Sea lice, members of the family Caligidae Burmeister, 1835, are ectoparasitic copepods found on most groups of marine fishes. Adult caligids typically inhabit the body surface, buccal cavity, or gills of their hosts. These parasites are abundant in natural ecosystems and can cause substantial pathogenesis in aquaculture (Mustafa et al., 2000). Caligids are of major economic significance because several species have emerged as serious pests in marine fin-fish aquaculture in coastal waters around the world, including cold temperate and subtropical environments (Johnson et al., 2004). These copepods are perhaps most well-known as problematic pathogens in salmon farms, where they are responsible for over $1 billion in losses each year (Boxaspen et al., 2022). It is not uncommon for dozens or even hundreds of individuals to be found on a single fish, feeding on mucus, tissue, and blood and causing substantial pathologic changes (Johnson et al., 1996, 2004; Bravo, 2003; Treasurer and Bravo, 2011; Kodama et al., 2021; Rodger et al., 2022).

Caligids are characterized by the possession of a shield-shaped cephalothorax incorporating the first to third pedigerous somites (i.e., leg-bearing body segments). The cephalothorax typically forms a suction cup, enhanced by a marginal membrane, closed off anteriorly by the paired frontal plates, and delimited posteriorly by the broad, apron-like third swimming legs (Ohtsuka et al., 2021). This sucker is used for host attachment, and the suction force can be rapidly generated and released for attachment or free movement as needed (Ohtsuka et al., 2021). However, there are several interesting and rather extreme deviations from this body plan, which are imaged here with confocal laser scanning microscopy (CLSM) and revealed in new detail.

The most recent key to the genera of Caligidae is from the detailed monographic study of Dojiri and Ho (2013), who considered 31 genera to be valid, but many recent taxonomic changes have occurred since. Dojiri and Ho (2013) synonymized many genera and noted that boundaries between several others were poorly defined. Since then, there have been several revisions, and the genera Metacaligus Thomsen, 1949, Sciaenophilus van Beneden, 1852, and Sinocaligus Shen, 1957 have all been synonymized with Caligus Müller, 1785 (Özak et al., 2017, 2024; Boxshall and Barton, 2023). Although Dojiri and Ho (2013) treated Midias Wilson, 1911 as valid, we agree with Kabata (1979) and others in regarding Midias as a synonym of Caligus. We also formally recognize Markevichus Özdikmen, 2008 as a junior subjective synonym of Caligus. In total, we recognize 27 genera as valid, which collectively include 503 species. Over 75% of caligid species are members of the 2 largest genera, Caligus (276 species) and Lepeophtheirus von Nordmann, 1832 (124 species), followed by Anuretes Heller, 1865 with 21 species (Walter and Boxshall, 2024). The remaining 24 genera have fewer than 10 species each, and 10 genera are monotypic.

Here, we present a new key to genera of Caligidae supported by images generated with CLSM and macrophotography for nearly all valid genera. The goals of this study were to facilitate the identification of the 27 genera, to highlight the morphological diversity of Caligidae, and to discuss limitations in data supporting several taxa. Molecular phylogenetic analyses are needed to establish more stable and robust generic boundaries, but because appropriate specimens for sequencing are not available for most genera and species, a more formal systematic revision awaits the collection of suitable material for sequencing.

MATERIALS AND METHODS

Specimens of 25 of the 27 valid caligid genera were imaged; only Arrama Dojiri and Cressey, 1991 and Dartevellia Brian, 1939 were not examined. The following specimens were imaged with CLSM: Abasia platyrostris Pillai, 1963 (NHMUK 2017.173), Alanlewisia fallolunulus (Lewis, 1967) (NHMUK 2010.658), Alebion gracilis Wilson, 1905 (NHMUK 2010.974), Anchicaligus nautili Stebbing, 1900 (NHMUK 2010.979), Anuretes heckelii (Krøyer, 1863) (NHMUK 1979.592), Belizia brevicauda Cressey, 1990 (USNM 241670), Caligodes laciniatus (Krøyer, 1863) (NHMUK 2017.192), Caligus diaphanus von Nordmann, 1832 (NHMUK 1975.180), Caritus serratus Cressey, 1967 (USNM 120354), Echetus typicus Krøyer, 1864 (USNM 107877), Euryphorus brachypterus (Gerstaecker, 1853) (NHMUK 2014.637), Gloiopotes huttoni (Thomson, 1890) (from a “sailfish,” locality unknown), Hermilius longicornis Bassett-Smith, 1898 (NHMUK 2017.334), Kabataella indica Prabha and Pillai, 1983 (USNM 268272), Lepeophtheirus pectoralis (Müller, 1776) (NHMUK 1975.482), Mappates plataxus Rangnekar, 1958 (NHMUK 2017.346), Paralebion elongatus Wilson, 1911 (NHMUK 1994.716), Parapetalus occidentalis Wilson, 1908 (NHMUK 1984.13), Parechetus carangis (Bassett-Smith, 1898) (NHMUK 1999.628), Pseudanuretes papernai Kabata and Deets, 1988 (USNM 231871), Pseudechetus fimbriatus Prabha and Pillai, 1979 (USNM 274345), Pupulina flores van Beneden (1892) (USNM 1087132), Synestius caliginus Steenstrup and Lütken, 1861 (NHMUK 1994.1212), and Tuxophorus caligodes Wilson, 1908 (NHMUK 1954.9.20.7).

