesuckerrowfromthosewith
similar length within a taxon. two rows (Fig. 1) illustrates that sucker size
Onthe plotofthe numberofsuckersonthe alsoinfluencestherelationshipbetweensuck-
hectocotylus versus hectocotylus length (Fig. ers. On the short arms of young octopuses
2), most males of taxa typified by a single with small suckers, each small sucker at the
sucker row have fewer suckers on the hecto- armtipoccupiesalargeproportionofthetotal
cotylus than do specimens with two sucker space. On longer arms with larger suckers,
rows. On the hectocotyli of two males of E small suckers at the arm tip occupy propor-
cirrhosa, one with one sucker row and one tionatelylessspace,thelargesuckersalready
with zigzag sucker arrangement, however, in place dominate. The threshold curves with
thenumberofsuckersequalsorexceedsthat increasing size as a result of growth.
on hectocotyli of octopuses with two rows. Sucker growth may also explain why some
The male type ofA. schultzeiwith two sucker hectocotylized arms violate the pattern seen
rows, has asfewsuckerson the hectocotylus in normal arms (Fig. 2). Hectocotyli develop
as do males with one sucker row. Hectocotyli as normal arms uptoa point; ifmorethan the
with one sucker row, otherthan those of Ele- critical number of suckers recruit, double
done, always plotbeneaththecurvethatsep- sucker rows form. Small hectocotyli plot as
arates normal arms with one from those with predicted by normal arms (Fig. 2), and they
two sucker rows; hectocotyli with two sucker are directly comparable; the comparison,
rows plot on both sides of the curve. however, becomes invalid with growth. The
hectocotylus carries an apparently species-
specific numberofsuckers, often manyfewer
DISCUSSION thanonnormalarms(Toll, 1988;Villanuevaet
al., 1991). Although hectocotyli are shorter
The hypothesisthat sucker crowding is as- with fewer suckers than are other arms, the
sociated with the formation of double sucker arm and suckers continue to grow, as evi-
rows is supported. In most of the octopus denced by within species variation in hecto-
specimensconsidered, ifthe numberofsuck- cotyluslength(Fig.2;Toll, 1988;Villanuevaet
ers exceeds a cntical limit dependent on arm al., 1991). If the suckers on the hectocotylus
length, the suckers form double rows. The become larger than those on normal arms,
consistency ofthis limit, orthreshold (Fig. 1), their size may maintain the double sucker
among the octopuses considered suggests rows, despite their reduced number.
that a physical constraint affects each of the If a double sucker row is associated with
taxa considered; the five exceptional speci- sucker crowding, and large suckers occupy
mens reveal the effect of other factors. more space than small suckers, then a com-
Infourspecimens of Pareledone, the suck- paratively few very large suckers could form
356 VOIGHT
40 80 120 180 240 280
Length of the Hectocotylized Arm (mm)
FIG. 2. Plottedarethenumberofsuckersonthehectocotylusversushectocotylus length. Symbolsdefined
as in Figure 1. The curve separates normal arms with two rows from normal arms with one sucker row.
double rows. This mechanisnn has been sug- two rows or in zigzags (Joubin, 1905;
gested to create double sucker rows in male Gravely, 1908). Octopus, defined by having a
specimens of the cirrate octopods Opistho- double sucker row, contains specimens with
teuthis depressa and O. japónica (Sasaki, suckers arranged in zigzags or nearly single
1929; Taki, 1963). suggest that this mecha- rows (Robson, 1932). That exceptions occur
I
nism also produced the double sucker rows in diverse genera suggest that the character
on the type of A. schultzei. The number of is artificial.
suckers on the arms of the type predicts that Second, the muscles attachingthe suckers
it will have a single sucker row, but the 6-8 to the arms are very similar in specimens of
dramaticallyenlargedsuckerson each arm of Eledone and Octopus (Guérin, 1908; Kier &
Aphrodoctopus schultzei (Roper & Mangold, Smith, 1990). Guérin (1908: 59) predicted
1991) may occupy enough space that most thateliminatingsomeofthesuckersandelon-
suckers occupy more than one row (Hoyle, gating the axis of the arm, that is reducing
1910: plate Va, fig. 1; Roper & Mangold, sucker crowding, would shift the sucker ar-
1991: fig. 4). rangementfromtworowstoone. The present
Sucker number varies more within groups resultssupporthispredictionand indicatethat
sharingthesame numberofsuckerrowsthan these genera differ only superficially in this
it does between groups. Such groups may character. Detailed studies of other genera
thus be arbitrary units. Three lines of evi- and of developmental series have yet to be
dence support this statement. First, although accomplished.
