Embryonic Development of the American Lobster (Homarus americanus): Quantitative Staging and Characterization of an Embryonic Molt Cycle

17 Pages·1991·7 MB·English

by  S M Helluy

Reference: Biol Bull. 180: 355-371. (June. 1991)
Embryonic Development of the American Lobster
(Homams americanus): Quantitative Staging and
Characterization of an Embryonic Molt Cycle
S. M. HELLUY AND B. S. BELTZ
Department ofBiological Sciences. Wellesley College, Wellesley, Massachusetts 02181
Abstract. The growth of a single brood of lobsters in older lobsters (Aiken, 1973). The quantitative staging
(Homarus americanusMilne-Edwards 1837) maintained of lobster development from extrusion to hatching, and
atconstanttemperature isstudiedfromthenaupliarstage thedescription ofthe embryonic molt cyclewill facilitate
to hatching, and the sequence ofappearance ofmorpho- future investigations on particularaspects oftheembryo-
logical, anatomical, and behavioral characteristics ob- genesis ofHomarus such as neural differentiation.
served. A percent-staging system based upon Perkins' eye
index (1972) is presented, and ten equally spaced embry- Introduction
onic stages are illustrated and characterized at different Studies on lobsters and othercrustaceans have made a
levels of resolution: whole eggs, dissected embryos, an- significant contribution to our understanding of neural
tennulae and telsons. The tegumentary and setal changes organization andthecontrolofbehavior(seeWieseelai.
in the telson show that a complete molt cycle takes place 1990). There is increasing interest in examining the on-
in the egg starting at about 12% embryonic development togenesis of particular behaviors and the cellular archi-
(El2%) with the molt ofthe nauplius into the metanau- tecturethat isthe basis forthose behaviors(Kravitz, 1988;
plius and endingjust after hatching when the metanau- Govind. 1989; Sandeman and Sandeman, 1990). Re-
plius moltsintoafirst stage larva(LI, first zoea). At E30%, search on neural development at the embryonic level in
the cuticle beginsto separate from the setae in the telson; Homarus is flourishing (Cole and Lang. 1980; Beltz and
D
this signals the start of Orach's (1939) stage of the Kravitz, 1987; Beltz el a/.. 1990; Helluy and Beltz, 1990;
metanaupliar embryonic moltcycle. Atthattime, the first Meier and Reichert, 1990), but progress hasbeen limited
sign oforganogenesis ofthe LI, the formation ofthe en- by the lack of adequate documentation on the general
dopod oftheantennulae, becomes visible; presumed sen- development ofthisorganism in theegg, aswell asby the
sory neurons and their axons are observed at the tip of absenceofastagingsystem forthetotal embryonic period.
the exopod ofthe antennulae where a giant sensillum is These two problems are addressed in this paper.
differentiating. DuringD thesetaeofthe first larval stage Recent developmental studies in Homarus have dealt
are forming proximally and medially in the bilobed telson primarily with the perihatching period (Davis, 1964; En-
under the metanaupliar cuticle. At E90%, these setae are nis, 1975; Charmantierand Aiken, 1987), and larval and
retracting, and the embryo has entered stage D,. After postlarval life (Phillips and Sastry, 1980; Charmantier,
hatching (100%), the telson of the free metanauplius 1987), whereas most ofthe literature concerned with the
(prelarva) shows the characteristics ofstage D:_3 and ec- prehatching period dates back to the nineteenth century
dysissoon follows. Thearresteddevelopment observedat (Bumpus, 1891; Herrick, 1895). The latter studies are a
constanttemperatureintheexperimental broodoccurred remarkable achievement of patient and detailed obser-
at stage D ofthe metanaupliar molt cycle, whereas de- vation and are illustrated by elegant drawings (Herrick,
velopmentwasresumedastheembryosentered stage D, . 1895), but the modern microscopic and photographic
D
These changes in developmental pace from to D! in methodsused inthisstudyarenecessarytoprovideadded
the embryonic molt cycle are parallel to those occurring resolution. The nineteenth century studies also tend to
focus on early embryogenesis while providing little orno
Received 3 December 1990;accepted 8 March 1991. information about middle and late development in the
355
356 S. M. HELLUY AND B. S. BELTZ
egg, and the embryonic molt cycle. A deeper knowledge Materials and Methods
of lobster embryology could also provide more insight
and understanding ofstudies that examine particular as- Lobsterandegg maintenance
pectsofdevelopment, suchastheinfluenceoftemperature Egg-bearing female lobsters Homarus americanus
on growth rate (Templeman. 1940; Perkins. 1972). pop- (Crustacea, Malacostraca, Decapoda, Reptantia, Asta-
ulation dynamics(Schuuret al. 1976; HepperandGough, cidea, Nephropidae) were obtained from the Massachu-
1978), the chemical composition and calorific content of setts State Lobster Hatchery on Martha's Vineyard, Mas-
