Background on Anamixidae

*Note: References to the new genus described in an upcoming monograph have been suppressed for reasons of consistency in the Taxonomic Code.

I. INTRODUCTION

This research presents a monographic treatment of a tropical group of amphipods, the Anamixidae. Past experience has shown that any cohesive monographic treatment must include not only detailed taxonomic descriptions, but ecological and biogeographical data as well. Partial or incomplete taxonomic inventories from major biotic provinces can seriously impair the ability of higher level treatments to generate competent systematic and phylogenetic hypotheses. With few exceptions, tropical marine invertebrates are poorly known, yet knowledge concerning their evolutionary histories is vital to understanding how, and by what processes, these animals have come to exist in their present habitats.

The author first became interested in the Anamixidae with the discovery of their remarkable transformation process in the summer of 1982 (Thomas and Barnard, 1983). Efforts to collect Caribbean anamixids and document their ecology were initiated and carried out over the next few years. While working through unsorted museum collections, a number of additional new species were discovered. In addition, the lack of anamixid or distribution records from the Australasian region required at least a cursory collecting effort to secure material before undertaking a worldwide monographic effort of a group that occurs primarily in reef systems.

Amphipods of the family Anamixidae are commensals in sponges and ascidians and occur throughout the world's tropical and subtropical marine systems. They exhibit an unusual life history pattern involving two highly dissimilar developmental stages that occur simultaneously in the host. These two stages are so different they were previously assigned to separate families. Initial, or, "leucomorph" developmental stages (male and female) of different anamixid species are nearly identical, while the transformed, or "anamorph" stage (always male) is distinct for each species. In a remarkable transformation, leucomorph males pass via a single molt to anamorph males. This transformation is accompanied by a number of extreme morphological changes that explain their placement in separate families prior to this discovery. Leucomorphs outnumber anamorphs in the host approximately 10-1. Nearly all leucomorph females are ovigerous. It is not known which male stage (or both) interacts reproductively with the female. The fate of post-brood females remains unknown.

Needs for syntopic treatments of tropical amphipods have been reported by Barnard (1976,1980). Kensley (1980) noted the paucity of tropical amphipod taxonomy in hindering the generation of biogeographic hypotheses. Despite the increased efforts of amphipodologists over the last 10 years, the species description curve is increasing steeply rather than stabilizing (Barnard and Karaman, 1991). This situation is unlikely to change in the near future. In 1976 J.L. Barnard estimated that it would take a team of experts 30+ years to delineate the tropical Amphipoda, based on a count (at the time) of 111 described species. More recently, Barnard and Karaman (op. cit.) place the world tropical amphipod species count at 1,095 with another 1,200-1,500 species undescribed. The Indo-Pacific is the richest in amphipod diversity with 814 described species in 207 genera (Barnard, and Karaman, op.cit.)

With recent taxonomic efforts in tropical Amphipoda focused at the descriptive level, there exists no comprehensive analytical treatment of any kind within the group. The paucity of phyletic monographs at the family level and proliferation of faunal compilations widely scattered in the literature has hindered advances in classification of the group (Barnard and Karaman, op.cit.). Clearly, if there is to be progress in the systematics of tropical amphipods, taxonomists must incorporate descriptive efforts in family-level monographs of selected tropical amphipod taxa. The research presented herein, based on comprehensive collections, rigorous taxonomic descriptions that include detailed character analysis and redescription of existing species, and computer analysis, provides the first such treatment for a group of tropical amphipods.

One of the most interesting problems that biogeographers face today is how island faunas are derived, (e.g. by vicariance or dispersal). Vicariance models for Central Pacific and Indian Ocean shorefishes have been proposed by Springer (1982,1988). Case and Cody (1987) suggest that numerous models may eventually be required to explain patterns of colonization and extinction, and caution workers in generalizing beyond the bounds of their taxon and the island system in which it has been studied. In their study of Indo-Pacific Insecta, Schuh and Stonedahl (1986) report that cladistic biogeography has begun to have an impact on empirical studies, but that application of this methodology required the presence of two kinds of data sets: (1) monophyletic groups whose taxa have restricted distributions and (2) cladograms for groups that resolve relationships of taxa at the level at which they are endemic. While the method of cladistic biogeography has been applied to some insect and plant groups, it has yet to be widely implemented with regard to marine invertebrates. The ultimate key to unraveling the evolutionary relationships of tropical island faunas relies on our ability to identify immediate ancestors and their distributions. Only then can we speculate on biogeographic or historical origins. Because we have lacked the raw data in the form of the specimens themselves, progress in generating cladograms for biogeographic analyses has been virtually non-existent.

The ultimate topic of interest is the evolutionary descent of the Amphipoda in space and time. With testable hypotheses we can attempt to determine whether, as has been suggested, the tropics are primarily an ultimate repository of advanced and specialized taxa, and whether the boreal/antiboreal regions may contain evolutionary center(s) for the various amphipod taxa (Barnard and Karaman, op. cit.). The evolutionary filter effect that may have occurred during various transgressions of the tropical frontier from one hemisphere to another, and the effects, if any, that such processes may have played in determining modern day tropical amphipod distributions is also an issue. To be of any lasting value, efforts to produce phylogenetic hypotheses must include an adequate accounting of tropical species and their distributions.