the genera Eledone and Pareledone are de- Third, the distribution of points relative to
fined by having a single sucker row, speci- the critical limit separating specimens with a
mens of both can have suckers arranged in single row from those with double suckers
SUCKER ARRANGEMENT ON OCTOPODID ARMS 357
rows(Fig. 1) reflectsthearrangementofsuck- ated with double sucker rows is consistent
ersonmostspecimens. Pointslyingjustabove withdataavailableforspecimensofthedeep-
thecurve(Fig. 1) representspecimensofCis- water genus Benthoctopus (Voight, unpubl.).
topus indicus that have suckers arranged di- Availablespecimensanddata(Russell, 1922)
agonally, or nearly in a single line (Robson, for Bathypolypus arcticus and B. faeroensis
1929), as predicted bythe plot. Specimensof show that despite their suckers being few in
Eledonearejustbelowthecurveifthesuckers number and small in size (Voight, in press)
form asingle rows; specimensofthisspecies theyalsoform double rows. Ifthe mechanism
withsuckers in azigzag arejustabove it. The forming double rows can be shown to differ
continuous distribution of points reflects the between Bathypolypus and the octopuses
continuous nature of the character. considered here, double sucker rows would
If, as suggested here, the spatial relation- be shown to be convergent in the Octopo-
ship among the suckers determines their ar- didae, as predicted by Robson's (1932) clas-
rangement, different strategies may serve to sificationofthefamilyand my preliminarycla-
influence that relationship. Chief among dogram (Voight, 1990).
these strategies may be differentiation of If the number of sucker rows is unreliable
sucker sizes along the arms. for phylogenetic reconstruction, could the un-
If octopuses have dramatically more than derlyingcharactersuiteofsuckernumberand
thecritical numberofsuckersrequiredtoform arm length indicate close evolutionary rela-
double sucker rows, why do the suckers only tionships, e.g. between Octopus and Ele-
form double rows? Although individuals with done? Higher order names have been as-
three sucker rows per arm are currently con- signed, not to reflect relationships, but to
sidered developmental anomalies (Toll & Bin- groupoutwardlysimilartaxaby readilyappar-
ger, 1991), Owen (1881) named the genus ent characters (e.g. Joubin, 1918). Anato-
Tritaxeopus for specimens with three sucker mists who perhaps believed that the generic
rows. Owen, who suggested that sucker ar- names indicated distinctly different taxa have
rangement was continuous among the Oc- comparedthesegenerabuthaverarelyfound
topodidae, stated that because Tritaxeopus significant differences (Girod, 1882; Guérin,
differed as much from Octopus in sucker row 1908; Kier& Smith, 1990).
number as did Eledone, it merited equal tax- Withoutan independent meansofpostulat-
onomicrecognition. Owen's(1881) reportthat ing relationships, and aware that a similarity
286 suckers occupy the 584 mm-long third in the relationship between sucker number
armofhisnowmissingtypespecimen iscom- and arm length can be produced by changes
parable to specimens included here with in either character, conclusions are prema-
shorter arms (Fig. 1) and two sucker rows. ture. The number of suckers in Octopus bi-
The rarity of specimens with multiple maculatus and O. bimaculoides, very similar
sucker rows may be associated with sucker species thought to have diverged only re-
sizedifferentiation. Inspecimenswithasingle cently (Pickford & McConnaughey, 1949), dif-
sucker row, the suckers occupy a compara- fermorethan among speciesof Octopusand
tively narrow size range. Especially in speci- Eledone (Fig. 1), suggesting thatthis charac-
mensofPareledone, theterminal suckersare ter does not necessarily reflect evolutionary
large compared to those on the tips of arms history.
with two sucker rows. In shallow-water octo- Eliminating the numberof suckers rows as
puseswithtwosuckerrows, thesuckers near a taxonomic character does not affect most
the margin of the web are distinctly the larg- currently recognized genera. The genus
est;distally, suckersizedeclines dramatically Pareledoneshould bedefinedto reference its
but continuously. Because few suckers are few suckers on each arm rather than one
large, the amount of crowding is reduced, as sucker row; its definition, however, may still
is the crowding associated with the many be based solely on plesiomorphic, or ances-
smallsuckers. Bypartitioningsuckersize,two tral, characters (Voight, 1993). Eledone
discrete sucker rows may be maintained de- remains as a distinct taxon; its members
spite the presence of hundreds of suckers. share the apparent synapomorphies of male
Why multiple sucker rows appear to be heteromorphic arm tips formed by the lateral
avoidedbyoctopusesmayrelatetofunctional extension of sucker buds, the reduction or
difficulties or that increased nervous and absence of a calamus, the anterior fusion of
muscular control are required. the branchial retractors and, pending more
That increased sucker density is associ- data, in uterofertilization (Perez etal., 1990).
358 VOIGHT
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