For CLSM, specimens were stained overnight in a saturated solution of Congo Red in 100% ethanol, rinsed in changes of distilled water until no stain could be seen diffusing, and prepared as temporary mounts in a 50% solution of glycerin and distilled water on a glass slide under a coverslip. For thicker specimens, the coverslip was suspended above the specimen with pieces of paraffin or, for the largest specimens, with rubber washers glued to a slide to create a deep well. Specimens were examined with a Leica TCS SP5 equipped with a Leica DM5000 B upright microscope and the Leica Application Suite Advanced Fluorescence software LAS AF 2.2.1 (Leica, Wetzlar, Germany). We used a 561 nm excitation wavelength from a DPSS 10 mW 561 nm laser set at 80% power and collected the emitted fluorescence in 2 channels: 570–630 nm artificially colored green and 630–715 nm artificially colored red. A series of image stacks was collected, and the final images were obtained by maximum projection of the overlaid channels using the same Leica software or ImageJ v2.14. For some specimens, multiple stacks or multiple fields of view were needed to produce high-resolution images of the entire specimen. Multiple stacks were combined using the concatenate tool in ImageJ or manually using Adobe Photoshop v25.0, and multiple fields of view were combined using the ImageJ pairwise stitching plugin (Preibisch et al., 2009) or with Adobe Photoshop.

RESULTS

Figure 1.

Caligid genera, dorsal view, confocal laser scanning microscopy (except F). (A) Abasia platyrostris. (B) Alanlewisia fallolunulus. (C) Alebion gracilis. (D) Anchicaligus nautili. (E) Anuretes heckelii. (F) Avitocaligus assurgericola (macrophotography, holotype MNHN Cp-2185). (G) Belizia brevicauda. (H) Caligodes laciniatus. (I) Caligus diaphanus. (J) Echetus typicus. (K) Euryphorus brachypterus; arrowheads indicate dorsal plates on fourth pediger. (L) Gloiopotes huttoni. (M) Hermilius longicornis. (N) Kabataella indica. (O) Lepeophtheirus pectoralis. (P) Mappates plataxus. (Q) Paralebion elongatus. (R) Parapetalus occidentalis. (S) Parechetus carangis. (T) Pseudanuretes papernai. (U) Pseudechetus fimbriatus. (V) Pupulina flores. (W) Synestius caliginus. (X) Tuxophorus caligodes; arrowheads indicate dorsal plates on fourth pediger. Color version available online.

Figure 2.

Confocal laser scanning microscopy of caligid genera with ventrally folded cephalothorax (Arrama not shown). Hermilius longicornis: (A) dorsal, (B) ventral, (C) lateral. Abasia platyrostris: (D) dorsal, (E) ventral. Kabataella indica: (F) dorsal, (G) ventral, (H) leg 4 ventral with segments numbered. Color version available online.

Figure 3.

Confocal laser scanning microscopy for comparison of legs 1 and 3. (A) Leg 1 with 2-segmented endopod; Alebion gracilis, arrowheads indicate segments. (B) Leg 3 with 3-segmented endopod in A. gracilis with segments numbered, arrowhead indicates 1 of many modified spines characteristic of Alebion. (C) Leg 1 with unsegmented endopod in Caligus hirsutus, endopod indicated with arrowhead. (D) Leg 3 with 2-segmented endopod in Caligus epidemicus with segments numbered. Color version available online.

Figure 4.

