Table Of ContentReference: Biol. Bull. 198: 379-386. (June :<>()(
Vestigial Ophiopluteal Structures in the Lecithotrophic
Larvae of Ophionereis schayeri (Ophiuroidea)
P. SELVAKUMARASWAMY* AND M. BYRNE
Department ofAnatoim and Histology, F13: University ofSydney: N.S.W., 2006: Australia
Abstract. Evolution of echinoderm development from a larval morphological traits have been combined with mo-
feedingto a non-feeding mode can be examined by studying lecular characters to produce a better understanding of the
non-feeding larvae with structures that appear to be vestiges evolution ofdifferent life histories (Wray, 1996; Hart et /..
derived from a feeding ancestral state. The lecithotrophic 1997; McHugh and Rouse, 1998). For echinoderms, plank-
larvae of the Australian brittle star Opliionereis schayeri totrophy is considered to be a plesiomorphic character. The
possess such features, and the early development of this loss ofcomplex larval feeding structures in the evolution of
species was documented by light and scanning electron non-feeding development involves major alterations in lar-
microscopy. The embryos undergo irregular cleavage, re- val morphology, which are thought to be irreversible
sulting in the formation ofdifferent sized blastomeres, with (Strathmann, 1974). Thus, the occurrence of ophiuroid and
subsequent development through a wrinkled blastulu stage. echinoid larvae with vestigial larval feeding structures (e.g.,
The lecithotrophic larva of O. schayeri possesses several pluteal arms, skeleton, ciliated band) indicates that they
vestigia] ophiopluteal structures, including a continuous cil- have evolved from an ancestral form that had a feeding
iated band, a larval gut, and a larval skeleton. The ciliated larva. The transition from planktotrophy to lecithotrophy is
band is a reduced expression ofthe continuousciliated band predicted to be reversible, however, when maternal nutri-
typical of ophioplutei. The larval gut is a transiently com- tional investment is altered due to selective pressures, with-
plete system, but an esophageal plug and rapid closure of outaresultingchange in larval morphology (McEdwardand
the blastopore renders it nonfunctional. The larval skeleton, Janies, 1997).
though reduced, consists of four rods corresponding to the Planktotrophic development in ophiuroidsoccurs through
bacotdeyr,istpiocstoefraolnatoeprhailo,plauntteeursol.atDeureal,toaanhdetpeorstoocrharlonryodisn lcahravra-l aonpm8e-natrmiendopohpihuiroopildusteiussa(sTsaobcilaete1d).wLietchitahortarnogpehiocfdleavrevla-l
skeletogenesis, the postoral rods develop early and simul- forms including planktonic ophioplutei with a reduced
taneously with the other rods. Compared with the larvae of number of arms, vitellaria larvae, and benthic and brooded
other lecithotrophic ophiuroids, the larva of O. schayeri is larval forms that may or may not possess ophiopluteal
oneofthe mostreduced ophiopluteal formsreportedtodate. features (Table 1). Thus far. eleven ophiuroid species are
known to have lecithotrophic larvae with vestigial larval
Introduction
structures, seven with planktonic development, one with
The diversity of developmental modes in marine inver- benthic development, and three with brooded development
tebrates has intrigued investigators for many years (Wray. (Table 1 ). This list suggests that lecithotrophic species with
1995). This diversity is presumed to be associated with life developmental features intermediate between planktotrophy
history evolution, but the direction of the evolutionary and lecithotrophy are uncommon. This may be due to evo-
change, whetherfromplanktotrophy tolecithotrophy orvice lutionary instability of intermediate larval forms (Vance.
versa, has been a topic of debate. Phylogenetic analyses of 1973)ormayreflectinadequate sampling. Furtherexamination
of ophiuroids with lecithotrophic larvae and comparison of
development in closely related species is needed to determine
R*eTcoeivewdh3oAmugusctorr19e9s8p:onadcecnecpeteds2h5ouFledbruabrey 20a0d0d.ressed. E-mail: the processes and pathways underlying the loss ofsuperfluous
[email protected]. larval structures during the evolution of lecithotrophy.
