Aesthete canal morphology in twelve species of chiton (Polyplacophora)

2007·12.5 MB·English

by  Christine Z Fernandez

THE VELIGER
TheVeliger49(2):51-69 (July 2. 2007) < CMS. Inc., 2006
Aesthete Canal Morphology in Twelve Species of Chiton (Polyplacophora)
CHRISTINE FERNANDEZ'
Z.
University of California, Los Angeles, CA 90095
MICHAEL VENDRASCO-
J.
Department of Earth and Space Sciences, 595 Young Drive East, University ofCalifornia,
Los Angeles, CA 90095-1567, USA
AND
BRUCE RUNNEGAR
Department of Earth and Space Sciences, Institute ofGeophysics and Planetary Physics and Molecular Biology
Institute, 595 Young Drive East, University of California, Los Angeles, CA 90095, USA
Abstract. Epoxy casts were made ofthe aesthete canal system in chiton valves (shell plates) from twelve species,
representing four families and the three major modern suborders ofthe Polyplacophora. In this study, Mopaliidae
wasrepresented by Mopaliamuscosa, Mopaliaacuta, Nuttallochitonhyadesi, andPlaciphorella velata; Tonicellidae by
Lepidochitona hartwegii and Nuttallma californica; Ischnochitonidae by Lepidozona cooperi, Lepidozona mertensii,
Lepidozona pectinulata, Ischnocliiton te.xtilis, and Ischuochiton vaviegatiis; and Lepidopleuridae by Lepidoplewus
cajetanus. The casts reveal a diversity oflarge and small-scale canal forms in the chitons studied. However, members
of each suborder and family share fundamental features of the aesthete canal system, which suggests that epoxy
casting of the aesthete canals provides a set of characters useful in future taxonomic and phylogenetic studies of
chitons. The casts also reveal a greater connectivity in the total aesthete canal system than is widely realized. For
instance, canals in the apical area connect to those in the slit rays, the ventral area below thejugum, and the dorsal
surface of the valve. Canal morphology also seems to be influenced by the shell layer in which canals occur. For
example, those canals that exist within the articulamentum are much more flattened in cross section than those that
occur in the tegmentum.
INTRODUCTION canals throughmuch ofitsvolume, and the distribution
and nature of the canal elements varies between the
Chiton valves (shell plates) consist ofa thin outermost valve areas in at least some chitons (Fischer & Renner,
organic periostracum layer and three underlying
1979). Although most discussion of aesthete canals in
aragonitic layers: tegmentum (upper), articulamentum
chitons has focused on the tegmentum layer where the
(middle), and hypostracum (lowermost). The principal
canals are densest, the presence of pores in the jugal
shell layers are the tegmentum, which bears the shell sinus and slit ray regions of the ventral surface of the
sculpture, and the underlying denser articulamentum,
valves indicates that the aesthete canal system infil-
whose marginal projections form the insertion plates
and sutural laminae (Baxter & Jones, 1981). Marshall t&i^ates the articulamentum layer in certain areas (Baxter
Jones, 1981).
(1869) was the first to describe the tissue-filled canal Moseley (1884, 1885) was one ofthe first to desci-ibe
system that penetrates the chiton tegmentum, pointing
aesthete tissues in fine histological detail, noting the
out that fine vertical canals at the surface connect to occurrence of two distinct size classes: he named the
bulbous cavities that in turn lead to horizontal canals larger ones megalaesthetes and the smaller ones
that run at the interface between the tegmentum and
micraesthetes. In addition, he discovered "eyes," or
articulamentum. The tegmentum is penetrated by ocelli, characterized by a refractive lens and the
presence of pigmentous cells seen in much larger
chambers within the valves of certain chiton species
'Present address: 3311 Calle Resales, Santa Barbara,
California 93105. USA. (Moseley, 1885), and Boyle (1969) nearly a century
-Presentaddress: InstituteforCrustal Studies, Universityof later confirmed the presence of photoreceptors within
California, Santa Barbara. California 93106, USA. these "eyes."
Page 52 The Veliger, Vol. 49, No. 2
anterior
posterior
Figure 1. Drawing ofchiton valve showingterminology (dorsal valve surface on left; ventral on right). Key: JS =jugal sinus; SL
= suturallaminae; DL = diagonal line; S = slit; SR = slitray;JA =jugalarea; PA = pleuralarea; LA = lateralarea; AA = apical
area; VJT = ventraljugal triangle.
