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Myctophidae

There can be little doubt of monophyly of the Myctophidae as it is currently recognized (Paxton, 1972, 1979; Moser et al., 1984), and in the course of this study a number of novel myctophid synapomorphies have been identified (see below). The widespread notion of myctophids as being "of an unremarkable fishy shape" but strikingly characterized by "numerous light organs" (Hulley, 1995) hardly does justice to this morphologically specialized and diverse lineage. ³ It can be argued that preoccupation with photophores and photophore patterns within the family has diverted attention away from the probably equally informative and certainly extraordinarily rich variety of morphological innovation exhibited by this interesting clade.

Found in the upper 1000 meters of all oceans of the world, the Myctophidae are currently comprised of some 230-250 species classified into 30-35 genera (Paxton, 1979; Eschmeyer, 1990; Nelson, 1994). Evidence for monophyly of this assemblage is manifold. In fact, I find the clade to be bristling with synapomorphies in just about every anatomical system examined. This is an enviable situation for any morphological systematist to encounter, but unfortunately the space limitations imposed by a review of this nature necessitate brevity. Therefore, before briefly considering the intrarelationships of the family I will describe a selection of characters that I believe lend strong support for the hypothesized monophyly of the myctophid clade.

Already the following myctophid synapomorphies have been identified:

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Subdivision of the first levator internus muscle into two heads, each inserting independently onto the second pharyngobranchial element (Fig. 1B; see also page 407)

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Reduction of the dorsal hypohyal element (HH1) (Figs. 2D and 2E; see also page 407)

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Presence of a distinct gap between the epihyal branchiostegals and the first two ceratohyal branchiostegals (Figs. 2D and 2E; see also page 407)

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Presence of a cylindrical median rostral cartilage firmly bound to the mesethmoid and only loosely attached to the buccal jaws (Fig. 15D,E; see also page 415)

Additionally, the following selection of characters is presented in corroboration of myctophid monophyly:

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Medial row of serially arranged toothplates arrayed along the length of the first branchial arch (Figs. 17D, 18A, and 18C)

Typically among teleosts an inner and an outer row of denticulate gill rakers are arrayed along the length of the first ceratobranchial (and often also the hypobranchial and epibranchial elements) (e.g., Fig. 18B). While in some outgroup taxa, supernumerary toothplates are irregularly interspersed between the serially arranged gill rakers; uniquely in myctophids the first branchial arch carries a medial row of serially arranged oblong toothplates (e.g., Figs. 17D and 18C). With the exception of the two genera of Gonichthyini, Gonichthys and Centrobranchus, a medial row of toothplates has been located on the first branchial arch of all the myctophid taxa examined. Similar toothplates are not found in neoscopelids nor in any other outgroup taxon examined. Therefore, based upon this limited distribution, the presence of a row of serially arranged, oblong, median toothplates, arrayed along the length of the first branchial arch, is interpreted here as a synapomorphy of the Myctophidae.

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Maxillary control mediated via an A1β muscle component of the adductor mandibulae complex (Fig. 19B)

Among neoteleosts, the maxilla is controlled either by an external Al division of the adductor mandibulae, which inserts along the maxillo-mandibular ligament (ligamentum primordium) and attaches anteriorly on the maxilla via a separate tendon (tA1), or by an internal A1β muscle system. The A1β muscle, which originates medial to the A2 section, inserts onto the maxilla near the maxillary fulcrum via a tA1β tendon and has no connection or association with the maxillo-mandibular ligament.

An A1 system of maxillary control is widespread among teleosts (Rosen and Patterson, 1969; Winterbottom, 1974; Fink and Weitzman, 1982), while an A1β system has a more limited distribution among neoteleosts (Rosen, 1973). Fink and Weitzman (1982) reviewed maxillary control mechanisms in neoteleosts and discussed the phylogenetic significance of the occurrence of an A1β muscle system. They concluded that an A1β system has arisen independently a number of times within the Neoteleostei and cited as one example the presence of an A1β system in myctophids (Fig. 19B; see also Jollie, 1954, fig. 25; Rosen and Patterson, 1969, plate 55; Rosen, 1973, fig. 40; Winterbottom, 1974, fig. 4), noting the absence of A1β (and the presence of an A1 system) in neoscopelids (Fig. 19A; see also Winterbottom, 1974, fig. 3).

While there is considerable variation within the Myctophidae in the degree of subdivision and complexity of the A2 component of the adductor mandibulae, as well as in the degree of muscular encroachment of A1β onto the maxilla, a simple Alp muscle is invariably present. As an A1β is absent in neoscopelid and aulopiform outgroups (see also Rosen, 1973), I concur with Fink and Weitzman (1982) in their assessment of the derived nature of the A1β system of myctophids and suggest here that the presence of a simple A1 β system of maxillary control is a synapomorphy of the Myctophidae. Interestingly, among basal acanthomorphs a complex A1β system is found in Polymixia (Rosen and Patterson, 1969, plate 53; Rosen, 1973, fig. 41), and certain paracanthopterygians also have an A1β system of maxillary control (Rosen and Patterson, 1969; Rosen, 1973), although in both of these cases the morphological details of the Alp systems differ from that found in myctophids.