Confocal laser scanning microscopy of lunules and lunule-like structures in the Caligidae. (A) Caligus schlegeli exhibits relatively large lunules. (B) Caligus confusus exhibits typical lunules. (C) Caligus furcisetifer has the smallest lunules in Caligus. (D) Alanlewisia fallolunulus female possesses lunule-like structures that are not true lunules. Color version available online.

Figure 5.

Confocal laser scanning microscopy of posterior processes on caligid genera. (A) Paralebion elongatus. (B) Caligodes laciniatus. (C) Pseudechetus fimbriatus. (D) Pseudechetus fimbriatus ventral. (E) Synestius caliginus. (F) Parapetalus occidentalis. (G) Parechetus carangis. Abbreviations: GC, processes on genital complex; Abd, processes on abdomen. Color version available online.

Figure 6.

Confocal laser scanning microscopy of caligid genera with dorsal expansion of cephalothoracic shield partially covering pediger 4. (A) Anuretes heckelii. (B) Pseudanuretes papernai. (C) Lepeophtheirus atypicus. (D) Mappates plataxus. (E) Caligus epidemicus. (F) Belizia brevicauda. (G) Belizia brevicauda, ventral view. Color version available online.

DISCUSSION

Here, we review questionable evidence supporting the validity of some caligid genera and speculate on potential evolutionary relationships based on new morphological interpretations. Boundaries between many genera are poorly defined and have often been further eroded by the discovery of species exhibiting intermediate combinations of character states. The few morphological features in caligids that have been examined in a molecular phylogenetic context have often been found to be homoplasious. For example, the segmentation of leg 4 was historically used to distinguish the genera Markevichus, Pseudocaligus, and Pseudolepeophtheirus, which possess a highly reduced leg 4 or lack leg 4 altogether. Although variability in the segmentation of leg 4 remains one of the most useful features for identifying species, molecular phylogenetic studies have revealed that leg 4 has been reduced to a single segment at least 3 times independently (Freeman et al., 2013), providing support for existing synonymies of Markevichus and Pseudocaligus with Caligus and Pseudolepeophtheirus with Lepeophtheirus.

The presence or absence of the sternal furca is also patchy throughout the Caligidae, and we consider it very likely that this structure also exhibits a high degree of homoplasy. This situation is analogous to that in the genus Dissonus Wilson, 1906 (family Dissonidae), the species of which may possess a typical sternal furca or a modified sternal stylet or may lack the sternal furca altogether (Boxshall et al., 2008). Given that what little we know about morphological evolution in caligids suggests there are many instances of convergent evolution, in the interest of taxonomic stability we have avoided making large taxonomic changes until we have evidence from molecular phylogenetic analysis to support or refute clades. We caution that taxonomic changes in this diverse and globally important group should not be made without robust support.

Perhaps the most problematic set of generic distinctions relates to Anuretes, Pseudanuretes, Mappates, and Lepeophtheirus. These genera are traditionally distinguished from each other primarily by the extent of the elongation of the free margin of the thoracic zone of the cephalothorax in the female; however, there is significant overlap in this feature between the genera (e.g., Fig. 6A–D). Dojiri and Ho (2013) included a detailed discussion of this history and evidence, or lack thereof, supporting their separation. These genera already have a complex taxonomic history given that Anuretes has already been synonymized with Lepeophtheirus and then resurrected (Ho and Dojiri, 1977; Dojiri and Ho, 2013) and that 3 of the 9 currently accepted species of Pseudanuretes (Walter and Boxshall, 2024) were considered as questionable by Dojiri and Ho (2013) because their inclusion would require broadening of the generic boundaries. Indeed, it is challenging to key out these genera because virtually every feature used to distinguish Anuretes is also found in some species of Lepeophtheirus or Pseudanuretes. Because there are no molecular data for any species of Anuretes, Pseudanuretes, or Mappates, we have elected to retain these 3 genera until a molecular phylogenetic analysis can better address the boundaries separating them.