the eggs (Pandian 1970a, b; Sasaki, 1984; Sasaki et al. sachusetts, and kept in recirculatingartificial seawater. In
1986), or the differentiation ofparticular organs or sys- addition, eggsdetached from the mother'sabdomen were
tems, such as heart and gut (Burrage, 1978; Burrage and provided bythe New England Aquarium in Boston, Mas-
Sherman, 1979), and, again, nervous system. sachusetts, where lobsterswere reared in filtered, temper-
The principal featuresinvolved in the reproduction and ature-controlled seawater. These detached eggs were
early development of the lobster Homarus americanus maintained in our laboratory in free-floating net enclo-
arewell known. Aftercopulation, spermatozoaarestored sures in artificial seawater. We found that hanging the
by the female for several months until oviposition and clumps ofeggs with surgical thread, and allowing them
fertilization occur (Aiken and Waddy, 1980). In New to float, led to good survival rates. Three tanks were
England waters, eggdevelopment spansabout 10 months, maintained at temperatures of 10 2C, 18 2C. and
from egg extrusion in July or August to hatching the fol- 20 2C, to slow or accelerate the rate ofdevelopment
lowing May orJune (Bumpus, 1891; Herrick, 1895). Fol- ofthe eggs, at a salinity between 27 and 32 ppt in a 12:
lowingextrusion, the eggs are carried on the abdomen of 12 lightdark cycle.
the mother, attached to the pleopods. Homarus has rel-
atively large, telolecithal eggs. Superficial cleavage leads The experimental brood
to the formation ofa blastoderm, and the central mass
ofyolk remains undivided (Bumpus. 1891). After only a We have not had any success promoting egg extrusion
fewdays, the naupliarorganization isapparent. The nau- in females held in recirculating tanks, probably because
plius, which is a developmental hallmark ofcrustaceans, ofthevarietyofcomplexenvironmental factorsnecessary
ischaracterizedbythepresenceofamedianeyeand three for this event (Waddy and Aiken, 1984). Therefore, in
pairsofappendages: the antennulae, antennae, and man- mid-October, the egg-bearing female containing the
dibles (Shiino, 1988). The metanauplius arises from the youngest eggs was chosen from a collection ofapproxi-
differentiation and growth of the postmandibular ap- mately 200 gravid females collected by fishermen forthe
pendages. Homarus hatches as a mature metanauplius State Lobster Hatchery. Theearliest stage observed in the
(prelarva. prezoea) that molts rapidly into the first larval experimental broodwasacleavagestage.Theapproximate
stage (Davis, 1964). There are three pelagic larval stages date ofextrusion was calculated as follows: in Temple-
swimmingwiththefeatheryexopoditesofsixpairsoftho- man's (1940) experiments, the period from the late nau-
racic limbs. A metamorphosis leads to the formation of plius to the first appearance ofpigment in the lateral eyes
a postlarva (fourth stage) with most ofthe adult charac- (26 days) lasted about 45% ofthe time required for the
teristics (Charmantier, 1987). The postlarva, which swims development fromextrusion toappearanceofeyepigment
ina fullyextended posture usingitspleopods. latersettles (58 days) at 12-13C, and 41% (11/27) in Herrick's ex-
on the substrate. The duration oflarval life, in the order periments (1895, p. 56) at 21C. In the lobster (Temple-
ofa few weeks, depends largely on temperature. man, 1940; Perkins, 1972) and in insects (Bentley et al.,
For the present study, behavioral, morphological, an- 1979), developmental events are more condensed or ex-
atomical, and morphometric data were gathered from panded in time depending on temperature, but the pro-
whole eggs and dissected embryos. A percent-staging portion ofthetotal duration ofembryogenesisdevotedto
scheme usingthe size ofthe pigmented area in the lateral each developmental event does not change with temper-
eyes [the eye index (Perkins, 1972)] was adopted. Sub- ature. In the present studyat 18C, thedevelopment from
sequently, ten equally spaced developmental stages were late nauplius to the first appearance ofeye pigment took
documented in detail with the eggs ofdifferent females. 9days; therefore, byextrapolation from the data ofTem-
Particular attention was given to the growth and differ- pleman (1940) and Herrick (1895), the period from ex-
entiation ofthe antennulae and telson. The antennulae, trusion to first appearance ofeye pigment would be pre-
which are lined with aesthetascs (olfactory sensilla) in dicted to last 20-22 days. Thus, the estimated date of
postembryonicanimalsfrom second larval stageon, were extrusion was calculated to be 21 days prior to the ap-
examinedtogain insight intotheontogeny oftheolfactory pearance ofeye pigment. Note that extrusion did occur
sensory apparatus. The telson was studied to elucidate in the wild in water at seasonal temperatures.