Micraestiietes typicallyconsist ofa singlecell with its channels; and slit ray channels. Multiple branch
nucleus lodged within the megalaesthete chamber. channels open along the anterior margin of the
Megalaesthetes are larger, multicellular organs gener- tegmentum. These branch repeatedly along their hori-
ally composed of several secretory,cells, microvillous zontal pathway through the tegmentum, with branches
cells(centralcells), oneormorephotoreceptorcells, and subsequently leading up to megalaesthete and mi-
peripheral cells (Fischer, 1988; Eernisse & Reynolds, craesthete canals just below the valve surface. Slit ray
1994). However, megalaesthete cellular composition channels open along the lines ofpores (slit ray) on the
variesbetween speciesand at least onespecies, Tonicella ventral surface ofvalves that have slits in the insertion
mamiorea, lacks photoreceptor cells altogether (Baxter plates,asoccursinmostchitonspecies. BaxterandJones
et al., 1987). OcelH, which occur in much larger spaces (1981) argued that these channels pass dorsally and
that are thought to be modified megalaesthete cham- posteriorly from the ventral openings through all shell
bers, have so far been found injust a few chiton species layers to open at the dorsal valve surface. Jugal area
andaresparselydistributedonthevalvesurfaceinthose channels form a triangular area ofpores on the ventral
species (Moseley, 1885; Boyle, 1969; Boyle, 1976). valvesurfaceandaresimilartoslit raychannelsbecause
The function of the typical (non-ocelli) aesthetes theywerealsothoughttopassthroughallvalveareasto
been debated, with proposalsincludingchemoreception open on the dorsal surface (Baxter and Jones, 1981).
(Fischer, 1988); mechanoreception (Moseley, 1885); Moseley (1885) proposed that the structure and
periostracumreplenishment and secretion (Boyle, 1974; arrangement of the "eyes" in those chitons that have
Baxter et al., 1990); secretions serving protective them could be useful in phylogeny. More broadly,
functions (Fischer, 1988); and photoreception (Mose- Leloup (e.g., 1934, 1936, 1937, 1940a, 1940b, 1942,
ley, 1885; Blumrich, 1891; Omelich, 1967; Boyle, 1972; 1948, 1952) incorporated many drawings of the
Fischer, 1978, 1988). An early electron inicroscopy patterns of micraesthetes and megalaesthete chambers
study (Omehch, 1967) and a more recent immunocy- in his taxonomic descriptions of numerous chiton
tochemical study (Reindl et al., 1997) have shown that species. More recent workers (e.g., Boyle, 1974; Baxter
aesthetes clearly contain neuronal structures, suggest- & Jones, 1981, 1984; Baxter et al., 1987; Fischer, 1988;
ing a sensory function. The presence ofphotoreceptive Currie, 1989; Baxteret al., 1990; Currie, 1992; Sturrock
cells and ocelli in larger aesthete complexes, along with & Baxter, 1993; Reindl et al., 1997) have also
the fact that many chitons have been shown to be documented aesthete and aesthete canal morphology
positively or negatively phototactic (e.g., Crozier, 1920; in different chiton species. These studies have shown
Omelich, 1967; Boyle, 1972; Fischer, 1988; Currie, that the fine scale form ofthe aesthetes and the canals
1989), has led many to view photoreception as one of that house them vary between species and so represent
the primary functions of aesthetes, although it seems characters that could be used in phylogenetic analyses.
likely that aesthetes serve multiple roles (Haas and In fact, aesthetepore densitiesand arrangements onthe
Kriesten, 1978; Fischer, 1979, 1988). valve surface have already been used in phylogenetic
Baxter & Jones (1981, 1984) described threemain types studies (e.g., O'Neill, 1985; Bullock, 1985). However, it
ofaesthete canal (channel) systems that occur in distinct still seems true that, as Currie (1992:3) wrote, "The
areas of the valves of Lepidochitona cinereus and paucity of information on aesthete morphology and
Callochiton achatinus: multiple branch channels, found distribution in a range of chiton species/families, and
in the lateral and pleural areas (see Figure 1 for an indeed habitats, clearly remains a hindrance to our
illustration of chiton valve terminology); jugal area understanding ofaesthete function and evolution."
C. Z. Fernandez et al., 2006 Page 53
Epoxy casts have been used to infer the shape of with sand in the intertidal. One individual of Mopalia
tunnels in shells with endolith borings (e.g., Golubic et acuta (Carpenter, 1855) (SBMNH 83160) was collected
al., 1970; Vogel & Marincovich, 2004) and shell pores half buried in sand on the side of a rock in Doheny
in limpets (Reindl and Haszprunar, 1994). In addition, Beach, Orange County, California; the other (SBMNH
Haas & Kriesten (1978) used this method to obtain 369432) was collected from Oceanside, near Camp
details of the aesthete canal system in Chiton alboli- Pendleton, on cobble reefon the underside ofa rock in
neatus, revealing short micraesthete canals that feed the intertidal zone during a —0.37 m tide. One in-
into elongate megalaesthete chambers that connect to dividual of Placiphorella velata Carpenter MS, Dall,
asmall numberoflarge horizontalcanals. Vendrascoet 1879 (SBMNH 83161) was collected fromthe intertidal
al. (2004) used the epoxy casts of a chiton valve to zone in Pacific Grove, Monterey Peninsula, California;
compare with the tunnels in valves ofmultiplacophor- theother(SBMNH 369440)isfromtheSpencerThorpe
ans, an unusual type ofPaleozoic chiton. collection, found in Timber Cove, Sonoma County,
The purpose of this study was to provide more California on a rock in the intertidal zone. Nuttallochi-
information about: (1) the morphology of the total ton hyadesi (de Rochebrune, 1889) (SBMNH 83157)
aesthete canal system; (2) how aesthete canal patterns had been dredged from a depth of 384-494 m at
differ between a set ofchiton species; and (3) whether 56"06'S, 66°19'W off the coast of Tierra del Fuego.
