Ontogeny and distribution of Hygophum benoiti (Pisces ...

Journal of Plankton Research Vol.16 no.8 pp.977-991, 1994
Ontogeny and distribution of Hygophum benoiti (Pisces,
Myctophidae) of the North Western Mediterranean
M.P.Olivar and I.Palomera
Instituto de Ciencias del Mar, CS1C, Paseo Joan de Borbo sin, Barcelona 08039,
Spain
Abstract. Two species of the lanternfish genus Hygophum, H.benoiti and H.hygomii, are present in
the Mediterranean. Differentiation of the larvae from these two species has been problematic. An
attempt to identify such larvae in the plankton samples of the North Western Mediterranean led to a
broader study that included the examination of a large number of plankton samples collected in the
region since 1976. The large number of Hygophum larvae obtained during July 1992 in the North
Western Mediterranean (that proved to be H.benoiii), together with collections of larvae from
different geographic areas (especially H.hygomii from the Southwest Indian Ocean), allowed a
detailed morphometric comparison of the larvae of H.benoiti and H.hygomii. Developmental stages
of H.benoiti from yolk sac to post-flexion larvae are described. Information on the temporal
occurrence of H.benoiti in the plankton, as well as the spatial distribution of this species during the
survey of peak abundance (June/July 1992) in the North Western Mediterranean, is presented.
Introduction
The genus Hygophum comprises nine species, of which only two, H.benoiti
(Cocco, 1838) and H.hygomii (Liitken, 1892), are present in the Mediterranean
Sea. References to the presence of adults of both species in the Mediterranean
are numerous (Zugmayer, 1911; Sanzo, 1918a,b; Taning, 1918; Karlovac, 1953;
Goodyear et al., 1972; Donato etal., 1977), with H.benoiti being more abundant
than H.hygomii (Taning, 1918; Sanzo, 1981b; Goodyear et al., 1972; Berdar and
Cavaliere, 1979). None of these studies included samples from the Catalan coast
and Gulf of Lions, where much of the material for the present study was
collected. Both species are also known from the North Atlantic Ocean (Backus
et al., 1977; Nafpaktitis et al., 1977), and H.hygomii has been recorded from the
Southern Atlantic, Southern Indian and Southern Pacific Oceans (Nafpaktitis
and Nafpaktitis, 1969; Hulley, 1981).
Larval characteristics of the genus Hygophum allowed Moser and Ahlstrom
(1974) and Moser et al. (1984) to differentiate three different morphological
types of Hygophum. The species H.benoiti and H.hygomii belong to their
'moderately slender' morphotype. Taning (1918) and Sanzo (1918a,b) described
juveniles and post-flexion larvae of both species, and subsequently larval series
of H.hygomii (Tortonese, 1956; Pertseva-Ostroumova, 1974; Shiganova, 1977;
Berdar and Cavaliere, 1979) and H.benoiti (Cavaliere and Berdar, 1977) were
described. Separation of the species was based mainly on the pigmentation
pattern. The pattern is somewhat variable in the two species, and the
melanophores are small and often lost after some time in preservative. In spite
of the published information, the identification of larval stages of both species
remains problematic (Taning, 1918; Dekhnik and Sinyukova, 1966; Pertseva-
Ostroumova, 1972; Palomera and Rubins, 1979; Hamann etal., 1981; Mas6 and
© Oxford University Press 977