BACKGROUND

Most of what we know of amphipod taxonomy, ecology, and community structure shows a strong temperate bias. Tropical marine systems offer promising avenues of research, but advances are often highly unpredictable because so little is known. Coral reefs are geographically discrete, widespread biological systems that exist within a narrow range of biological and physical conditions. Coral reefs present a complex mosaic of macro and microhabitats (Hay, 1981) e.g., live coral, coral rubble, turf algae, sessile invertebrates, and reef-derived sediments, that allow ecological comparisons to be made. However, the small size of tropical amphipods (2-4mm) combined with difficulties in obtaining representative collections from reef sites has hindered taxonomic and systematic progress within the group.

Cryptic and commensal reef species form highly localized populations, further minimizing dispersive capabilities. For this reason, the author has chosen to focus research efforts primarily on cryptofaunal and commensal amphipods, with the assumption that their morphologies and distributions more clearly reflect evolutionary events than do epifaunal forms that can be widely dispersed and exhibit high phenotypic plasticity (Schram, 1986; Conlan, 1988).

Adequate representative samples of amphipods can be procured only by in-situ collection from specific substrates with the use of SCUBA techniques. Investigators have relied instead on material collected by others as incidental to collecting efforts targeting other organisms, or sorted from large scale faunal surveys by non-specialists. To obtain representative samples of smaller benthic crustaceans, specific collecting programs must be undertaken. This habitat-intensive approach to collecting has opened a new era in the systematic study of shallow-water marine amphipods and other small invertebrates, and has provided much new and exciting material for study.

The need for systematic advancement in tropical marine areas is pressing, especially in light of increased human pressure on most tropical environments of the world. The taxonomic status of tropical organisms, including amphipods is a subject of great concern in view of their wide usage as materials for study in environmental impact surveys and for sophisticated ecological and physiological studies (e.g. Hay et al., 1987). While tropical marine systems are poorly known in terms of taxonomic biodiversity, there is a high demand for information about their inhabitants (Wilson, 1985). Clearly, scientists must make a concerted effort to document tropical biodiversity if we are to succeed in understanding, and eventually managing, these complex and increasingly impacted systems.

REVIEW OF THE ANAMIXIDAE

Prior to this research there were 13 described species of anamixids, in two genera, Anamixis and Paranamixis. These two genera were distinguished by the presence (Anamixis), or absence (Paranamixis), of first gnathopods. The majority of what is known of anamixid biology and ecology has been reported by the author and co-workers (Thomas, 1979,1981; Thomas and Taylor, 1981; Thomas and Barnard, 1983. Prior to the discovery of the unique transformation process in anamixids (Thomas and Barnard, 1983), leucomorph stages of anamixids were assigned to a separate family, the Leucothoidae. Anamorphs are characterized by having extremely reduced and fleshy mouthparts, in having coxa 1 reduced or vestigial, and in having greatly enlarged second gnathopods and coxae 2-4 that act as a lateral shield to enclose the animal during feeding. Leucomorphs also show various modifications and reductions in structure, but possess only slightly reduced, spinose mouthparts and morphologically unique second gnathopods. Investigations by the author have shown that the transformation molt is not terminal, and can be followed by several instars. Before discovery of the transformational process leucomorphs had been assigned to a number of species in the genus Leucothoides in the Leucothoidae. Leucomorph stages of major anamixid groups are, in most instances, nearly identical and cannot be readily separated at the species level with the exception of the more primitive groups where both stages have morphologically distinct first and second gnathopods. In cases where the transformation process has been documented, or both stages collected in-situ from their invertebrate hosts, specific identifications can be made. These combinations of circumstances have produced an extremely tangled taxonomy.

Documentation of the transformation process in anamixids created a sizable taxonomic problem, because there were now a dozen described species of Leucothoides with no possibility to associate them with their anamorph counterparts. This can only be accomplished by collecting the stages in-situ from their specific hosts, or by confirmation through rearing experiments that document the transformation process (Thomas and Barnard, 1983). To date, the author has documented the transformation process or confirmed host specificity for four species of anamixids. For detailed explanations regarding rearing methodologies and host confirmation procedures, see Thomas and Barnard (1983). Developmental information on all other taxa is lacking. It must be emphasized that occurrence of both stages in the same general collection does not necessarily confirm taxonomic relationships because most reef areas contain several anamixid species. Problems in the family with unassociated morphologies restrict primary taxonomic emphasis within anamixids to anamorphs, but because leucomorphs represent early developmental stages, they can be highly informative in determining generic limits and relationships from an ontogenetic perspective.

DISTRIBUTION

The genus Paranamixis appears confined to the Pacific and Indian Ocean basins, while Anamixis is widespread, with representatives in all tropical oceans. Members of the more plesiomorphic new genus are distributed as possible Gondwanaland relicts across the Indian and Pacific Oceans and the Caribbean Sea (Newman, 1991).