Ten genera appear closely related to Caligus based on morphology: Anchicaligus, Belizia, Caligodes, Caritus, Echetus, Parapetalus, Parechetus, Pseudechetus, Synestius, and Tuxophorus. Despite the occasional dramatic elongations or lateral extensions of body somites (e.g., “necks,” waists, and foliaceous processes on the abdomen or genital complex), the appendage structure across these genera is remarkably conserved. They share the same segmentation on all appendages except for leg 4 (there are also rare fusions of the terminal 2 exopodal segments of leg 3 in a few species of Caligus) and share the same unique evolutionary novelty—the paired lunules on the frontal plate of the dorsal cephalothoracic shield (Kaji et al., 2012). Essentially all these genera are erected based on a difference in body somites relative to Caligus. The following 6 genera are monotypic and based on only autapomorphies relative to Caligus: Anchicaligus, Belizia, Echetus, Parechetus, Pseudechetus, and Synestius. The robustness of all 10 of these genera awaits to be tested in a molecular phylogenetic context. Regardless of their validity, these taxa are very interesting from an evolutionary perspective, and resolution of their phylogenetic relationships will give insights into the evolution of some remarkable body structures (e.g., “necks,” foliaceous processes, and enlarged lenses in the nauplius eye), attachment methods and sites (e.g., specialization for attachment to the operculum or to the wall of the buccal cavity by Echetus, Parapetalus, Parechetus, and Pseudechetus, with a mesoparasitic style in Echetus), and host associations (e.g., the colonization of Nautilus Linnaeus, 1758 by Anchicaligus). We suspect that the divergent morphology of at least some of these taxa is not the result of a unique phylogenetic history but that they are terminal branches of larger clades within Caligus, and their divergent morphology may be linked to changes in hosts and/or attachment sites.

The distinction between Belizia and Caligus is similar to that separating Anuretes, Pseudanuretes, and Mappates from Lepeophtheirus in that Belizia is distinguished primarily by the extension of the posterior margin of the thoracic zone of the cephalothorax so that it overlaps and conceals the fourth pedigerous somite in dorsal view (Fig. 6F). Cressey (1990) and Dojiri and Ho (2013) further distinguished Belizia based on the presence of only 4 setae on the caudal ramus, the sternal furca being reduced to 2 sclerotized knobs (Fig. 6G), the presence of 4 setae on the terminal endopodal segment of leg 3, and the velum not distinctly separated from the first endopodal segment of leg 3. However, we do not find these derived differences particularly robust. The sternal furca is reduced or absent in at least 10 species of Caligus. Although most species of Caligus have 6 setae on the terminal endopodal segment of leg 3, several species have 5, Caligus longipes (Moon and Kim, 2012) and C. laciniatus have 4, and Caligus uniartus (Ho, Kim, Cruz-Lacierda and Nagasawa, 2004) has 3 (Ho et al., 2004; Moon and Kim, 2012). The velum of leg 3 is not much different from that of most species of Caligus except that it is a bit smaller; this is not so surprising given that Belizia, at 1.6 mm body length, is 1 of the smallest caligids and has a small leg 3. We suspect molecular phylogenetic analyses will show that Belizia is a morphologically unusual species of Caligus, but we retain it as a valid genus at present.

Caritus also closely resembles Caligus. Caritus serratus, the type species, was redescribed from type material by Dojiri and Ho (2013) and was distinguished from Caligus by the absence of a posterior process on the proximal antennal segment, the absence of the postantennal process, the absence of the sternal furca, the possession of a mandible with an elongate terminal section, the possession of modified exopodal spines on leg 2, the absence of setae on the terminal endopodal segment of leg 3, and the relatively large lobe-like leg 5. This latter character is not particularly robust because several species of Caligus and Lepeophtheirus have an enlarged lobe-like leg 5, as discussed in detail in the remarks on Midias, above. Furthermore, numerous species of genera such as Abasia and Caligus lack a posterior process on the proximal antennal segment, a postantennal process, or sternal furca and occasionally lack 2 of these 3 features. In fact, the antennal posterior process is very reduced in Echetus typicus, and this species also lacks a postantennal process and sternal furca. Although no species of Caligus lacks all setae on the leg 3 terminal endopodal segment, several species have fewer than 6, and C. uniartus has only 3. Caritus tolii Rangnekar, 1984 is relevant to this discussion. Although poorly described, it appears to have the typical complement of 6 setae on the terminal endopodal segment of leg 3, and it appears to lack the large lobe-like leg 5 of the type species (Rangnekar, 1984), further eroding the distinction between Caritus and Caligus. If C. tolii truly lacks the posterior process on the proximal segment of the antenna and lacks the postantennal process, the sternal furca, and the lobe-like leg 5, that would leave the genus distinguished by only modified spines on the exopodal segments of leg 2. We consider that Caritus tolii should be placed in Caligus, as proposed by Morales-Serna et al. (2024). Indeed, based on these morphological interpretations, we infer that the 2 species of Caritus are not sister taxa and that both are nested within Caligus, but this hypothesis requires testing with molecular data.