aesthete canal patterns correlate best with taxonomic One specimen of Lepidozona mertensii (SBMNH
relationships or environmental factors. 369438) was collected from Port Gamble, Washington,
USA in the intertidal. One specimen of Lepidopleurus
MATERIALS AND METHODS cajetanus (SBMNH 83154) was collected at 2.5^ m
depth under stones in Galeria, Corsica, Mediterranean;
Individuals of Mopalia muscosa (Gould, 1846) (Santa the other individual (SBMNH 83155) on the coast of
Barbara Museum ofNatural History (SBMNH) 83143 Croatia under rocks on sand in the lower intertidal. All
and 83144), Lepidochitona hartwegii (Carpenter, 1855) chitons used in this analysis were adults.
(SBMNH 83146 and 83147), and Nuttallina califomica Valves from at least two individuals from each
(southern form; previously Nuttallinafluxa) (SBMNH species were used, except for the rare Nuttallochiton
83148-83149) were collected by CZF and MJV in the hyadesi. Both the northern and southern form (the
middle to lower intertidal of Palos Verdes, California latter was previously named Nuttallina fluxa) of
on the surface of rocks. Individuals of Lepidozona Nuttallina califomica were used. Valves from only one
pectinulata (Carpenter in Pilsbry, 1893) (SBMNH individual of the northern form was examined,
83152 and 83153) were also collected from Palos although two individuals of the southern foi'm were
Verdes, from under submerged stones in the lower processed. One to three intermediate valves of each
intertidal. Individuals of Lepidozona cooperi (Dall, individual were embedded and examined.
1879) (SBMNH 83150 and 83151), one specimen of Whole specimens were boiled so the valves could be
Nuttallina califomica (Nuttall MS, Reeve, 1847) removed from the flesh. Even isolated shell plates were
(SBMNH 83156), and one specimen of Lepidozona boiled in ordertoclean them. The isolated intermediate
mertensii (von Middendorff, 1847) (SBMNH 83145) valves of all species were soaked in bleach for up to
were collected by MJV from the rocky intertidal of 24 hr and placed in a sonicating bath for 25 min at
Cambria, California. Specimens of Mopalia acuta, room temperature to dislodge remnant organic mate-
Placiphorella velata, Lepidozona mertensii, Ischnochiton rial and other debris. Valves were dehydrated through
textilis, andIschnochiton variegatuswere obtained from an ethanol series and then embedded in epoxy using
the SBMNH and isolated valves of Nuttallochiton a method modeled after (Golubic et al., 1970). A low
hyadesi were provided by the Los Angeles County viscosity embedding medium was created using the
Museum (LACM). All other chitons were obtained Embed 812 kit from Electron Microscopy Sciences.
from shell dealers. The Embed 812 kit consisted ofEmbed 812 embedding
One specimen of Ischonochiton textilis (Gray, 1828) resin, Dodecenyl Succinic Anhydride (DDSA), Nadic
(SBMNH 83158) was collected from intertidal rocks in Methyl Anhydride (NMA), and Benzyldimethylamine
Chelsea Point, Port Elizabeth in the Eastern Cape, (BDMA). They were combined in the following
South Africa; the other (SBMNH 369435) is from the proportions: 44.1% Embed 812, 35.3% DDSA, 17.6%
George Hanselman collection, also collected from Port NMA, and 2.9% BDMA. The valves were submerged
Elizabeth, South Africa at 0-2 m. One specimen of in resin and placed under a vacuum in a desiccating
Ischnochiton variegatus (H. Adams and Angas, 1864) chamber for 24 hr and then cured in an oven at 60°C
(SBMNH 83159) was collected under rocks in the for 24 hr. The cured blocks were trimmed using
intertidal zone in DeMole Point, South Australia; the a Buehler low speed saw or with a dremel tool using
other (SBMNH 369437) was collected from Port a thin-bladed saw. Cuts were made around the edges of
MacDonnell in South Australia, under small rocks the valves, making sure to intersect the valve on all
Page 54 The Veliger, Vol. 49, No. 2
sides. The blocks were placed in 10% HCl for another aesthete pores on the dorsal valve surface. The canals
24 hr or until all of the calcium carbonate dissolved have a meandering, somewhat interweaving pathway
away, then rinsed with distilled water, cleaned with from megalaesthete bulb to valve margin.