M.P.OIivar and I.Palomera
Palomera, 1984; Sabates and Maso, 1990; Olivar and Fortuno, 1991; K.E.
Hartel, personal communication).
A large number of Hygophum larvae collected in the North Western
Mediterranean during June-July 1992 supplied enough material to undertake a
through study of the morphometry, as well as the pigmentation, of these larvae.
Comparison of these larvae with other Hygophum larvae collected in different
areas of the world led to the discovery of new characters by which larvae of the
two species can be distinguished. In this paper, we describe our Mediterranean
specimens, identified as H.benoiti, and provide diagnostic characters that
differentiate them from H.hygomii larvae.
Also, we discuss the temporal occurrence of H.benoiti in the North Western
Mediterranean based on the presence of larvae from several cruises conducted
from 1977 to 1992, and the spatial distribution during June/July 1992.
Method
The high number of H.benoiti larvae collected in the North Western Mediterranean
during June-July 1992 (n = 1747) provided adequate material to
describe the early stages of the development of this species. The survey covered,
for the first time, the area between 39°30'N 00°30'E (south of the Ebro Delta)
and 43°30'N 08°E (Gulf of Lions). Stations were sampled from inshore waters
with depths of ~40 m to offshore waters with depths of ~2000 m. This allowed
the spatial distribution of larvae of H.benoiti along the Catalan coast and the
southern coast of France to be presented (Figure 1). The survey consisted of a
series of transects perpendicular to the coastline in which the stations were
placed every 18 km, from very shallow waters (20 m depth) to beyond the slope
(>2000 m depth). Samples were obtained with a Bongo net fitted with 0.3 mm
mesh, hauled obliquely from 200 m to the surface, where the bottom depth
41
39
EBROR.
10 # E
Fig. 1. Location of plankton stations sampled during June/July 1992 in the North Western
Mediterranean.
978

Ontogeny and distribution of H.benoUi in NW Mediterranean
permitted. The volume of water filtered was estimated by means of a flowmeter
mounted centrally in the mouth of one of the nets. Larvae were preserved in 5%
buffered saline formalin and measurements were made at least 6 months after
collection and fixation at sea. Measurements were performed to an accuracy of
0.1 mm. Larval nomenclature follows Kendall et al. (1984).
The larvae described here are deposited in the Ichthyoplankton Collection of
the Instituto de Ciencias del Mar de Barcelona (ICICMB). Among the 1747
larvae collected in the cruise, several measurements were taken from a selection
of 60 larvae, covering all the larval size range [from 2.9 to 11.2 mm body length
(BL)]. The following measurements were recorded: BL, the distance along the
midline of the body from the tip of the snout to the tip of the notochord in preflexion
larvae, and to the posterior margin of the hypural elements in postflexion
stages; pre-anal length (PA), the distance along the midline of the body
from the tip of the snout to the vent; head length (HL), the distance from the tip
of the snout to the posterior margin of the cleithrum; head depth (HD),
maximum depth of the head; body depth at pectoral (BDP), depth of the body at
the base of the pectoral fin; dorsal fin distance (DD), the distance along the
midline of the body from the tip of the snout to the origin of the dorsal fin; eye
diameter (ED); eye depth (EDepth), eye depth including choroid tissue. The
allometric relationships between the various body measurements and BL were
calculated by use of the equation v = ax b , where x is BL, y is the other
measurement being related, b is the allometric factor and a is the expected value
of y at x = 1 (Gould, 1966). Confidence intervals (CI) were calculated at the
95% level of significance.
In order to study spatial distribution, the number of individuals collected in
the June/July 1992 samples were standardized to numbers per 10 m 2 of surface
sea. Temporal occurrence of H.benoiti larvae was based on samples obtained in
several surveys carried out on the Catalan coast from 1976 to 1992 (Table I).
Owing to the different methodology and bathymetric area covered, spatial
distribution and abundances during these cruises are not straightforwardly
comparable.
For comparison, a large number (n = 1668) of H.hygomii larvae from the
Southwest Indian Ocean were available (Beckley and van Ballegooyen, 1992).
The methods of collection and preservation were similar to those described
above for H.benoiti: oblique hauls with a Bongo net and fixation in buffered
formalin at 5%. The methods for measurements and morphometric analysis of
H.hygomii larvae were the same as those described above for H.benoiti. Other
Hygophum larvae from different museums or collections of the world have also
been examined: H.hygomii from the Eastern Mediterranean (Centre de
Recherches Marines, Jounieh, Lebanon), from the Central Western Atlantic
(Museum of Comparative Zoology, Cambridge, MA), from the tropical
Northeast Atlantic [Zoologisches Institut and Museum of Hamburg, Germany,
and our material in the Instituto de Ciencias del Mar (ICM)] and from the
southern Benguela region (our material in the ICM), and for H.benoiti from the
Central Western Atlantic (Museum of Comparative Zoology, Cambridge) and
the tropical Northeast Atlantic (Zoologisches Institut and Museum of Hamburg,
979