379
SFLVAKUMAK \SW \xn \\DNI in KM
I'
lahle 1
Developmental mnjc\ in llu- ti. irh <-m/>/i</.w'\ ;i Iri-ilhoin'//< .\/><-ciV.\ f/i< rrfni
Developmental mode Larval form SpCCICs
Planklotrophic N-arni ophiopluleus -ecies M.uHndc. 140". Moitensen. 1421; Narasimhamuni. 1933:
OKcn. I'M2. Hendler. 1975; Mladcnov. 1985;
Yaiiiashita. 1985; llciullci. 1'i'H
l.ecithotrophic Reduced ophiopluleus
(plunkiomci
6 arms Amphiurafiliformu Fell. 1445
2 arms Ophiura aftun\. .\mi>hinru iliiujci. Fell. 1445; Fenaux. 1963: Mladenov. 147')
Ophitilhrn oei /iill
Armless Ophiopluteus r/<j/x;/r</<7 Fell. 1445
Vitellaria (with larval structuresi "/>'""''/'" iricta. \1oiieiisen. l4i.S; Hendler. I4S2
(>l'/niini'ii'i\ iinnulahi
\itcll.ina inn larval structures) Opliiurii hrc\i\/>niii. (ira\e. I4DO. Moiteiiscti. 1421. I enauv 1969; Stancyk.
197.V Komalsu and Shnshaku. I'i'K
Op/iioili'riiw liiii'^niiinla.
OpIuoU'i'it clt en'iv
Ophiolpocusjaponicus
Lecitholrophic iWith larval structures) Hendler. I97S
benthic
i
iNo laival siiuctutcsi Kirk's ophiuroid Fell. 1441
Lecithotrophic (With larval sti Amphipholis joponica, Muiakami. 1940; Fell. 1946; Mnrlensen. 1421
ihrooded) .\mphipln>lt\ .\iiiiiimiiln.
(>/>lii<>nln\ hi:\ucli.\
INo larval structures) t.-n-<\ i>/i\inen. Hvine. 1991; Hendler. 1991; -Morlenscn.
O/>hi/>nnii\ vnii'iiin. and
>68 other species
: Previous studies on Opinion/rn species.
The Australian ophiuroid ()/>liii>iicn'i\ \fliuyrri develops kept in the dark for half an hour. This was repeated every
ihrouiih a lecithotrophic larva that retains a suite of larval half hour for 2-3 h. For larval culture, fertili/ed eggs were
feeding structures. It spawns 241 /nm diameter, lipid-rieh rinsed three times in tillered seawater ( 1 /urnI. placed into
eggsand completes development in 6-7 days (Moloney and 5()()-ml beakers at densities of 5 eggs/ml, and kept con-
Hsrne. 11J1J4: Selvakumaraswam) and Byrne. 1945: Selva- stantly stirred at ambient temperatures (19-23C). The
kumaraswamy and Byrne. 1998. 1999). The development of seawater was changed every second dav by aspiration
four other lecithotrophic (>i>liunicici^ species has been de- through an X()-/nm mesh.