bleach, and split apart into a dorsal and ventral cast. The ventral casts show that a large number of
The resultant casts were gold sputtered and most were aesthete canals terminate on the ventral surface ofthe
examined under SEM using a LEO 1430 with an shell plate, even though Lepidupleunis lackstheslit rays
accelerating voltage of 10-15 kV. and associated ventral slit ray canals. There is an
The cladistic analysis using aesthete characters was abundanceofcanals intheventraljugal triangle, at and
performed using PAUP 4.0b10 (Swofford, 2002). An near the apical area, and in the region underneath the
exhaustive search was completed using maximum lateral areas.
parsimony; all characters were unweighted and all
character states were unordered. Mopalia muscosa (Acanthochitonina: Mopaliidae)
Assignments to chiton orders and families were The dorsal casts (Figures 2a-c) reveal nearly
based on Sirenko (1997), although alternative taxo- straight, primary horizontal canals (—20-35 jam in
nomic schemes are mentioned in the Discussion ofthis diameter) that run from the posterior to the anterior
paper. margin through all valve areas. The canals are closely
Isolated valves and epoxy casts ofeach species in the spaced (—m12m-20 |j.m between primary canals; —20
analysis were deposited in the chiton collections ofthe canals per along the horizontal plane) and in some
Santa Barbara Museum ofNatural History. cases adjacent horizontal canals merge with each other.
At least two layers ofprimary horizontal canals can be
RESULTS seen as they near the anterior margin of the valve.
There are also many smaller-diameter (—10-20 |J.m),
In this section, the trend of the canal system is short, connecting canals, above and sub-parallel to
described in a manner consistent with the direction of primary horizontal canals, that merge with the primary
valve growth (see Baxter & Jones, 1981, 1984) and the canals at regular intervals. The connecting canals, and
flowofsensoryinformation. Theopenings ofthecanals in some instances the primary horizontal canals,
on the dorsal surface of the valve are taken to be the connect to the dorsal valve surface via a megalaesthete
origin and the places wherethe canals enterthe body of chamber fed into by a megalaesthete or micraesthete
the chiton (e.g., at the anterior and lateral eaves) are canal. Micraesthete canals are —2-3 \x.m in diameter.
described as the exit. The portions ofthe valve surface Megalaesthete canals begin as a short length of canal
referred to in the text are shown in Figure 1. All with a diameter of —6-7 jo^m, before gently flaring out
directional indicators (anterior, posterior, dorsal, ven- to a diameter of —15-25 jam as they continue down
tral) are meant with respect to thevalve in life position. towards the horizontalcanals. At thetop ofthecasts of
The two pieces ofthe aesthete canal cast are referred to the micraesthete canals and megalaesthete chambers
as dorsal and ventral, also defined based on life are cup-shaped protuberances that appear to be filled-
position. in subsidiary (—6-8 |J.m diameter) and apical (—10-
11 |j.m) caps, respectively.
Lepidoplewus cajetanus In thejugal area, somehorizontal canals haveamore
(Lepidopleurina: Lepidopleuridae) flattened appearance and turn down towards the
The dorsal casts (Figures 5f-h) reveal micraesthetes ventral valve surface. On the ventral casts (Figures
(—4-6 jam diameter) that feed into a bulbous mega- 2d-f), the corresponding area (herein referred to as the
laestehete chamber —30-60 |am diameter), that then ventral jugal triangle) has short lengths of similarly
(
connects to a single, narrow (—10 |j.m diameter) flattened canals. These ventral canals occur along valve
horizontal canal that leads to the anterior or lateral growth lines; —2-5 canals per growth line can be seen
margin. Megalaesthetes are arranged in anterior- on the cast. A number of canals pass from the apical
posterior rows along the ridges of the central area. area into the ventral jugal triangle.
The megalaesthetes are much denser, and less orga- Some canals (visible on the dorsal casts) near the
nized, in the ridged lateral areas. The megalaesthete boundary of the pleural and lateral areas, along the
bulbs typically have a sub-cylindrical shape in the diagonal line, are turned downwards. The canals in this
centralarea, often with a constriction at the base where region originate dorsallyand mergewith each other(on
they attach to, or become, the horizontal canal. In average, 2-3 canals merge) as they near the ventral
some cases the bulbs have a more expanded tent-like valve surface. On the ventral cast, a single line of
shape and have agreaternumberofmicraesthetes(>10 horizontal canals is present parallel to the slit ray; on
vs. —7) feeding into them. There is a distinct spacing of Mopalia muscosa valves, slit rays correspond exactly to
—
10 |jm between adjacent aesthete complexes in the the diagonal line.
central area, leading to a clustered appearance of The ventral casts show the apical area canal system
.