howtheroundbilobedtelsonoftheembryoistransformed Observations were made on the experimental brood
2C
into the triangular telson ofthe first larval stage. kept at 18 for five months (mid-October to mid-
EMBRYONIC DEVELOPMENT OF THE LOBSTER 357
March, see Table I). The female died in mid-January, Table I
when theeggswereat66% development: shewasstripped Dalesofobservation ottheexperimentalbroodol Homarus
ofeggs and the spawn was suspended in nets in the tank. amencanusmaintainedat 1SC. age. eyeindex, and
Theeggswereagitated daily totry to replace the vigorous ppeerrcceenntt-ostfatgoitnalglsivsmteemf.roTmheextdroutstieodnltionehsaitgcnhailnsgtahnedtrpaenrscietnitonolbeetyweeen
beatingofthe pleopods ofthe mother. In theexperimental indexathatching
brood,the majorityoftheeggsattained the hatchingstage
butvery fewactually hatched into free metanaupliae; still
fewer molted into first larval stages. Those larvaethatdid g
emerge were perfectly normal animals. The smoothness
ofgrowth curves ofthe experimental brood (see Results)
and numerous observations on the progeny ofother fe-
males confirmed that the free eggs of the experimental
brood followed a normal course ofdevelopment afterthe
death ofthe mother.
Five live eggs from the experimental brood were ex-
amined every two or three days for the first two months,
then onceaweek until hatching. Assoon asthe heartwas
formed, the heart beat was confirmed in each embryo to
ensure that the observed eggswere alive. Duringeach ob-
servation period, the width and length ofthe pigmented
area in the lateral eyes(Fig. 1) and the greatest axisofthe
eggweremeasured in intacteggs: followingdissection, the
length ofthecephalothorax was measured ventrally from
the median eye to the anterior margin ofthe abdomen
(Fig. 2). Behavioral observations, such as antennal
twitching or tail flipping during dissection, were also
noted. Photographs ofwhole eggs and dissected embryos
were taken with a Zeiss stereomicroscope.
Developmental staging system
A developmental scale was designed that used the eye
index (Perkins, 1972) as a marker ofdevelopmental pro-
gress. Theeyeindex isdefinedastheaverage ofthe length
and the width of the brown screening pigment spot (in
micrometers) inthe lateral eyes. The first measurable eye
pigment spot had an eye index of70 ^m (Perkins, 1972;
present study). Therefore, development prior to the ap-
pearance of eye pigment was characterized using time
rather than the eye index. The estimated duration ofde-
velopment oftheexperimental broodwas 159days(Table
I). Eye pigment first appears at 13.2% of the total time
from extrusion to hatching, while the eye index at first
appearance ofpigment (70 jum) is 12.2% ofthe eye index
ofthe experimental brood at hatching (578 /^m) (Table
I).Thesevaluesindicatethatthereislittledifferenceduring
early embryogenesis between staging based on time and
that based on the eye index; time-staging was used prior
to, and eye index-staging after 15% development (Table
I). Later in embryogenesis, because ofthe period ofde-
velopmental arrest (see Results), stagingbased upon time
is no longer valid; the morphometric marker (eye index)
must then be used.