ECOLOGY

Host associations for anamixids (when known) are the interior of small asconoid sponges and the branchial cavity of solitary and compound tunicates where the amphipods utilize host-generated currents to filter fine particulates. Where associations are known, anamixids appear highly specific in the choice of host, and are either restricted to a particular host species, or in the case of multiple host species, are restricted to similar-sized interior feeding chambers. Unfortunately, host and ecological data are only available for a handful of species (Thomas and Barnard, 1983).

Information from host data, feeding behavior, and distribution patterns can be valuable sources of information in phylogenetic studies. Observations from natural history studies and in-situ observations can aid in interpreting convergent morphologies in certain structures. An example of this approach are attempts by the author to evaluate the presence of similar lateral ridges on the cephalic margin of anamixid species in different genera such as A. vanga and P. kanu. Ecological studies have shown these ridges are present only in those species inhabiting large solitary ascidian hosts, thus, this character probably reflects convergent adaptations to similar feeding strategies and not similar evolutionary histories. Such ecological information can assist in hypotheses addressing the evolutionary significance of a given morphology. In this regard the author agrees strongly with Brusca (1984) who stated similar views regarding isopods.

SELECTED TAXONOMIC CHARACTERS

The following characters are some of the more important for taxonomic purposes in the Anamixidae:

Head region: The presence or absence of a lateral keel on the head which serves to align the head against coxa 2 when the head is flexed ventrally during feeding. The morphology and microstructure of the ventral keel located in the mid-region on the ventral portion of the head. The relative length of antennae 1 and 2, and enlargement of the first peduncular segment of antenna 1.

Mouthparts: Standard light microscopy provides limited information on the microstructure of the mouthpart field, except in the case of the elongate and hyper-developed maxilliped palp and prominent ventral keel. With the exception of the maxilliped palp and ventral keel, the remaining mouthparts are reduced, fleshy and transparent in anamorph stages and are best observed by SEM or differential interference contrast microscopy. The maxilliped, enlarged and modified for food-handling, exhibits various reductions and degrees of fusion in the inner plates, while the outer plates bear vestigial inner lobes in the more plesiomorphic taxa.

Coxal morphology: The general size and dominance of various coxae, their shape, and ornamentation can be important features. The morphology and amount of reduction in coxa 1 is important at the generic level. The morphology, and ornamentation of the margins of the relatively larger coxae 2-4 are important at the species level, as is the relative dominance of coxae 2-4.

Gnathopods: Gnathopod 1 is present and reduced in most species of Anamixis, except for A. jebbi n. sp., where it is vestigial in transformed males. In Paranamixis, gnathopod 1 is lacking in all post-transformational anamorphs. However, the author has documented tiny vestiges of gnathopod 1 in 2 species of Paranamixis, assumed to persist at the transformational molt itself, a remnant of the leucomorph stage. All subsequent stages of Paranamixis are characterized by the complete lack of gnathopod 1 in post-transformational anamorphs. When gnathopod 1 is present, the general morphology and dimensions of article five (carpus) are important at the generic and specific levels. Patterns of armament and terminal ornamentation of article six are informative at the specific level. Anamorphs of the new genus possess a distinct gnathopod 1 with a basally-expanded carpus, and a linear gnathopod 2 with a geniculate carpal lobe, differing from all other species and underscoring the taxonomic importance of these appendages. Leucomorphs of the new genus also exhibit a unique configuration of gnathopods 1 and 2, further distinguishing them from all other juvenile anamixid morphs, providing a means of identifying the presence of the new genus in collections, even when anamorphs are lacking. Gnathopod 2 is greatly enlarged and modified for filter feeding and possesses a number of morphological and morphometric characters of value such as accessory ornamentation (spines, cusps, ridges, long setae) of article 2 (basis); the presence or absence of a serrate carpal ridge (either lateral or medial) on article 5 (carpus); the shape, armament, and morphometrics of article 6 (propodus); the shape, morphology and armament of the dactyl (article 7). The large second gnathopods bear mediofacial setae on the propodus that are used in filter feeding. Members of the new genus have two rows of this specialized setae, while Anamixis and Paranamixis both have a single row. Leucomorph stages possess unique second gnathopods that differ substantially from anamorph forms, being relatively smaller in size and having a prominent transverse palm.

Uropods: Anamorphs differ from leucomorphs in having the outer rami of uropods 2 and 3 reduced to about half the length of the inner ramus. Two species of Paranamixis have uropods and peduncles completely lacking spines. All other taxa have spines on the rami and peduncles of uropods 1-3.

Some characters may be especially difficult to observe under normal light microscopy and previous descriptions may be deficient in many cases in enumerating these features. For example, the lateral serrate ridge on the basis of gnathopod 2 in P. clarki is hard to distinguish without Nomarski or phase contrast microscopy. The condition of the apices of the inner plates of the maxilliped is another situation that requires careful examination to accurately assess. Caution must be used in the analysis and interpretation of minute or hard to observe structures. This is especially true in constructing character transformation series for cladistic analysis. There is no substitute for careful and thorough examination of material, however, there are times when specimens are not available for examination and existing descriptions and illustrations lack the minute detail now required in taxonomic descriptions. In these circumstances, the taxonomist must make the best use of available character data.

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