C. Z. Fernandez et al., 2006 Page 55
Figure 2. SEM images of casts of aesthete canal systems of Mopalia muscosa (SBMNH 83143) (a-f), Nutlallochiton hyadesi
(SBMNH 83157)(g, h), and Mopaliaacuta(i,j). Imagesd-fareofaventralcast. All otherimagesareofdorsalcasts, a. Horizontal
canals in pleural area and part oflateral area. b. Close-up ofthejugal area showing flattened canals, c. Close-up ofmegalaesthete
chambersandmicraesthetecanals, d. Slit raycanals, e. Apicalareaandventraljugaltriangleshowingcanalspassingfromtheapical
area into the ventraljugal triangle. Canals exiting at the lip ofthe apical area can also be seen. f. View ofapical area showing
micraesthetecanalsthatoriginateattheapicalarea. g. Overviewofvalvecast showinglittledifferentiation ofvalve areas, h. Close-
muipcrsaehsotwhientgecmaengaallsae(sSthBeMteNHch8a3m1b6e0r).s,j. OivVeireviwewofofhhoarlifzoonftavlalvceancaalsst (sShoBwMiNngH m3e6g94a3l2a)e.stKheeyt;e Mcha=mbmeergaslaaensdtheitnecocmhpalmebteerl;y mcas=t
micraesthete canal; SRc = slit ray canals; all others abbreviations are shown in Figure 1
Page 56 The Veliger, Vol. 49, No. 2
^^s^M^^'^,:, \..ir d
Figure3. SEM imagesofcastsofaesthetecanalsystemsofPlaciphore/la vekita(a~e), Lepidochitoiuiharnvegii(f-h),and Niittalliua
californica (i-k). Images c-e, k are ofa ventral cast. All other images are ofdorsal casts, a. Overview ofhalfofcast showing little
differentiation of valve areas (SBMNH 369440). b. Close-up of horizontal canals on same specimen, showing megalaesthete
chambers and micraesthetecanals, c. Overview ofhalfofcast showingcanalsat slitrayregionandapical area and partialcanalsat
the ventral jugal triangle (SBMNH 83161). d. Magnification of canals at apical area passing into slit ray region.
C. Z. Fernandez et al., 2006 Page 57
in great detail. Numerous micraesthete-sized canals Placiphorella velata (Acanthochitonina: MopaHidae)
originate at the dorsal valve surface, merging with The dorsal casts (Figures 3a, b) reveal primary
a larger subsidiary canal or with each other then with horizontal canals (—21-34 |a.m in diameter) that run
a subsidiary canal before then merging with a primary from the posterior to the anterior margin through all
hapoirciazlonatraela.cAanaflewtchaatnalesxictsanatbethseeenanctreorsisoirnglifproofm tthhee vhaolrviezonatraelasc.anaTlhsertehroiusghsimgnuicfihcanotf tshpeacvialnvge binettewrieoern,
apical area into the slit ray region. but at the anterior margin, there are —24 canals per
Mopalia acuta (Acanthochitonina: Mopaliidae) mm with a spacing of —22-40 (im between canals
along the horizontal plane. At least two layers of
The dorsal casts (Figures 2i, j) reveal primary primary canals can be seen. At regular intervals along
horizontal canals (—25-35 ^m diameter) that run from the length ofprimary canals, there arejunctions where
the anterior to the posterior margin through all valve short canals (—20-25 |a,m in diameter) from the
areas. There is a single principal layer of primary megalaesthete chambers merge with the primary canal
canals, with a density of —19 canals per mm and in a manner similar to that seen in Mopalia acuta and
a spacing of —8-17 \x.m between the canals within this the other mopaliids. Numerous micraesthetes (—3-
layer. Indistinct, elongate megalaesthete chambers 4 ixm in diameter) feed into each elongate, indistinct
(—8-17 |j.m diameter) feed into the main horizontal megalaesthete chamber along its length. No obvious
canals after a short length. The chambers merge along differences in aesthete canal morphology in the
the length of the primary canals at regular intervals. different valve areas can be detected in the casts.
Numerous micraesthetes (—1-3 |j.m in diameter) feed On the ventral casts (Figures 3c-e), there are a few
into each megalaesthete chamber all along its length. partial canals preserved at the ventral jugal triangle.