358 S. M. HELLUY AND B. S. BELTZ
imposed by the precision of the ocular micrometer. ican lobsteregghasbeen calleda"post-nauplius" by Her-
Twenty micrometers represents 3.5% ofthe total devel- nck(1895)or"postnauplius" by Helluyand Beltz(1990).
opment scale. To characterize each ofthe ten stages and In the present study "metanauplius" isused, aterm com-
the early postembryonic stages, the same protocol de- monly assigned to the form developingjust past the nau-
scribed earlier forthe experimental brood was used. Pho- pliarstage(Wear, 1974; Williamson. 1982;Shiino, 1988).
tographs ofwhole eggs (Figs. 3, 4) and dissected embryos The form that is released from theeggenvelopesand rap-
(Fig. 5) were taken with a Zeiss stereomicroscope. In ad- idly molts into the first larval stage is usually called a
dition, antennulae (Fig. 6) and telsons (Fig. 7) were also "prelarva" or"prezoea"; however, theterm "mature me-
severed, examined fresh, and photographed usinga Zeiss tanauplius" seems more biologically relevant (see Dis-
IM35 photoinvertoscope equipped with modulation con- cussion). The first larval stage ofHomarus is sometimes
trast optics (Hoffman, 1977). referredtoasa "mysis" (Shiino, 1988)basedonthe num-
Drach (1939) and Drach and Tchernigovtzeff (1967) berofitsappendages, orafirst"zoea"becauseitlocomotes
designated the phases ofecdysis in crustaceans by letters with its thoracic appendages (Anderson, 1982; William-
from A to E: A and B are the postmolt periods, C the son, 1982). Finally, "embryogenesis," "prehatch," and
intermolt, Dthe premolt period, and E theecdysisproper. "egg development" are used interchangeably, although
In the present study, this system was used to characterize the latter part ofeggdevelopment in Homarus isdevoted
the molt cycle of Homarus that occurs within the egg to larval organogenesis rather than to embryogenesis
envelopes. The period ofthe embryonic molt cycle was strictly defined.
determined by matching setal changes in the telson of
embryos(Figs. 7 and 8)withthe setal changesinthetelson Results
previously documented in larvae ofHomarus americanm
(Rao el al., 1973; Sasaki, 1984) and in the pleopods in /. Timing ofdevelopment, sequence ofevents,
juveniles (Aiken, 1973; 1980) during molt cycles. The andcharacterization ofembryonic
subdivisions ofstage D (D , D,, D:_,) and their distin- andearlypostembryonic stages
guishing features are those described by Sasaki (1984).
Inthe followingaccountoftheembryogenesisofHom-
arus americanus, the sequence ofdevelopmental events
Terminology
and morphometric data on eye index (El) and cephalo-
More than 70 terms have been used to refer to the thoracic length (Figs. 1, 2) were obtained by studying the
various embryonic and larval stages ofdecapods (Gore, experimental brood, whereas the illustration and char-
1985). The form that arises from the differentiation and acterization ofequally spaced embryonic and early post-
growth ofthe postmandibular appendages in the Amer- embryonic stageswasachieved bystudying manyclutches
Eye
EMBRYONIC DEVELOPMENT OF THE LOBSTER 359
360 S. M. HELLUY AND B. S. BELTZ
60
H
70 80
EMBRYONIC DEVELOPMENT OF THE LOBSTER 361
theanterioredgeoftheseappendages in a bundleofa few Sequenceofeventsfrom 40% to50% development. The
micrometers(Fig. 6D). Serial plasticsections haveshown red chromatophores have invaded the appendages and
that the bundle of axons projects to the olfactory lobes thegrowingcarapaceby E43%andtheabdomen byE49%.
(in the deutocerebrum), which measure about 40 nm in 50% development (El = 285 urn; Figs. 3E, 5E, IF). By
diameteratthatstage(unpub. results). Theclusterofneu- E50%, some embryos perform very cleartailflipsafterre-
rons is very similar to the cluster ofbipolar sensory neu- moval ofeggenvelopes; also, the first caeca ofthe paired
rons that innervate each aesthetasc (olfactory sensillum) digestive glands (hepatopancreas) are seen, with the
in the antennulae of spiny lobsters (Griinert and Ache, stereomicroscope, attheanteriorendofthemidgutwhere
1988). At E30% also, the endopod of each antennula it comes in contact with the mass ofyolk. The rostrum
tippedwithapointedseta, isvisibleunderthecuticle(Fig. ofthe differentiating LI is folded ventrally between the
6D). Theendopods are freed afterhatchingwhen the me- opticlobesandisvisibleupondissection. Thegapbetween
tanauplius molts into a first larval stage (LI). The ap- the cuticle and the six or seven most distal and lateral
pearance of the endopod of the antennulae is the first setae on each side ofthe telson has widened, but the tips
visible sign ofthe formation ofthe LI under the cuticle ofthesesetaeare still in contact with thecuticle(Fig. 7F).