Interestingly, Midias (=Caligus), Parapetalus, Parechetus, Pseudechetus, and Synestius share a suite of characters not just with Caligus but specifically with the Caligus confusus-group. With the addition of C. lobodes here, there are now 25 species in the confusus-group (Boxshall, 2018), and the group is defined by the following set of characters: antenna with posterior process on proximal segment (process typically spatulate); accessory tine present on postantennal process; maxillulary posterior process bifid; apron of leg 3 with raised cuticular rib and circular array of large denticles (also known as a rosette); outer margin spine on first exopodal segment of leg 3 stout and strongly recurved; leg 4 with 4 segments and with terminal 3 segments bearing spines I, I, III (Boxshall, 2018). In addition, the distal segment of the antennule is nearly always elongated, the setae on the posterior margin of the exopod of leg 1 are usually somewhat reduced in size, and members of the confusus-group typically parasitize species of the Carangidae. Although species assigned to the confusus-group typically possess this entire suite of character states, some species lack 1 or more of them.

We find it remarkable that many of these confusus-group character states are shared with Parapetalus, Parechetus, Pseudechetus, and Synestius. These genera are distinguished by superficially obvious differences involving processes and expansions on the abdomen or genital complex (Fig. 5C–G). We suspect that these gross differences in body form, currently used as generic discriminants, may not be robust with respect to phylogeny, especially given the larger suite of shared characters detailed below. We have synonymized Midias with Caligus here but have maintained the other genera until their phylogenetic relationships can be more formally evaluated with molecular data.

Of these 4 confusus-like genera, Pseudechetus most clearly resembles the C. confusus-group. The sole species, Pseudechetus fimbriatus, is distinctive because it possesses an elongated waist (also referred to as a neck) between the fourth pedigerous somite and the genital complex and has 6 elongate posterior processes: 4 on the genital complex and 2 on the abdomen (Fig. 5D). Despite these differences in the overall appearance of the body, P. fimbriatus possesses all of the confusus-group characters listed above, is known from only carangid hosts, and further resembles C. confusus itself in that it possesses a postantennal process that has 2 accessory processes (a posterior tine and an additional anterior-lateral tine) rather than the typical single accessory process (Fig. 5D).

Parechetus is easily identified by its possession of an elongated waist and single pair of long processes on the genital complex and abdomen (Fig. 5G), but it also possesses many of the confusus-group features. As redescribed by Pillai (1985), the type species P. carangis has a bifid postantennal process, a leg 3 protopod with a small rosette, a leg 3 exopod with a stout recurved spine, and a leg 4 typical of the confusus-group. It also possesses a long antennule distal segment and has reduced setae on the exopod of leg 1 (in this case, reduced to a single seta), plus it is known from only the carangid Carangoides ferdau. Its possession of the remaining confusus-group characters cannot be confirmed because Pillai (1985) did not figure the maxillule or the rib on the apron of leg 3.

The status of Parechetus constrictus Kirtisinghe, 1964 is uncertain. It is currently treated as a synonym of P. carangis (Walter and Boxshall, 2024), but Pillai (1985) did not formally propose a synonymy. He suggested only that the constricted neck in Kirtisinghe's P. constrictus might be an artifact, and if so they would likely be synonyms (Pillai, 1985). Dojiri and Ho (2013) also considered that these species were likely synonyms. However, the fact that the NMNH specimen of P. fimbriatus imaged here (Fig. 5C) has a constriction in just the same area of the “neck” indicates that this character might not be an artifact. Pillai (1985) specifically stated that he could not synonymize the 2 taxa because he had not looked at Kirtisinghe's types, so both should be treated as valid until further review.

The single species of Synestius, S. caliginus, possesses 4 elongate posterior processes on the genital complex (Fig. 5E) but also exhibits many confusus-group features. The distal segment of the antennule is elongate; the postantennal process has an accessory process; leg 1 has reduced setae on the posterior margin of the exopod; leg 3 bears a rosette, rib, and robustly recurved exopodal spine; and leg 4 is of the typical form (Dojiri and Ho, 2013). Beyond the 4 large posterior processes on the genital complex, S. caliginus differs from typical confusus-group species in several notable ways: the postantennal process has an accessory process that is a anterio-lateral knob rather than the typical posteriorly directed pointed process; the maxillulary process is simple rather than bifid and is less elongate; and it is known to parasitize hosts belonging to the families Stromateidae and Lutjanidae but not Carangidae (Walter and Boxshall, 2024).