scribed, with two species possessing some pluteal features The larval skeleton was visnali/ed as follows. Embryos
and the other two complete!) lacking them (Table 1 I. The anil larvae were fixed for 1 h in 2'i paratormaldehyde
selective pressures underlying the developmental diversity buffered in 0.2-^im filtered seawater. and (hen stored in 70%
in (iplniiroids are unknown, and the larval forms in these elhanol. Thc\ were then made translucent h\ dehydration
o()n[)hhio<iwinTlio-si\s ostpeclairevsalarsetrcuoctmupraersemda\herheavteo porcocvuirdreedinwsiitghhitns aalncdohcolle,ariancgcoirndainIg: I tsooluAtmieonmiofyabenaznydl bleinm/leolale(1a9n9d2)h.enT/.hyel
this elade. In this studs the early larvae of (). \clui\rri are cleared larvae were examined with cross polari/ed light. For
described, vvnh emphasis on (he development ofits vestigial scanning electron microscopy (SEM) and light microscopy
ophiopluteal structures (I M). emhivos and larvae were lixed tor I h in 2.5'*
gluteraldehyde. buffered in 0.2-jnm filtered seawater. rinsed
Materials and Methods in 2.5r/c sodium hydrogen carbonate, postlixed for 1 h in 2CA
Ofihionereis xchaveri was collected from several sites in osmium tetroxide buffered in 2.5% sodium hydrogen car-
New South Walesduring its summerspawning season, from bonate, rinsed in distilled water, ami dehydrated. Specimens
January to April, in 1996 and 1997. Animals were induced for SEM were then critically point dried, coaled wnh 20 nm
OL
to spawn by combined light and temperature shock. About of gold, and examined with a .11 .*>(' scanning election
20 indiWviCduals were transferred repeatedly between seawa- microscope set at I5 kv . Specimens lorresin embedding and
tei at and at ambient temperature (19-23C). then (IAll serial sectioning were further dehydrated, infiltrated,
LARVAL DEVELOPMENT IN AN OPHIUROID 381
Figure 1. Fertilisation andcleavage ofOp/iioiiereis schaveri. (a) Fertilized eggs have araised fertilization
membrane (FM) and large perivitelline space (arrow), (b) Fertilized eggs and four-cell embryos. Scale bars =
100 tun.
and embedded in Spurr's resin at 80C for 20 h. Larvae Formation ofa reduced ophiopluteus
were serial sectioned at 0.9 /im and stained with 1% tolu-
idine blue in 0.595- ethanolamine. Lan'al gut. Twenty-five hours after fertilization the gas-
trulae elongated, and the blastopore started to close (Figs.
2g, 3c). An anterior ventral depression marking the position
Results
of the stomodeal invagination was also evident (Figs. 2g,
3a), but this opening was much smaller than the mouth of
Early development
typical ophioplutei. Larvae sectioned at this stage had a
Fertilized eggs had a 50-jum-wide perivitelline space digestive tract with an incomplete lumen; the esophagus
between the oolemma and the fertilization envelope (Fig. was open to the exterior via the stomodeum, but its connec-
la, b). Although looked for, polar bodies were never ob- tion to the stomach was blocked by what appeared to be an
served, indicating that meiosis is completed during ovula- esophageal plug (Fig. 3a, b). The larval gut was filled with
tion and spawning. Cleavage was initially equal and radial mesenchyme cells, and opened posteriorly via the blas-
(Fig. 2a). After second cleavage, however, the division topore (Fig. 3c). Closure of the blastopore after 25 h.
planes became offset, resulting in an irregular arrangement however, resulted in formation ofa larval gut that was only
of the blastomeres (Fig. 2b). Beyond third cleavage, cell transiently complete, and thus the larvae did not feed. Sub-
division became asynchronous, with some cells cleaving sequently, the gut became a simple sac.
before others (Fig. 2b). The irregular arrangement of blas- Ciliated band. After 36 h and further elongation, a dis-
tomeres remained evident to the blastula stage. About 12 h tinct ventral ridge appeared near the posterior end of the
after fertilization, the embryos took on a slightly contorted larva, and the ciliated band subsequently formed along this
shape as they developed into wrinkled blastulae. The wrin- structure (Fig. 2g). Formation of the ventral ridge trans-
kles were shallow, forming slight indentations in the epi- formed the late gastrula into an early larva that was bi-
thelium (Fig. 2c). The blastulae subsequently smoothed laterally symmetrical through the anterior-posterior axis
(Fig. 2d), the epithelium became uniformly ciliated, and the (Fig. 2g).
embryos started to rotate within the fertilization envelope. The larva (335 /xm in length) developed a single contin-
The blastulae hatched about 16 h after fertilization and uous ciliated band about 42 h after fertilization (Fig. 2h. i).