ofthe metanauplius. All postmandibular appendages are Other setae are growing more medially and more proxi-
present and the trunk islined with six pairsofprominent mally beneath the cuticle ofthetelson. By now, thedistal
appendages: a pair ofthird maxillipeds and five pairs of ends ofthe third maxillipeds, as well as the telson, reach
wapapleknidnaggelesg,s.wDhuircihngarteheunmiertaamnoauusp,licaarnnpohtaseb,e tsheepatrrautnekd antSeerqiourelnycteootfheevelnevteslforfomth5e0o%ptiucntliolbehsa.tching. There are
from each other. The tips ofthe third maxillipeds reach nofotohbeveimobusrycohafnrgoemsEi5n0t%heugnteinlerhaaltcehxitnergn(al10m0o%r)p.hoHloowg-y
a level between the point ofinsertion ofthe antennulae
ever, the embryo grows dramatically and the structures
andtheanterioredgeoftheopticlobeswhereasthetelson
typical ofthe LI are forming progressively beneath the
isatthe level oftheanterioredge ofthe optic lobes [stage
cuticle ofthe metanauplius.
"Q" ofBumpus (1891)]. In the telson, the metanaupliar 60% development (El = 342urn:Figs. 3F. 5F, 6F, 7G).
cuticle begins to separate from the side ofthe setae but At this stage the yolk occupies about halfthe volume of
the tip ofthese setae is still attached to the cutDicle (Fig. the egg. At least 1 setae are formed on each side ofthe
7D); this signals the start ofthe premolt stage n ofthe
telson (Fig. 7G).
metanaupliar molt cycle (see II, below). 70% development (El = 399 n>n: Figs. 3G, 5G, 7H.
"RS"eoqfueBnucmepoufseviesnrtesafcrhoemd3be0t%weteon40E%30d%evaenldopEme3n7t%.wSthaegne 8oBf)t.heThfiersftulllarcvoalmpsltaegmeeinstproefsseenttaeo(n14thoert1e5lsoonn euancdhersitdhee)
the tip ofthe third maxillipeds is at the level ofthe an- metanaupliar cuticle; the median spine begins to differ-
tennae and the telson grows beyond the optic lobes. By entiate (Figs. 7H, 8B).
E37%, the eye pigment spots have become oval rather 50% development (El = 456 urn: Figs. 3H, 5H, 6G,
than crescent-shaped, and red pigment granules line the 71). Inthetelson, onlythemost medialsetaeareincontact
sides ofthe nerve cord. with the metanaupliar cuticle. These setae have not yet
40% development(El= 228urn;Figs. 3D. 5D, 6E, IE). assumed the shape ofspines, and the embryos are still in
A giant sensillum (260 /urn) is visible as a long straight stage DO ofthe metanaupliar molt cycle (Fig. 71).
rod inverted at the tip ofthe exopod ofeach antennula. 90% development (El = 513 \im; Figs. 4A, 51, 6H, 7J.
Setae are present at the extremities oftrunk appendages. 8C). The egg is now enlarging rapidly, and its largest axis
mm
Red chromatophores are seen on the sides ofthe nerve measuresabout2.0 (Fig. 2).Theyolkisturningyellow
cord, on the anterior edge ofthe optic lobes, and on the (Fig. 4A). The telson manifests a number of dramatic
growingcarapace. Thethird maxillipedsreach theanterior changes(Figs. 7J, 8C). Thecuticle hasliftedentirely from
edge ofthe optic lobes, and the telson reaches anteriorly the setae and also from the epidermis on the lateral sides
to the optic lobes. This stage is moreadvanced than stage ofthe telson. The two most lateral setae are now pointed
"R." the most advanced stage described by Bumpus and sharp like spines, and they begin to retract. About a
(1891). third of each seta is visible beneath the tegument. The
Figure 3. Unfixed, intact eggs ofHomarus americanus at (A) 10, (B) 20, (C) 30, (D) 40. (E) 50, (F) 60,
(G)70, and (H) 80% embryonicdevelopment. The figuresin the lowerleft corners referto the percentage
ofdevelopment. Inall photographs, thedorsalside isatthetop,andtheheadandtelson oftheembryoare
ontheright. At 10%development(E10%), theembryoisseenasasmall haloatthebottom partoftheegg.