M.P.OIivar and I.Palomera
Table I. Presence of H.benoiti larvae in the different cruises carried out in the North Western
Mediterranean
Month/year Latitude Presence
August 1984 40°30'-42°30'N +
September 1983 40°30'-42°30'N +
September/October 1984 40"30'-42°30'N +
October 1976 38°30'-42°30'N +
October 1982 41°30'-42°30'N +
October 1983 40°00'-41°30'N +
October 1983 40°30'-42°30'N +
December 1981 41°30'-42°30'N
January 1982 41°30'-42°30'N
March 1982 41°30'-42°30'N
April 1983 40°30'-42°30'N
May 1983 40 o 00'-41 o 30'N +
May 1983 40°30'-42°30'N +
May 1990 39°0O'-41°30'N +
May 1992 42°05'-42°00'N +
June 1983 40°30'-42°30'N +
June 1992 42°05'-42 1> 20'N . +
July 1983 40°30'-42 o 30'N +
June/July 1992 39°30'-43°30'N +
and our material in the ICM). Comparative material of H.proximum was also
obtained from Indo-Pacific collections from the waters around Australia
(Australian Museum) and Pacific collections from the waters around Japan
(Faculty of Fisheries, Kagoshima University).
Results
Based on pigmentation pattern, which was mainly useful in recently collected
material, we identified the larvae collected in June-July 1992 as H.benoiti
(Figure 2). Morphometric data on H.hygomii larvae from the Southwest Indian
Ocean used for comparative purposes (Figure 3) also showed differences
between H.benoiti and H.hygomii, which permitted us to re-identify all larvae of
Hygophum collected in previous cruises carried out in the North Western
Mediterranean and determine the spawning period of H.benoiti.
Morphology and morphometry
Larvae of H.benoiti were moderately slender, especially in pre-flexion stages.
As a proportion of body length, body depth at the pectoral base and head depth
showed significant allometric increases, with development (Figure 4). Relative
body depth and head depth were slightly smaller in H.benoiti than in H.hygomii
larvae (Figure 5).
In H.benoiti, the gut was slightly curved with a prominent fold-terminated
section. Pre-anal length versus body length was relatively constant with
development (Figure 4). The anterior section of the gut did not have transverse
rugae and was conspicuously thinner than the posterior section. The anterior
section was relatively longer in pre-flexion and flexion larvae (representing ~l/3
980

Ontogeny and distribution of H.benoiti in N W Mediterranean
Fig. 2. Larval development of H.benoiti. (A) 2.9 mm; (B) 5.5 mm; (C) 7.8 mm, (D) 9.2 mm.
of gut length) than in later stages (Figure 2). Pre-anal length versus body length
was one of the important characters that differentiates H.benoiti from
H.hygomii (Figure 5). Pre-anal length was significantly shorter (P < 0.0001) in
the latter (always 60% of BL). Furthermore,
in H.hygomii the gut was more curved and the foregut was shorter (25-27% of
cleithnim to anus distance) than in H.benoiti (32-34% of cleithrum to anus
distance).
Head length in H.benoiti was almost isometrical with development: from 25%
981

M.P.OIivar and I.Palomera
B
Fig. 3. Larval development of H.hygomii. (A) 5.3 mm; (B) 7.0 mm; (C) 9.9 mm; (D) 13.0 mm.
of BL at 3 mm to 29% at 11 mm. Small teeth were discernible in the upper jaw
in early larvae as small as 3 mm.
Pre-dorsal length in H.benoiti decreased with development. A significantly
negative allometnc relationship was observed between pre-dorsal length and
body length (Figure 4). For the size range considered, pre-dorsal distance varied
from 56 to 49% of BL. In H.hygomii,.this distance was smaller (changing from
50 to 44% of BL for the same range of sizes) (Figure 5).
Eyes were elliptical during all larval stages studied in both species (Figures 2
and 3). A conical mass of brown choroid tissue, similar in the two species, was
always visible on the ventral edge of the eye.
Notochordal flexion started at ~5 mm and was completed at 5.5 mm in
H.benoiti. In H.hygomii, notochordal flexion started at ~6 mm BL and was
completed at 7 mm BL.
982