The situation in Parapetalus is more complicated than that of the previous 3 genera because almost since its inception it has comprised a heterogenous group of species loosely united by degrees of expansion of the lateral margins of the abdomen. In the last decade, 7 species previously assigned to this genus have been transferred to Caligus (Boxshall, 2018; Boxshall and Barton, 2023; Walter and Boxshall, 2024). The 4 remaining species are Parapetalus dewani Pillai and Hameed, 1981, Parapetalus longipennatus Rangnekar, 1956, Parapetalus occidentalis, and the type species Parapetalus orientalis Steenstrup and Lütken, 1861. Each possesses a different number of confusus-group characters. Parapetalus longipennatus exhibits the most: it has an antennule with an elongate distal segment; an antenna with a broadening posterior process; a post-antennal process bearing an accessory process; a bifid maxillulary process; a leg 3 bearing a rib, rosette, and exopod armed with a robust recurved spine; reduced setae on leg 1; and is so far known only to parasitize carangid hosts (Pillai, 1985; Walter and Boxshall, 2024). Parapetalus dewani exhibits the next largest suite of confusus-group characters: long antennular distal segment; postantennal process with an accessory tine (although the maxillulary process is not bifid); leg 3 bearing a rosette and robust recurved exopodal spine; and reduced setae on leg 1 (Pillai and Hameed, 1981); it is unclear from available descriptions whether this species possesses a rib on the apron of leg 3. Parapetalus dewani is known from only Rachycentron canadum (Rachycentridae), and although this host is not in the Carangidae it is a member of the order Carangiformes.

Parapetalus occidentalis and P. orientalis share a more reduced suite of confusus-group features. They lack an accessory process on the postantennal process, and they have a simple rather than bifid maxillulary process, but they are reminiscent of the confusus-group in possessing an elongate distal antennulary segment, reduced setae on the exopod of leg 1, a rib on leg 3, a strongly recurved or oblique spine on the exopod of leg 3, and a leg 4 typical of the group (Ho and Lin, 2004; Dojiri and Ho, 2013). Parapetalus occidentalis parasitizes R. canadum, and P. orientalis parasitizes carangids. Given the suite of shared characters, it is tempting to transfer the remaining species of Parapetalus to Caligus. However, in the interest of taxonomic stability, until the diagnostic utility of the various confusus-group characters can be evaluated in a phylogenetic context or the evolutionary relationships of these species can be directly tested, we have elected to retain these species in Parapetalus given their unusual body plans and mosaic of character states.

The variety of posterior foliaceous and digitiform processes among species of Parapetalus, Parechetus, Pseudechetus, and Synestius (Fig. 5C–G) raises the question: what is their function? Parasites in general, and indeed many caligids, tend to have enlarged reproductive systems that increase their egg production capacity, but the processes on the genital complex of these genera are very flattened and do not appear to contain reproductive tissue. These posteriorly directed processes may serve to protect egg strings, perhaps from the activity of cleaner fish, which has been suggested for similar posterior processes in other groups of copepods such as the Lernanthropidae (Boxshall et al., 2020). All of these genera are relatively large caligids, so the expansions and processes may contribute additional surface area for respiration, but species of Pupulina and Gloiopotes and even Caligus curtus are substantially larger and lack such processes. These 4 genera have modified attachment sites relative to most caligids and live on the interior of the operculum or in the buccal cavity rather than on the body surface or gills. These regions may be more sheltered from shearing forces, which is particularly relevant because these copepods tend to parasitize rapidly swimming fishes. Under reduced shearing forces, the bodies of these copepods may have a broader adaptive landscape. Lastly, it is interesting to speculate that there may be a developmental element to this process. Many body parts in the confusus-group have additional processes or extensions: the posterior process of the maxillule and the postantennal process both tend to have accessory tines, and the rib on the apron of leg 3 often has a bifid tip. In Pseudechetus, the sternal furca is also bifid. Could there be a signaling pathway that leads to the development of additional axes of growth such as those listed above, and if so might this same process be involved in forming the foliaceous processes on the genital complex and abdomen in Parapetalus, Parechetus, Pseudechetus, and Synestius?