Theeyepigmentisvisibleinthelateraleyes(le)byE20%.Theredchromatophores(ch)alreadypresentby
E40% arelabeledat E60%. The intestinalgranules(ig)areparticularlyclearat E70%. Scalebar: 500/jm.
362 S. M. HELLUY AND B. S. BELTZ
Figure4. PerihatchingdevelopmentofHomarusamericanus. Inallthesephotographsofunfixedspec-
imens, dorsal side is at the top, and anterior is right. (A) 90% embryonic development. (B) Embryojust
priorto hatching(100%, blue embryo): note the blue tinge ofthe hemolymph, the blue stomach (st) and
the red chromatophores (ch) in which the pigment isstill concentrated. (C) Hatchling: the outer (oe) egg
envelopehasburst,andthetelson (te) ispiercingtheinnereggenvelope; theredpigmenthasspreadinthe
star-shapedchromatophores(ch). (D) The metanauplius(prelarva, prezoea) is now freeofboth outer(oe)
andinner(ie)eggenvelopes. (E) Earlyfirst larval stage(first zoea): theexuvia(ex)ofthe metanauplius has
beensloughed. (F)Maturefirstlarvalstage: rostrum,abdominalspines,andotheracuminatestructuresare
nowerect. Scalebars: 500nm.
EMBRYONIC DEVELOPMENT OF THE LOBSTER 363
epidermis forms papillae around each seta, and appears the first larval stage after hatching (metanaupliar molt),
scalloped. Retraction ofsetaeandscallopedepidermisare a complete molt cycle is observed in the setal changes of
characteristic ofstage D, (Sasaki, 1984). the telson. The cuticle begins to lift away from the telson
100% development (El = 570 urn; Figs. 4B. 7K). At at about E30% (Fig. 7D) when the metanauplius enters
this stage, just prior to hatching, the egg (2.2 mm) is stage DO. The shape ofthe 6 + 6 setae on the telson of
brightly colored (Fig. 4B). The stomach is deep blue and themetanaupliusiswelldefinedon thiscuticle. Setaeform
thehemolymphpaleblue.Theyolk,which hasbeennearly then medially and proximally, and by E70% (Fig. 7H,
entirely absorbed, is yellow or pale green. The two pairs 8B), the full complement ofsetae ofthe first larval stage
ofyolk caeca that were filling the eggearlier are attached (15 + 15) is formed. Between E80% (Fig. 71) and E90%
to the digestive tube dorsally by this time, between the (Fig. 7J), dramatic setal and tegumentary changes occur
pyloric stomach and the numerous tubular digestive asthe metanauplius enters stage D,. Particularly striking
glands. The bilateral spines ofthe telson are entirely re- isthetransformation ofthemost lateralsetaeintostraight
tracted, whereas the setae are only partially so. and sharp spines (compare Figs. 71 and J, 8B and C).