HP (mm)
BDP (mm)
4.0
5.0
HD • 0.0594 SL 1 ' 6662
Cfb • 0.1113
• 0.9740
1.6600
BDP • 0.0602 SL
Ob • a 1204
r • 0.0671
4.0
3.0"
3.0
2.0"
2.0
1.0
1.0
n • 46
n • 55
3.0 4.0 5.0 6.0 7.0 8.0 0.0 10.0 11.0
SL (mm)
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
SL (mm)
O
DD (mm)
PA (mm)
7.0
641
• a8682 SL°'
00
••' +
6.0
0.9314
r •
1.1108
PA • 0.6342 SL
Ob • 0.0381
r - 0.9916
^
¥~~**~~
6.0
ir
I
.+••
•*"""" +
' .. .^ •
4.0
•31
n
n • 60
9.6"
8.6"
7.5"
6.5"
5.5-
4.5-
3.5"
2.5"
S.
-•— H
10.0 11.0
2.0 —i—
H
H
6.0 7.0 8.0 9.0
SL (mm)
4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
SL (mm)
Fig. 4. Morphology of H.benoiii larvae. Relationship between body length (SL) and body depth at pectoral (BDP), head depth (HD), pre-anal length (PA)
and dorsal distance (DD). Continuous line: fitted curve. Dashed lines: confidence intervals (at the 95% level of significance) for estimating y.

HP (mm)
BDP (mm)
1.4060
3.0+ HD-0.0822 SL
Ob - 0.0842
r • 0.9784
1.6707
3.0 + BDP-0.0699 SL
Ob • a0838
r -0.0800
2.0 •
2.0-
1o
1.0-•
1.0-•
n-52
n-ee
0.0
3.0 4.0 5.0 6.0 7.0 8.0 0.0 10.0 11.0 12.0 13.0
SL (mm)
0.0
3.0 4.0 5.0 6.0 7.0 8.0 0.0 10.0 11.0 12.0 13.0
SL(mm)
DD (mm)
O7062
6.0 •}• DO-0.8908 SL
Ob • 0.0920
r • 0.S469
PA (mm)
8.0
1.0192
7 0 •• PA-0.5577 SL
Qb - 0.0209
6.0+ r-0.8952
5.0-
n • 31
n-92
11.0 12.0 13.0
3.0
7.0 8.0 ' 0.0 10.0
SL(mm)
4 1-
2.5 3.5 4.5 6.5 &6 7.5 8.5 0.5 10.5 11.5 12.6 13.5
SL(mm)
Fig. S. Morphology of H.hygomii larvae. Relationship between body length (SL) and body depth at pectoral (BDP), head depth (HD), pre-anal length
(PA) and dorsal distance (DD). Continuous line: fitted curve. Dashed lines: confidence intervals (at the 95% level of significance) for estimating y.