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Bony connection between the descending process of the third hypobranchial element with the urohyal (Figs. 20C-20G)

Lauder (1983) identified what he believed to be a specialization of myctophiform fishes in which thedescending processes of the third hypobranchial extend laterally to form a rigid bony connection with the urohyal. Stiassny (1986) noted that while Lauder was correct in highlighting the peculiarity of the myctophid urohyal-hypobranchial 3 connection, he had incorrectly attributed this derived condition to neoscopelids (e.g., Fig. 20B). While all myctophids are characterized by the presence of a bony overlap of the elongate hypobranchial 3 elements that are firmly bound to either side of the median urohyal (see, e.g., Figs. 20C-20G), this condition is unique to that clade and does not occur in neoscopelids (e.g., Fig. 20B) nor in any non-myctophiform outgroups (e.g., Fig. 20A). Therefore, I concur with my initial interpretation (Stiassny, 1986) that the development of a rigid bony connection between the third hypobranchial and the median urohyal is a synapomorphy of the Myctophidae.

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Bony articulation between the urohyal and the second, or second and third, basibranchial element (Figs. 20C-20G)

Stiassny (1986, fig. 7) illustrated, but did not comment upon, a novel bony articulation between the urohyal and the basibranchial series in Myctophum. Further review of this character reveals that a similar articulation is present in most myctophid taxa (e.g., Figs. 20C-20F), although in a few taxa (e.g., Notolychnus) bone-to-bone contact is lacking, but an articular facet is retained on the urohyal (Fig. 20G). A similar articulation is lacking in neoscopelids (e.g., Fig. 20B) and in non-myctophiform outgroups (e.g., Fig. 20A). Once again, this distribution suggests that the presence of an articulation between the basibranchial series and the urohyal element, mediated via an articulation facet developed on the dorsal face of the urohyal, is a synapomorphy of the Myctophidae.

Although a detailed analysis of myctophid intrarelationships is beyond the scope of the present investigation, I comment briefly on a chapter by Paxton Paxton, 1972; Moser and Ahlstrom, 1970, 1972, 1974; Moser Paxton et al. (1984) as that study is the most recent and comprehensive analysis of relationships within the myctophid clade. Paxton et al. is an abbreviated synthesis and reanalysis of data initially presented in a prior series of papers ( et al., 1984). The authors acknowledged that their previous interpretation of myctophid intrarelationships was not strictly phylogenetic, being instead based on notions of overall similarity among the various taxa. As part of a more phylogenetically rigorous treatment et al. (1984) arrayed some 25 osteological, 17 larval, and 17 photophore characters culled from their previous studies, among representatives of 35 myctophid genera. The characters were coded in a data matrix with polarity assessment determined on the basis of a number of discussed criteria, and a phylogenetic diagram representing myctophid intrarelationships was also presented (Fig. 21A). However, the phylogenetic diagram of Paxton et al. was apparently not derived from the matrix, but rather the authors constructed a phylogenetic tree based on their "knowledge of the family" and subsequently "used the apomorphic states of the 59 characters to define the various branching points" (1984, p. 243). When the Paxton et al. matrix (1984, pp. 242-243) is subjected to parsimony analysis using PAUP version 3.1.1 (Swofford, 1993) 16 equally parsimonious trees result; the strict consensus of these is presented here as Figure 21B. As can be seen by a comparison of the matrix-derived consensus tree (Fig. 21B) with the Paxton et al. tree (Fig. 21A) there is some discrepancy in the resultant hypothesis of myctophid intrarelationships, most evident in the alternate placement of the odd diminutive genus, Notolychnus (see also Fraser-Brunner, 1949; Moser and Ahlstrom, 1970,1972,1974; Paxton, 1972, for a discussion of the problematic Notolychnus).

While I do not intend to dwell at length on the question of myctophid intrarelationships, nonetheless in the course of this study I have identified some additional characters that help to elucidate the phylogenetic placement of Notolychnus at the subfamilial level, and these are outlined briefly below:

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Loss of fused third epibranchial toothplate (EP3TP; Figs. 22A-22C)

R. K. Johnson (1982) noted that myctophids lack third epibranchial toothplates (EP3TP), a character state that he considered derived among "inioms." While I agree that the loss of a toothplate fused to the ventral face of the third epibranchial element is a derived character at the level of the Ctenosquamata, I find that the loss of fusion of this toothplate is restricted among myctophids to the myctophine genera (e.g., Figs. 22A-22C). In these taxa toothplates, or gill rakers, are present but never fused to the epibranchial element. In contrast, all of the lampanyctine genera examined retain a toothplate fused to the ventral face of the third epibranchial (e.g., Figs. 22E-22G). Similarly, an EP3TP is present in the eurypterygian outgroup taxa examined (e.g., Fig. 22D). Based on this limited distribution, the absence of a toothplate fused to the ventral face of the third epibranchial is interpreted as a derived character state.