Eclosion ofthemetanauplius (fiatcliling) (Figs. 4C, 61, These spines are invaginating as well as all the setae. In
7L, 8D). The outer egg envelope has burst. The red pig- addition, theepidermisbecomesscalloped, andthecuticle
ment disperses in star-shaped chromatophores (Fig. 4C). lifts from the sides of the telson. Just prior to hatching
Thegiant sensillum iseverted and projects from the exo- (E100%), retractionofspinesandsetaeismaximal. When
pods of the antennulae, but is still confined within the the outer egg envelope ruptures, the bulging of the epi-
cuticleofthemetanauplius(Fig. 61). Thespinesandsetae dermisaroundthesetaeDispronounced,andthe metanau-
ofthe telson begin to expand (Figs. 7Land 8D). The epi- pliushasenteredstage 2_3. Afterhatching, themetanau-
dermis becomes very distinct and forms a pronounced plius molts into the first stage larva and the cuticle which
bulging around the invaginated setae; theseare two char- is discarded still has the typical metanaupliar shape with
actFerrieseticmsetoafnsatuapgeliDus:_3(porfelSaarsvaak,ip(r1e9z8o4e)a.; Fig. 4D). The theGrimopwrtihntcuonf'etshean(d6 d+ev6e)lmoeptmaennatuapllpilaartesaeut.ameT.mhegreatest
imtetisanmaousptlliyusstiilsl,frbeuetd oocfctahseiotnwalolyextpeerrnfaolrmesggsetnrvoenlgopteaisl;- atxoiasboofutthe1.e8ggmimncraetasEe8s0g%radaunadllmyofrreomrap1i.d6ly toa2t.2E1m0m%
pflriopcs.esTsh.eWsietmhoinvehmoeunrtssafptrerestuhmeaebglgymfeacmiblirtaatneetshearmeolsthiend,g a1t8hCa,tcbhoitnhgt(hFeige.ye2).inIdnexth(eFieg.xp1e)rainmedntthaelcberpohoaldotrahiosreadciact
theMomlettaonfatuhpelimuestamnoalutpsliiuntsoaandfirestmelrargveanlcsetaogfef.irst larval tlheensgethvasrhioabwleeds caoluolgdarbiethmmeiacsugrreodwtthofarpoprmoxthiemaftiersltytdiamye
stage (Figs. 4E and F, 6J. 7M). The exuvia peels away 110. In Figure 2, cephalothoracic length appears linear
from the metanauplius. Slowly the rostrum, the abdom- because it isexpressed asa function oftheeye index. The
eggs ofthe experimental brood reached a developmental
inal spines, andalltheotheracuminate structuresbecome plateau at an eye index ofabout 474 (E82%). Until this
(erFeicgt..7TMh)e.tTehlseonseotpaeenesvlaigkienaatfea.nTihnteoeantdrioapnogdulaorfsetarcuhctaunr-e sitnatgeer,inddeivveildouaplmevnatrioafbitlhietyeggwsitwhaisnstyhnechbrroonoodu.sTwhitihslwitatlse
tennula is released (Fig. 6J), as well as the feathery exo- shown bythelowstandarddeviation oftheeyeindexand
tpeodnintuelsaoatfhthaetcshiixnpgaihrassoofnethgoiraancticseanpspielnlduamg(e5s5.0E/a*cm)haannd- eogfgtshewecreeph"aglroetehno"raaxnidc tleankgetnha(tFirgas.nd1o,m2.).HUonwteivle8r2,%a,ftaelrl
three setae at the tip ofthe exopod, and one seta at the E82%, the population was no longer homogeneous, and
tipoftheendopod(Fig. 6J)asreportedby Herrick(1895). the naked eye could distinguish by size and color three
In brief, the curvaceous metanauplius molts into an categories of eggs: "green," "yellowish" (Fig. 4A), and
angular larva ready to assume a pelagic life. "blue" (Fig. 4B). In addition, the eggs hatched overa pe-
riod ofabout a month, again indicating significant vari-
//. Metanaupliar embryonic molt cycle, growth curves ability between eggs in a single brood. After E82% it was
anddevelopmentalplateau no longer possible to choose eggs randomly for observa-
tion; toassign an approximate age toeach ofthese stages,
Metanaupliar embryonic molt cycle. At about El2%, "yellowish eggs" were examined on day 152 when the
an envelope is seen enshrouding the telson (Figs. 7B and majority ofeggs had reached this stage, and "blue eggs"
8A), stretchedatthetipsofthe 12 (6 + 6)bilaterallypaired (the hatching stage) were examined a week later. Indi-
setae on the telson of the nauplius. This envelope is vidualeggstookabouttwoweekstochangefrom "green"
thought to be the exuvia ofthe naupliar stage; it is flat to "blue" eggs at 18C.
and was formed in the nauplius when the tip ofthe ab-
domen had not yetacquiredanysetae. Themetanauplius Discussion
thatemergesatthe naupliarmolt hasbeen formingduring Inthepresentpaper, differentaspectsoftheembryonic
the naupliarstage. From this molt until theemergence of developmentofHomamsamericanusareexaminedfrom
he