Fin development
Ontogeny and distribution of H.benoiti in NW Mediterranean
In post-flexion H.benoiti larvae, the finfold was still present at the dorsal margin
of the body and was somewhat inflated anteriorly. The caudal fin developed at
small size and all rays were visible at ~6.5 mm BL. The anal fin base started
forming at ~5.5 mm BL; at 6 mm BL ~15 pterygiophores were present, at
~8 mm BL all pterygiophores were forming, and at ~9 mm BL all rays (19)
were visible. The dorsal fin started forming at larger larval size than the anal fin.
At ~6 mm BL a rudimentary dorsal fin base developed in the finfold, at 7.7 mm
BL 9 pterygiophores were visible, at 11.2 mm BL (our largest larvae) 8 or 9 rays
were visible. Pelvic fin buds were visible at ~9 mm BL. The adipose fin was
visible in larvae from ~11 mm BL.
Development of the caudal fin was similar in H.hygomii (all rays visible at
~7 mm). Development of the anal fin started at 5.5-6 mm BL and at 9-10 mm
BL all rays were formed. Like H.benoiti, the dorsal fin base started to develop at
slightly larger sizes, at 7 mm, and all rays were present at ~13 mm BL.
Photophores
The Br2 photophores developed at slightly smaller sizes,in H.benoiti larvae, at
~7 mm BL, and were the only photophores visible in our largest larvae
(~11 mm), which is typical for most myctophid larvae. In H.hygomii of the
Southwest Indian Ocean, the Br2 photophores appeared at ~7.5 mm BL, and
the same was observed in H.hygomii larvae from the Eastern Mediterranean (off
Lebanon).
Myomeres
The number of myomeres was similar in both species, with ranges from 34 to 37
(from 15 to 18 pre-anal) in H.benoiti, and 35-37 myomeres in H.hygomii.
Pigmentation
The pigmentation pattern that is diagnostic for the genus Hygophum (Moser et
al., 1984) was evident in all larvae examined: melanophores at the cleithral
symphysis and isthmus region.
Lateral gut spots were present in all H.benoiti larvae. Large melanophores
occurred over the terminal portion of the gut. Generally, two lateral gut spots
were present over the area of the transverse rugae, and one anterior spot was
present at the foregut. No pigment was visible on the lower jaw in pre-flexion
stages, but was present in some flexion and post-flexion larvae. At any stage of
development, melanophores could be found scattered over the pectoral fin rays,
as shown in H.hygomii larvae (Sanzo, 1918b; Tortonese, 1956; Pertseva-
Ostroumova, 1974; Berdar and Cavaliere, 1979; and our observations).
In H.benoiti larvae >9 mm, internal pigmentation was present over the
cleithral region. In the smallest larvae, ~3 mm, there was pigmentation on the
finfold. One melanophore was present in the dorsal finfold at about the level of
the anus, another melanophore was located on the post-anal membrane, ~5
985

M.P.OIivar and I.Palomera
myomeres posterior to the anus, and two groups of melanophores were present
at the dorsal and ventral borders of the caudal finfold. This pigmentation was
not always visible due to the difficulty of collecting small larvae with intact
fmfolds. In pre-flexion larvae, but not in flexion or post-flexion stages, 1-4 small
spots were sometimes present on the post-anal midventral line of the tail. This
pigmentation differed from the post-anal melanophore present in pre-flexion
and some flexion H.hygomii larvae. This large stellate melanophore often
extended onto the myosepta, and was always located between 6 and 8 myomeres
posterior to the anus [also noted by Berdar and Cavaliere (1979)].
The dorsal and ventral caudal finfold pigment which was sometimes noted in
pre-flexion H.benoiti was never observed in H.hygomii. In some post-flexion
stages of H.benoiti, a spot was visible at the tip of notochord, and in some larvae
a medial spot was located at the caudal bases. In a few larvae, both
melanophores were present. Finally, a group of many spots was scattered more
or less distally in the ventral rays of the caudal fin. The pigmentation in these
three areas was never present in larvae of H.hygomii.
The diagnostic characteristics of H.benoiti larvae that differentiated them
from H.hygomii were: moderately slender body, with the gut extending to
>60% of BL, a thin foregut that comprised —1/3 of the gut length in pre-flexion
and flexion larvae, and a dorsal fin that originated behind the midpoint of the
body. Most larvae had caudal pigmentation, but lacked both the stellate
melanophore on the ventral midline of the tail and the pigmentation on the
pectoral rays. In H.hygomii, pre-anal length and foregut length were shorter,
and the dorsal fin originated anteriorly to the midpoint of the body. Caudal
pigmentation was never present, but melanophores were generally scattered
over the pectoral fin rays and a large stellate melanophore was always present on
the ventral midline of the tail in pre-flexion larvae.
A controversial point is when the Br2 photophores appear. According to
Taning (1918), Cavaliere and Berdar (1977) and Berdar and Cavaliere (1979),
these photophores are present in smaller H.benoiti (~8.5 mm BL) than in
H.hygomii (~ 11 mm BL). However, Sanzo (1918a,b) states just the opposite:
Br2 appear later in larger H.benoiti [at 14 mm total length (TL)] than H.hygomii
(at 10.3 mm TL). The results presented here show no difference in the stage at
formation of Br2 photophores in either species. Tortonese (1956), who reviewed
Taning's (1918) and Sanzo's (1918a,b) descriptions, pointed out that the
progressive development of the different photophores is not identical in all
individuals.
A summary of the main features that differentiate the larvae of all the
Hygophum species of the 'moderately slender type' of the world is included in
Table II.
Larval distribution of H. benoiti in the North Western Mediterranean
Larvae of H.benoiti appeared in the plankton from May (northern spring) to
October (northern autumn). During the survey of June/July 1992, the highest
abundances of larvae of this species were obtained (Figure 6). No larvae of this
986