On the ventral casts, growth lines as well as a few There are also a number of slightly flattened canals
incomplete canals can be seen in the ventral jugal scattered along the lines parallel to the slit rays. Canals
triangle. In one specimen, there is a line of partially are also preserved along the entire apical area. The
preserved horizontal canals that parallel the slit ray, in canals turn upwards towards the dorsal valve surface
addition to the apical area canals. and are oriented parallel to the direction of valve
growth. They appear to originate as megalaesthete
Nuttallochiton hyadesi (Acanthochitonina: Mopaliidae)
chambers close to the dorsal valve surface with two or
The dorsal casts (Figure 2g, h) reveal primary more ofthese chambers merging into a single horizon-
horizontal canals (—30-50 (im in diameter) that are tal canal which exits at the apical area.
preservedthroughouttheentire shell. Thereisaspacing
of—20-50 (imbetweencanals,with —18canalspermm Lepidochitona hartwegii
along the horizontal plane. Micraesthetes (1-3 |j.m (Acanthochitonina: Tonicellidae)
diameter) feed into the elongate, indistinct mega- The dorsal casts (Figures 3f-h) reveal primary
laesthete chambers that feed directly, after a short horizontal canals (—35-45 |a.m in diameter) that run
distance, into the primary horizontal canals. This canal from the posterior to the anterior margin through all
pattern is similar to that seen in the Mopalia spp. The valve areas. There is a spacing of —15-35 |Lim between
megalaesthetechambershaveadiameterof—10-12 |j.m the primary canals, with —20 canals per mm along the
before widening to the same diameter as the connecting horizontal plane. The primaryhorizontal canals vary in
canals (—17-20 |J,m). At the region above the sht ray, diameter as they traverse the valve. At the posterior
the horizontal canals curve towards each other, merge, valve margin, many short lengths of subsidiary canals
and proceed downward towards the sht rays. can be seen merging with each other to form a larger
The ventral casts reveal a narrow line of slit ray primary canal. As these primary canals continue to
canals, apical area canals, but very few ventral jugal course towards the anterior margin, subsidiary canals
triangle canals. merge into them at regular intervals. Megalaesthete
e. Close-up ofapical area on same specimen showingcanals turned up towardsdorsal valve surface. A line ofcanalspresent at slit
ray regioncan beseen behind the apical area. f. Overview ofcast oiLepidochitonaliartwegii(SBMNH 83146). g. Magnification of
megalaesthete chambers and micraesthete canals (SBMNH 83147). h. Horizontal canals at posterior showing merging of
megalaesthete chambers (SBMNH 83146). i. View ofhorizontal canals in same specimen, showing megalaesthete chambers and
micraesthetecanals,j. OverviewofcastofNiittallinacaliforriica(southernform; previouslyN. fhi.xa) (SBMNH 83149). k. Overview
ofcast ofNiittallina californica (SBMNH 83156) showing flattened canals at the ventraljugal triangle, slit ray region, and apical
area. Key: Same as in Figures 1 & 2.
Page 58 The Veliger, Vol. 49, No. 2
chambers that are typically elongate and mdistinct, but the lines parallel to the slit rays, some of which are
occasionally with a slightly pear-shaped appearance, turned upwards towards the dorsal valve surface, and
originate at the dorsal valve surface with a diameter of others downwards towards the ventral valve surface.
—10-14 )am, then widen to a diameter of —25-30 jam, There does not seem to be any significant difference
before tapering off to —9.5-14 \xm in diameter. in the aesthete canal systems between the southern
Numerous micraesthete canals with a diameter of 6- form of this species (previously A^. fluxci) and the
9 p.m merge into the megalaesthete chambers. Each northern form.
micraesthete canal originates at a sub-spherical sub- In addition to the aesthete canal system, the dorsal
sidiarycap. Themegalaesthetes fuse into thehorizontal cast of one individual shows numerous borings pro-
canals after a short length. duced by endolithic microorganisms. The clusters of
There is some variation in canal morphology in the borings are scattered among the aesthetecanals located
different valve areas. In the lateral and pleural areas, near the posterior end ofthe valve.
there is a shorter distance between megalaesthete
chamberandprimarycanal than inthejugal area, where Lepidozona inertensii (Chitonina: Ischnochitonidae)
thereisalongerdistanceand wheremanyoftheprimary The aesthete canal morphology seen in the dorsal
canals turn down towards the ventral valve surface. casts (Figures 4a-d) correlates with the dorsal valve
Theventralcasts show portions offlattenedcanals in sculpture. Canalsarearranged indiscretezonesdivided
the ventraljugal triangle and a few partial canals along by linear regions that correlate with ridges on the valve
the lines parallel to the slit rays, as well as apical area surface. There is a v-shaped pattern of aesthete canals
canals. in the jugal area, a more parallel set of canals in the
pleural area, and some canals that run diagonally from
Niittallina californica (Acanthochitonina: Tonicellidae) the slits to the apex ofthe valve in the lateral areas.