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Elongation of the second basibranchial element (Figs. 20E-20G)

In most taxa examined the second basibranchial is a moderate sized element that is approximately twice the length of the first basibranchial (e.g., Figs. 20A-20D). However, this is not the case in the lampanyctine myctophids examined, and invariably in these taxa the second basibranchial is a markedly elongate element that is minimally three, and often four, times the length of the first basibranchial. A similar elongation of the second basibranchial has not been found in myctophines or outgroup taxa and the presence of an elongate second basibranchial is interpreted as a derived character state.

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Urohyal with elongate anterior process and reduced articulation facet (Figs. 20E-20G)

Primitively among eurypterygians the urohyal is a robust element with a short bifurcating anterior process (e.g., Figs. 20A-20B). In myctophids, with the exception of the Gonichthyini (e.g., Fig. 20D), the urohyal is a thin, blade-like element, and in the lampanyctine genera examined the anterior process is markedly elongate and distinctly demarcated from the body of the urohyal blade (e.g., Figs. 20E-20G). Additionally, in lampanyctines the articulation facet on the dorsal surface of the urohyal is short (e.g., Fig. 20I). In contrast in the myctophine genera the anterior processes are less elongate and are formed as a tapering continuation of the urohyal blade (e.g., Figs. 20C and 20D), and the articulation facet is elongate (e.g., Fig. 20H). The presence of an elongate, strongly demarcated anterior process and short articulation facet is tentatively interpreted here as a derived character state.

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Metapterygoid strut projects dorsally from the main body of the bone (Paxton, 1972, fig. 6B)

Paxton (1972) identified two conditions of a bony strut developed on the posterodorsal margin of the metapterygoid in myctophid fishes. This metapterygoid strut either projects dorsally and extends well above the dorsal margin of the bone (Paxton, 1972, fig. 6B) or it is restricted to, or lies ventral to, the dorsal margin of the bone (Paxton, 1972, fig. 6A). Paxton (1972, table 3) lists the distribution of these two character states among 32 myctophid genera. In assessing the polarity of this character, while some variation exists, it is evident that among neoscopelids and other outgroups the metapterygoid strut most closely approximates the second condition, i.e., the strut is situated ventral to the dorsal border of the bone and I therefore interpret the presence of a dorsally projecting metapterygoid strut as the derived character state.

When these four characters are coded (Table 1) and added to the Paxton et al. (1984, pp. 242-243) data matrix, and the augmented matrix is subjected to parsimony analysis using PAUP, 264 equally parsimonious trees (length 148) result ⁴; the strict consensus of these is presented here (Fig. 21C). Perhaps most relevant to the discussion at hand is the resolution of Notolychnus as the basal lampanyctine clade. In this scheme, the Myctophinae are monophyletic and uniquely diagnosed by the loss of toothplates fused to the third epibranchials, and a monophyletic Lampanyctinae (including Notolychnus) is uniquely diagnosed by the presence of an elongate second basibranchial and a specialized urohyal morphology.

Genus

EP3 + TP

Basibran-chial 1

Urohyal shape

Meta-pterygoid strut

Obviously, in such a cursory review I do not wish to convey the impression that the intrarelationships of the myctophid fishes are represented adequately by the diagram presented here as Fig. 21C, or that the Paxton Paxton et al. (1984) matrix is not without need of much critical review and revision. Rather, I add these data in the hope that the resultant analysis will stimulate further work on this interesting clade, and at the present time I can do little more than echo the sentiment of et al. (1984, p. 244) who ended their consideration of myctophid relationships in the hope that "new, less plastic characters and better definitions of polarity will help resolve the problems."

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There can be little doubt of monophyly of the Myctophidae as it is currently recognized (Paxton, 1972, 1979; Moser et al., 1984), and in the course of this study a number of novel myctophid synapomorphies have been identified (see below).

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A model of the Headlightfish Diaphus rafinesquii, a type of Lanternfish. Credit: Museo de Ciencias Naturales Senckenberg

Deepwater fishes hold an undeniable fascination in the hearts and minds of many an aquarist, but, alas, these bizarre denizens of the deep are impossible to acquire for the home hobbyist. However, what is often not appreciated is that there are several groups of mesopelagic fish which regularly ascend into the shallow waters around coral reefs and could, in theory, be targeted by aquarium collectors. The most abundant of these are the remarkable little bioluminescent fishes from the Family Myctophidae known commonly as Lanternfishes.