Ontogeny and distribution of H.benoiti in NW Mediterranean
Table II. Main characters that differentiate the larvae of Hygophum species of the 'moderately
slender type' in pre-flexion-flexion stages
H.benoiti
H.bruuni
H.hanseni
H.hygomii
H.proximum
%PA/BL
62-66
52-61
53-57
57-58
61-63
Pigmentation
Lower jaw Tail Caudal fin Pectoral fin
- In larvae

M.P.OIivar and I.Palomera
43
41
39
43
41
39
43
41
39
1000'
II I 1 I
•

Ontogeny and distribution of H.benoiti in NW Mediterranean
During June/July 1992, two patterns of distribution of H.benoiti larvae were
obvious. Larvae were most abundant in deep waters off the Catalan coast, but
were largely absent from the Gulf of Lions in the northern part of the survey
region. This difference could not be related to differences in temperature and
salinity between the Catalan and French coasts, since these environmental
factors showed little variation between the slope areas of the two sectors.
The main concentrations of H.benoiti larvae at deeper stations off the Catalan
coast match the oceanic habits of the adults (Taning, 1918; Goodyear et al.,
1972; Hulley, 1981). As pointed out by Sabates and Maso (1990), the circulating
pattern in the area has a major effect on the distribution of mesopelagic larvae.
Current flow in the region is southwesterly along the edge of the continental
shelf. Associated with this current is a typical shelf-slope front separating less
saline inshore waters from the open sea (Font et al., 1988). Just north of the
Ebro Delta, the current penetrates to the wide continental shelf (Font et al.,
1990), and the concentration of larvae closer to the coast in this area is probably
related to this circulation pattern. Thus, the nearshore distribution of H.benoiti
observed in this study may be closely related to both hydrographic and
topographic features in the area.
Acknowledgements
The authors wish to thank Dr H.G.Moser for the revisions and useful comments
on the manuscript, and advice in its preparation. Larvae used for comparison
were generously made available by the following persons, to whom we are
especially grateful: A.Aboussouan (Faculte des Sciences de Luminy, Marseille),
L.Beckley (Oceanographic Research Institute, Durban), K.E.Hartel (Museum
of Comparative Zoology, Cambridge), W.F.Hoffman (National Museum of
Natural History, Smithsonian Institution), H.Ch.John and C.Zelck (Zoologisches
Institut and Museum, Hamburg), J.Leis and T.Trnski (Australian
Museum, Sydney), T.Ozawa (Kagashima University), P.Rubies and A.SabatSs
(Instituto de Ciencias del Mar, Barcelona), R.Zeidane (Centre de Recherches
Marines, Lebanon). We are also grateful to G.Fuster and A.Carpena for sorting
the samples. We also appreciate the comments and suggestions made by
P.Rubies while preparing the manuscript. J.Corbera prepared the figures of the
larvae.
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Received on September 21, 1993; accepted on March 16, 1994
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