The dorsal casts (Figures 3i, j) show an irregular There is a single layer of primary horizontal canals
spacing (—25-40 |im between canals) of primary closeto the articulamentum layer. Theseprimarycanals
h—o2r4izocnatnaallscpaenralsmm(—a2l0o^ng0t)hJ.em hdoiraimzeotnetral) pplaacnek.edThaet fvarroymitnhisnizean(d—r1o5u-3n0d|tiomtihnicwkidatnhd)falnatdtesnhead.peT,hrearnegianrge
primary canals run in a posterior-anterior direction at —27 canals per mm along the horizontal axis, with
thejugal area and fan out diagonally at the pleural and a spacing of—20^0 )am between canals at the anterior
lateral valve areas. There are some canals near the margin. These primary horizontal canals connect the
posterior margin ofthe valve that flank thejugal area, large, bulbous megalaesthete chambers. Each separated
coursing inwards to form a v-shaped pattern. The most longitudinalzoneofbulbousmegalaesthetechambersin
conspicuous primary canals are flattened with many the lateral and pleural areas is approximately 5-7
tubular subsidiary canals (—12-15 jam in diameter) chambers wide. The ovoid megalaesthete chambers are
that feed into them. The subsidiary canals run above oriented sub-parallel (angled slightly upwards) to the
and subparallel to the primary canals. They begin at dorsal surface, with a maximum width of —35^5 jam,
the dorsal valve surface as a megalaesthete chamber tapering at each end to —7-10 (im in diameter. The
with a diameter of—6-10 |im and branch and widen to chambersseemtoeitherconnecttoaprimaryhorizontal
a range of diameters as they run into the primary canal or to the body of a megalaesthete chamber
canals. There are also numerous micraesthete canals adjacenttoit. Anumberofmicraesthetecanalsthatstart
—2.5-3.5 |im in diameter that merge all along the offatthevalve surface at 5-7 |J,m in diameterand taper
branching subsidiary canals. The megalaesthete cham- to 2-3 ]xm in diameter are located directly above the
bers are not well defined, making it difficult to define megalaesthete chamber and surround themegalaesthete
where the megalaesthete chamber ends and the sub- canal that originates at the dorsal valve surface.
sidiary canal begins. The ridges on the valve surface correspond with
At thejugal area, flattened horizontal canals appear symmetrical pairs ofmegalaesthete chambers that have
to turn downwards towards the ventral valve surface. numerous micraesthetes feeding into them. The mega-
Thesecanals appearto correlatewith flattened, upward laesthete chambers in these areas are slightly larger
oriented canals found on the ventral jugal triangle of than those in other regions ofthe valve.
the ventral cast. The lateral areas lack the clearly defined longitudinal
On theventral casts (Figure 3k), a layer oflarge, fiat zones ofmegalaesthete chambers seen in thejugal and
canals can be seen overlying a lower layer of smaller, pleural areas. Instead, there are closely spaced mega-
branching subsidiary canals in the apical area. Some of laesthete chambers surrounding rows ofpits that house
the large, flat canals extend from the apical area into large mound-shaped structures (assumed to be an
the ventraljugal triangle. Numerous micraesthetes that extremely large megalaesthete chamber) with numerous
originate at the apical area feed into subsidiary canals. micraesthete canals feeding into them. These pits
There are also smaller and less flattened canals along correlate with the enlarged granules arranged in rows
C. Z. Fernandez et al., 2006 Page 59
Figure4. SEM images ofcasts ofaesthetecanal systems ofLepk/ozoiuiniertensii(a-d), Lepidozcma cooperi(e, f), and Lepkluzona
pectinulata(g,h).Allimagesareofdorsalcasts,a. OverviewofcastofLepidozonamertensii(SBMNH 83145)showingdifferentiated
valve areas which correlate with valve sculpture, b. Megalaesthete chambers and megalaesthete pit located in lateral area in same
specimen, c. Longitudinal columns ofmegalaesthetes with some flattened primary horizontal canals in same specimen, d. Veiw of
micraesthete canals and megalaesthete chambers in same specimen, e. View ofLepidozona cooperi (SBMNH 83150) longitudinal
columnsofmegalaestheteswithmicraesthetecanals, f.Overviewofcast(SBMNH 83151)showingdifferentiatedvalveareas, g.View
ofLepidozonapecimilata(SBMNH 83152)showinglateralareawithhorizontalcanalsconvergingintoslitraycanals,h.Overviewof
cast (SBMNH 83153) showingdifferentiated valve areas. Key: Mp = megalaesthete pit; all others same as in Figures 1 & 2.