Benthosema sp. seen in Bali at 5 meters. Credit: manboon

Myctophids are a perfect example of an animal which should be more widely known among the general public. These small, unassuming creatures are prolific on a monumental scale and are widely assumed to be amongst the most numerous of marine fishes, forming upwards of 65% of the biomass in their mesopelagic realm. Shoals of lanternfishes can be so dense as to reflect back sonar (due to the hollows of their swimbladders); this was discovered during the second world war, when sonar operators noticed a false bottom to the ocean at several hundred meters that changed position during the course of the day—the so-called “Deep Scattering Layer”.

Lanternfishes undergo huge vertical migrations of hundred of meters as they follow their zooplanktivorous prey upwards at night.  Their journey typically begins shortly before sunset, and, at its peak, myctophids (and many other deepwater fish families) can be plucked from the surface waters, offering the possibility for their capture in the aquarium trade. In fact, researchers have been doing so for years in order to better study the luminescent properties of this group.

Lanternfish photophores have a modified lenticular scale to diffuse the light produced beneath it. Credit: Joan Costa/CSIC

Illustration of the Headlightfish Diaphus metopoclampus showing the serial photophores along the belly and the large luminous organ near the eye common to this genus. Source: Goode & Bean, 1896

Myctophid luminescence is the result of a luciferin-luciferase reaction—the same chemistry which creates the cold light of fireflies. This is fundamentally different from the other group of shallow-water bioluminescent fishes: the Flashlightfishes of the Family Anomalopidae, which have a specialized organ for culturing light-producing bacteria. The soft, bluish glow emitted by lanternfishes occurs in small circular organs known as photophores lining the undersides of the body, providing the fish with “counter-illumination”. This serves to mask the fish’s outline from below when seen against the small amount of blue light that reaches the mesopelagic depths.

Lampanyctus sp. observed at Osezaki, Japan in shallow water. Credit: Ryo Minemizu

The Mexican Lampfish (Triphoturus mexicanus) is the most abundant fish in the mesopelagic waters of Baja. Specimens observed in situ often adopt a vertical swimming orientation, which seems to negate the “counter-illumination” provided by the photophores. Credit: MBARI

There are roughly 250 species of lanternfish, and each is said to possess its own unique arrangement to its photophores, making these structures highly important for species recognition amongst both fish and scientist alike. Many also have additional patches of luminous tissue that serve a more direct role in intraspecific communication. In some, such as the Headlightfish Diaphus, the luminous organs occur in association with the eyes, while in others these organs are positioned on the caudal peduncle and are sexually dimorphic, with males having them dorsally and females ventrally. Unlike the serially arranged photophores, which emit a constant blue light, these other types of luminous organs are behaviorally controlled and only briefly flashed when communicating. One unusual observation was made when a researcher unwittingly held the phosphorescent dial of his watch near a captive lanternfish, which the fish proceeded to respond to with its own flash of light.

Benthosema lanternfish seen along a reef in Palau. Credit: manboon

The most abundant species in shallow tropical waters are typically Benthosema pterotum and B. fibulatum, though Diaphus, Myctophum and Hygophum are also commonly trawled near surface waters. Specimens captured in this manner, particularly for Benthosema, are reported to lose many of their scales and would likely prove difficult to keep in captivity long-term. Should specimens ever be targeted for the aquarium trade, they would likely need to be hand-netted and transported quite carefully to avoid damage. One trait working to this fish’s advantage though is their ability to tolerate low oxygen levels, an adaptation to similar conditions often found in the mesopelagic depths.

The typical lifespan for a lanternfish is said to vary from 1-5 years, though it’s likely captive specimens could prove longer lived. In the wild, myctophids are on the menu for many different animals, from larger deepwater fishes to penguins and dolphins, undoubtedly making it challenging for them to grow to a ripe old age. Their delectability relates to their high fat content—so fatty are these fishes that consumption in large quantities is said to cause diarrhoea. For this reason, as well as the difficulties in processing these small fishes, they have limited utility for our own species, being mostly used as fertilizers and animal feed.

The glowing lights of Diaphus. Credit: unknown

While lanternfishes may not be as aesthetically grotesque as some of the other specialized fishes with which it shares its deepwater home, they are nonetheless fascinating little fishes and one which could make for a truly unique aquarium display. Hopefully someday soon aquarium collectors can make these available, as I imagine I’m not the only one who’d like to keep a school of these glowing beasts.

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Deepwater fishes hold an undeniable fascination in the hearts and minds of many an aquarist, but, alas, these bizarre denizens of the deep are impossible to acq