swam with their anterior pole forward, in no particular The ciliated band ran from the anterior to the posterior end
direction or orientation. Gastrulation occurred about 21 h ofthe larva, marking its ventral and dorsal halves (Fig. 2i).
after fertilization by invagination of the vegetal plate (Fig. This was madeclearly visible by red pigmentcells that were
2e). During gastrulation, the epithelial cells changed shape associated with the ciliated band. The ciliated band also
from cuboidal (Fig. 2d) to columnar (Fig. 2f). Round cells looped ventrally to run along the ventral ridge, forming a
in the epithelium, which were situated towards the apical complete and continuous loop similar to that formed in the
surface, may be recently divided cells (Fig. 2f). Lipid drop- early stages ofembryogenesis in other ophioplutei (Fig. 2h.
lets in the eggs ofO. scluiveri were about 3 /u.m in diameter i). Although the larvae had this continuous ciliated band, a
(Moloney and Byrne, 1994) and were conspicuous in the characteristic of ophioplutei, pluteal arms were not formed
epithelial cells of gastrulae (Fig. 2f). (Fig. 2i).
382 ! SI I \ \KIA1ARAS\VAMY AND M HVRN1
Figure 2. Scanning electron micrographs ot the cail\ events ol de\clopineni in Ophionereisschayeri. (a)
Two-cellemI'iMI iIn()ltseielca\ageplanesandasynchronouscleavageresultsinhlasionieresol different si/.es.
i Wrinkled Nasinla with shallow wrinkled depressions (allowI. icll Cracked open hlaslula will) hollow
blaslocoel and ciliated epithelial cells, re) Uniformly cilialed LMsiiiila with vegelal hlaslopoie i.in.m). (f)
I i 'iniIn Imi11 siHIM.in:1 niliiuni.il cells with \rsihlc lipul ilroplets (.mowlie,ul) and loiniil cells ranow). (g)
a mil,i with closed hlastopore lammi and anterior stumodeal depression (111 l-.arh lar\a with a
dialed hand (arrowheads), (i) Side view ul .in eail\ l.n\,i showing the prominent ventral ridge and
the ciliated hand i rrrowheads) that bisects the lar\a into dorsal and ventral halves. Blastocoel. BL; cilia. C:
dorsal. D; teiiili/.iii..n nieriilnaiie. I'M; slornodeal depression. S; \enlral. V; ventral ridge. VR. Scale bars for
a-c, g-i = ' rfor1 Hi /xm.
Ijirvnl skeleton. The hilutcral larval skeleton lirst hecaine ridge and ciliated hand. The rods of the tetraradiate larval
evident in yastrulae as two birelrinjient spots on either side skeleton formed simultaneously, and consisted of the pos-
ot the hlastopore (Fig. 4a). As the gastrulae clongateil. ihe terior body rods (x = 52 /urn length. SE 7 jum. range:
skeleton quickly developed into a tetraradiate form (l-'ii:. 37-93 /mi. // 7), which anchored the three reduced larval
4b). The rods reached their maximum length 42 h after ,11ID mils The posterolateral rods (v -- 30 pun. SK := 4 /xm.
fertili/ation. when the larvae had developed the ventral i.iiiiv IX 4d /Jin. /; = 7) projected in the same plane as the
LARVAL DEVELOPMENT IN AN OPHIUROID 383
Figure3. Serial scmithin sections through a 28-h-old larvaofOphioneriesxchayeri. Sections are oriented
withtheannual,oranterior,endtowardsthetopofthe figure,andvegetal,orposterior,endtowardsthebottom,
(a) The stomodeum opens on the ventral surface, (b) The larval digestive tract is bipartite, consisting of an
esophagusthatisseparatedfromthestomachbywhatappearstobeanesophagealsphincter(arrow),(c)Thegut
is transiently open posteriorly i the blastopore. The gut is filled with mesenchyme cells (arrowhead).