onthedorsalvalvesurface. Themegalaesthetechambers portions of canals in the ventral jugal triangle are
are oriented more perpendicular to the valve surface flattened and periodically turn up towards the dorsal
than their counterparts in thejugal and pleural areas. valve surface. Canalsat the slit rays seem to bethin and
On the ventral cast, canals are evident at the ventral diagonally oriented towards the valve surface. Canals
jugal triangle, slit rays, and apical area. The small at the apical area are present along the entire width of
Page 60 The Veliger, Vol. 49, No. 2
the posterior valve margin. Flat horizontal canals surface occurin the uppermost tegmentum layer. These
appear to exit at the anterior lip of the apical area cavities connect to tiny elongate canals that form
and have many micreasthete canals which originate at a horizontal webjust below the shell surface. Fourway
the apical area feeding into them. intersections ofthese tiny tunnels are present, although
The slit ray canals consist of multiple rows that it is not clear if the intersections connect the large
expand outward from the apex to the anterolateral cavities to each other. The bulbous chambers (~40-
margin. Thejugal area canals are not abundant on the 50 |j.m in diameterat origin, wideningto ~53-60 jam in
ventral casts. diameter) have many small canals merging into them
all alongtheirheight. Thevalves ofthis individual have
Lepidozona cooperi (Chitonina: Ischnochitonidae) occasional large openings on the dorsal surface that
The aesthete canal morphology of this species correspond to the bulbous cavities.
(Figures 4e, f)is similarto that oiLepidozonamertensii,
Ischnochiton variegatus (Chitonina: Ischnochitonidae)
except L. cooperi has a slightly greater proportion of
horizontal canals. These canals are most noticeable On the dorsal casts (Figures 5c-e), there is evidence
along the anterior margin, and in the lateral areas. The oftwo ormorelayersofprimaryhorizontal canals. The
thin, meandering horizontal canals curve towards each top row of horizontal canals (~9-13 )im in diameter),
other in the lateral areas. spaced approximately —12-22 |im apart from each
The ventral casts show a greater extent ofjugal area other, can be clearly seen at the anterior valve margin.
canals preserved than in L. mertensii. There are —34 canals per mm along the horizontal
plane at the margin. The dorsal casts reveal that
Lepidozonapectinulata (Chitonina: Ischnochitonidae) micraesthete canals feed into teardrop-shaped mega-
The aesthete canal morphology of this species laesthete chambers that taper down to connect to the
(Figures 4g, h) bears similarities to that of the other primary horizontal canals. The primary canals are
members of Lepidozona in this study. However, this relatively thin, with low density, through much of the
species differs fundamentally from the other two valve interior. The horizontal canals appear to be
species in that it lacks the huge esthete chambers in straight in the jugal area, curve slightly in the pleural
granule spaces and, more noticeably, has an extensive areas (though to a lesser degree than in /. textilis), and
complement of large horizonatal canals throughout have strong curvature in the lateral areas.
much ofthe tegmentum interior. Thecurvature ofthese Both individuals examined showed the unusual,
canals in the lateral area is more regular in L. probably secondary (see Discussion) bulb and tunnel
pectinulata than in L. cooperi. system in the uppermost portion of the tegmentum as
described for the one individual of/. textilis. Above the
Ischnochiton textilis (Chitonina: Ischnochitonidae) primary horizontal canals, there is an interconnecting
The dorsal casts (Figures 5a, b) reveal acanal system web of tiny, criss-crossing tunnels. Periodically, the
similar to that of Lepidozona cooperi and Ledidozona small tunnels connect to the large, bulb-shaped cavities
pectinulata in havinghorizontal canals that converge at (—70-80 |j.m in diameter at the origin, widening to
the diagonal line and distinct, tear-drop shaped —80-120 )am in diameter). There are approximately 15
megalaesthete bulbs that feed into horizontal canals. bulbs per mm-.
Thereare —29 primary horizontalcanals (~9-23 (im in On the ventral cast, the ventral jugal triangle and
mm
diameter) per along the horizontal plane at the lines parallel to the slit rays contain broken offcanals.
anterior margin. Primary horizontal canals are spaced These partial canals are flattened and some are angled
~15-27 jam apart. These canals thin out towards the upwards towards the dorsal valve surface. There are
posterior margin, revealing rows of bulbous, sub- alsoa fewsmalldiametercanalspresentinsomeregions
conical megalaesthetes above them. Micraesthete ca- ofthe apical area. Some canals in the apical area pass
nals (~1^ |j.m in diameter) that originate at the dorsal into the ventraljugal triangle and slit ray regions.
surface merge into the ends of the megalaesthete
chambers. The horizontal canals show a strong curva- DISCUSSION
ture in the pleural and lateral areas, and are straight in
thejugal area. The casts reveal a remarkable degree of variation in
The ventral casts reveal a high density of flattened morphology of the aesthete canals between species,
canals present at the ventral jugal triangle and at the families, and suborders of the Polyplacophora. This
slit rays. Canals in the apical area are sparsely scattered method of epoxy casting provides a unique way to
along the posterior valve margin. observethe fullmorphologyofthe aesthetecanal system
In one individual (SBMNH 83158), an unusual, in chitons. Moreover, the results of a cladistic analysis
probably secondary (see Discussion) pattern of large, using only aesthete canal characters suggest that the
bulbous cavities oriented sub-perpendicular to the shell aesthete canal system provides a suite of characters