Blastopore, B; dorsal. D; esophagus. O; stomodeum, S; stomach, ST; ventral. V. Scale bars = 50 fum.
body rods. The other two arm rods, the anterolateral and ated band, and transiently complete larval gut. The non-
postoral rods, were approximately equal in length (.v = 25 pluteal features include a simple barrel-like shape and
jam, SE = 3 /am, range: 12-36 jam, n = 9) (Fig. 4c). The nonfunctional gut. The presence of vestigial larval struc-
posterodorsal rods oftypical ophioplutei neverdeveloped in tures in this lecithotrophic larva is evidence that the ances-
the larvae of Ophionereis schnveri. Subsequent develop- tral form had a feeding ophiopluteus. Ophiuroids with
ment resulted in the skeletal rods all developing thorns planktotrophic larvae typically have small (80-185 jam)
simultaneously (Fig. 4d, e). eggs (Hendler, 1991; Sewell and Young, 1997). Acquisition
With onset of rudiment formation in the midventral re- ofa larger egg in O. schaveri was probably a preadaptation
gion of the larva, 48-60 h after fertilization, the two body for the evolution of non-feeding development, with subse-
rods grew and met midway at the posterior end ofthe larva quent loss ofsuperfluous larval structures. This sequence of
to form what appeared to be a continuous rod (Fig. 4d, f). evolutionary changes is similar to the evolution of non-
The apparentjoining ofthe two posteriorbody rods did not, feeding development in echinoids with large eggs, where
however, seem to be accompanied by the presence oftrans- loss ofpluteal arms and acquisition ofa simple larval shape
verse orend rods, as in typical ophioplutei. Simultaneously. also occurs (Olson ft <//., 1993; Wray. 1996).
the adult skeleton appeared as six small birefringent spic- The ciliated band in Ophionereis schayeri larvae is a
ules in the rudiment, corresponding to the central and five reduced expression of the continuous ciliated band typical
radial plates of the juvenile dorsal skeleton (Fig. 4d. e. f). of ophioplutei. similar to that seen in the brooded lecitho-
The larval skeleton started to break up as the rudiment trophic larvae of Ophionotus liexactis (Mortensen. 1921).
formed, and pieces of the larval rods migrated into the By contrast, its congener, Ophionereis anmtlata, has an
rudiment and could no longer be traced by polarized light early barrel-shaped larva that is uniformly ciliated, with no
(Fig. 4d). The adult skeletal spicules were very different vestige of a ciliated band (Hendler, 1982). A continuous
from the larval skeleton. They had the multirayed shape band is characteristic of feeding echinoderm larvae and is
typical of the early juvenile skeleton, and they occurred as considered to reflect not only the functional requirements
plates rather than rods (Fig. 4e. f). of feeding and swimming, but also selective pressures
for a particular feeding clearance mechanism (Strathmann.
Discussion
1974). Retention of a ciliated band by the non-feeding
The larvaofOphionereis schayeri, although non-feeding, larvae of O. schayeri is similar to the reduction of the
appears to be in an intermediate stage ofevolution, since it ciliated band in the non-feeding larvae of some echinoids
has features characteristic ofboth planktotrophic (ophioplu- (Olson et /., 1993; Emlet, 1995). Like the larvae of O.
teal) and lecithotrophic development. Its ophiopluteal fea- schayeri, the brooded larvae ofO. hexactis have a digestive
tures include the vestigial larval skeleton, continuous cili- tract consisting of a stomodeum, esophagus, and stomach.
384 P. SKLVAKl'MARASWAMY AND M BYRNE
Figure 4. Cross pnlan/cd light micrographs showing l;ir\al skclctogenesis in Ophidiniii'. \, li,niii'i. (a)
(iastrula with first appearance ot a lar\al spiculc lanowheadl; ihe other spicule is not in the plane ol locus (hi
I ,iil> lonnation ol the letiaradialc hilatcial lanal skeleton (arrowheads). Id An early lana with tclraiadiaic
skeleton consisting ot anchoring postcnor body rods and three arms rods -posterolatcrals. anterolaterals, and
postorals. (d) Rudiment formation and appearance ol |u\emle plates (4 unlaheled .mowhe.ids) The two larval
bodj iods meet midwav atthe posteiiorot the lanaandseemtohecomecontinuous(arrow).Thelanalskeleton
is thoinv. slants to break up. and migrates into the rudiment, lei I ana viewed from the postciioi. sluming
bilateral thorns lanal mds lammhcadsi. with |ii\cmle plates .ml ol locus in the background tli I .11\a with
lanal skeletonstill presentandwell-formedjuvenileplates. A mulluaycdshape istypicaloljuvenileechmodcim
plates Anterolateral iods. Al.; body rods. H: juvenile plates. .1; lanal skeleton. I . posieiolaleial iods. PL;
postoial rods. PC); and slomodeal depression. S. Scale bais 50 fim.
In contrast to (). schuvcri. HOWCVLT. these larvae lack an Morlensen. 1921: Narasimhamurti, 1933; Olsen. 1942;
anus (Morlensen. \(>2\). A transieiills coiit|ilete iligestive Mladenov, 1985). Accelerated rod formation into a tclrara-
tract, as seen in the lai\ae nl <>. \< Inncri. apparently has not diate configuration shows that a heterochrony in skeletogen
been previously reported for other Icaihotrophic echino- esis has occurred in the evolution of development in O.
derm larvae. \ihd\cn. A similar pattern of skeletogenesis is reported for
The skeletal mils in Ophionereis scliavcn larvae are the lecithotrophic larvae of .\/n/>liiiini cliiajei (Fenativ
considerably smaller than those ol planktotrophie ophioplu- I9foi and ,\nii>lii<>pl(is uhilitiia (Ilendler. 1978). and for the
tei (Mortensen, 1921 ). and thev do not support pluteal arms. planklotrophic. 8-armed ophioplulei ofAmphipholis kochii.
Moreover, all fourrods hmlv. posterolaleral. anterolatetal. which undergo rapid development (Yamashita. 1985). Ac-
and postoral formed simultaneously, and developed their celerated skelclogcnesis mav he a mechanism to reduce Ihe
thorns simultaneously. This omit.isis with t\pical ophioplu- time to metamorphosis during the evolution of lecitholro
tei. in which the body rod. pn-.vmlaleral rod. and antero- phy. The skeleton is further reduced in the Iccithotrophic
lateral rod develop prior to the postoral and posterodorsal larvae of Ophiothrix nerstcdi and Ophionereis iiniuiUita.
rods(MacHrtde. I'K)7; Narasimhamurti. I'J^VOKen. ll)42: which lotin only three distinct skeletal rods (Mladenov.
Mlailenov. 1985). In addition, the (horns on the roils ol 1979: Hendlcr. 1982).
ophiophitei do not i.UN at the same tune (MacHride. 11X)7: PosterodorsaJ arm rods, the last to be formed in Ivpical
LARVAL DEVELOPMENT IN AN OPHIUROID 385
ophioplutei, are not present in Opliionereis schayeri larvae, Within the genus Opliionereis, O. schayeri is the fifth
or in other ophiuroid larvae with a vestigial skeleton species known to have lecithotrophic development (Table
(Mortensen, 1921, 1933; Fell. 1945; Fenaux, 1963: Mlad- 1 ). Like O. schaveri, O. \tjiiamulosa and O. tiniuiliita
enov, 1979; Hendler. 1982). This suggests a potential se- have planktonic larvae (Mortensen. 1921: Hendler. 1982).
quence of morphogenetic change, with those functionally whereas O. rivipara and O. olivacea have brooded larvae
related larval structures (e.g., pluteal arms, skeletal rods) (Mortensen, 1933; Byrne, 1991). Only one species, O. fas-
that are formed late in planktotrophic development being cintii, is known to have planktotrophic development (Sel-
the first to be lost in the evolution of lecithotrophic devel- vakumaraswamy, unpubl. data), and thus most likely pos-
opment. This kind ofincremental loss, as depicted in Fell's sesses the ancestral larval form forthe genus. The relatively
(1945) diagrammatic series of ophiuroid larvae with a de- complete set oflarval structures in the non-feeding larvae of
creasing number of arms and shorter developmental times, O. xchaveri (continuous ciliated band, tetraradiate skeleton,
is considered to be an important mechanism ofevolutionary and transiently complete larval gut), indicates it may be the
change (Anderson. 1987; Raff. 1996). The evolution of most recent member of the genus to have evolved lecitho-
lecithotrophy in O. xchaveri has involved reduction of sev- trophy. Based on morphology, O. aiunilata might be ex-
eral larval structures, including the skeletal rods, ciliated pected to be next in this evolutionary progression, because
band, and gut, whose development is unlikely to be genet- it has retained only a triradiate skeleton. O. squamulosa
ically linked. Reduction in these larval structures may have would be expected next as it has a stomodeum and vestiges
occurred independently in response to similar selective of the ciliated band. The extreme end ofthe morphological
pressures. continuum would be illustrated by O. olivacea and O.
The brooded larvae ofOphionotus hcxactis are similar to vivipara, whichcompletely lack larval structures. Non-feed-
the larvae of Opliionereis schayeri with respect to the form ing development thus appears to becommon in thisclade. A
of the ciliated band, the presence of a partially developed molecular approach is needed to determine whether the
digestive tract, and some skeletal rods (Mortensen, 1921). switch to lecithotrophy in O. schayeri was a single event or
Similarly, the brooded larvae ofAmphipholis sc/iiamata and a multiple one as demonstrated for the sea star genus Pati-
A.japanica and the benthic embryos ofAmphioplns abilittts riella (Hart et ai. 1997). The causes of selection for a
have areduced larval skeleton (Murakami, 1940; Fell, 1946; particular mode of development in certain lineages are still
Hendler. 1978). These findings show that phylogenetically relatively unknown, but they seem to be more prevalent in
diverse ophiuroids with a range of developmental modes some echinoderm lineages than others (Pearse and Bosch,
have retained vestiges of an ancestral feeding state. Other 1994; Byrne and Cerra, 1996).
species with lecithotrophic development, however, show no
ophiopluteal features (Table 1). The evolution to lecitho-
Acknowledgments
trophy in ophiuroids has involved the reduction and loss of
a suite of ophiopluteal structures, resulting in an increas- This work was funded by an Australian Postgraduate
ingly simple larval form and accelerated development. Sim- Award. The authors thank Tony Romeo and Francis Chee
ilarly, in echinoicls, reducing the time to metamorphosis for help with electron microscopy. Roland Smith and Clive
during the evolution of lecithotrophy involved complex Jeffery for help with photography, and all those who as-
developmental processes, resulting in a variety of morpho- sisted in collection of specimens. The manuscript was
logical changes including loss ofpluteal arms (Wray, 1996). greatly improved by comments from an anonymous re-
This study demonstrates that the internal events ofdevel- viewer.
opment in what appear, externally, to be relatively simple
lecithotrophic larvae warrant careful examination. Detec-
tion of the larval gut and skeleton in the larvae of Op/iio- Literature Cited
nereis scha\eri provided important information on the ex- Amemiya,S.,and R. B. Emlet. 1992. Thedevelopmentandlarval form
tent of evolutionary change in development. A larval of an Echmothurioid echinoid. Asthenosoma ijimai. revisited. Biol.
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this larval form evolved from an ophiopluteal ancestor Byrne, M. 1991. Reproduction, development and population biology of
(Mortensen. 1938; Hendler. 1982) and is not divergent, as the Caribbean ophiuroid Ophionereis olivacea. a protandric hermaph-
previously thought (Fell, 1945). Similarly, the discovery of rodite that broods its young. Mar. Biol. Ill: 387-399.
larval rods in the larvae ofAsthensoma ijimni and Helioci- Byrne,M.,and A.Cerra. 1996. Evolution ofintragonadaldevelopment
in the diminutive asterinid sea stars Patiriella vivipurn and P. pur-
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