Review Article |
Corresponding author: David J. Wildish ( wildishd@dfo-mpo.gc.ca ) Academic editor: Matthias Glaubrecht
© 2017 David J. Wildish.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wildish DJ (2017) Evolutionary ecology of driftwood talitrids: a review. Zoosystematics and Evolution 93(2): 353-361. https://doi.org/10.3897/zse.93.12582
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Marine driftwood, both when floating at the sea surface and after stranding in the supralittoral of a beach, has been inadequately sampled for talitrids throughout the world. It is probable that many more talitrids than the seven currently recognized as driftwood species are extant. Because they are obligate xylophages all seven species are considered to be specialized driftwood talitrids. They contrast with talitrids able to feed on either wrack or driftwood, as has been established experimentally in Platorchestia platensis (Krøyer, 1845). For the best known genus of specialized driftwood talitrids, Macarorchestia, there are two zoogeographic series: Northeast Atlantic: M. microphtalma – M. roffensis –M. martini and Mediterranean: M. remyi – M. pavesiae. Both geographic series are characterized by increasing dwarfism. Experimental studies suggest that dwarfism evolved to allow talitrids to occupy the small burrows made by gribbles (Isopoda, Limnoridae) in driftwood, and/or because driftwood was a poor quality food by comparison with wrack. The phylogenetic advantages of talitrids living in driftwood are that: they are protected from shorebird predation, they are provided with a long distance dispersal mechanism, and they have a relatively long term, albeit poorer quality, food source. Molecular genetic studies confirm that both Macarorchestia and Neotenorchestia have evolved by dwarfism from larger Orchestia ancestors.
Obligate xylophagous talitrids, dwarfism, interspecific squatting in gribble burrows, driftwood ecotope, facultative xylophagous talitrids
The Talitroidea are a superfamily of Gammaridean Amphipoda and among the families of which it is composed, the largest is the Talitridae (=talitrids) with over 250 species listed by Serejo & Lowry (2008). Talitrids are characterized by reduced uropod 3, with antenna 1 shorter than antenna 2, the mandible lacking a palp and with palp of the maxilla reduced (
Perhaps 2 to 20 times less than the likely total species number within the family (~500–5000) have been collected and formally described
Inadequate sampling effort is available to discover new species
Too few taxonomists are available to formally describe the new species already discovered (e.g.
The powerful molecular genetic methods now available have not been applied adequately to further talitrid taxonomy
Morphology-based phylogeny’s are likely to suffer from convergence problems (see below)
Classification of talitrids at levels higher than the species is likely to suffer from convergence problems if it is based only on morphological criteria (
An alternative way of classifying talitrids is by the ecological habitats, or ecotopes, that they occupy. Talitrids are found on all continents, with the exception of Antarctica, in the following ecotopes: eulittoral or supralittoral wrack, supralittoral sandy beaches, supralittoral marshes, driftwood and caves which open in the supralittoral (Table
Classification of primary Talitrid ecotopes, with some examples of the ecotypes occupying them.
Ecosystem | Primary Ecotope | Ecotype examples |
Marine/estuarine | Eulittoral wrack | Orchestia mediterranea |
O. aestuarensis | ||
Supralittoral wrack | Orchestia gammarellus | |
Mexorchestia sp. | ||
Supralittoral sand burrowing | Talitrus saltator | |
Megalorchestia sp. | ||
Supralittoral marsh | Orchestia grillus | |
Uhlorchestia uhleri | ||
Supralittoral driftwood | Macarorchestia remyi | |
M. roffensis | ||
Freshwater | Supralittoral estuarine/freshwater wrack | Cryptorchestia cavimana |
Terrestrial | Rainforest leaf litter | Orchestia gomeri |
Palmorchestia epigaea | ||
Cave living | Palmorchestia hypogaea | |
Minamitalitrus zoltani | ||
Grassland | Makawe hurleyi | |
Puhuruhuru patersoni | ||
Soil- burrowing | Keratroides albidus |
This is a review of the evolutionary ecology of those talitrids which are capable of living in both floating or stranded driftwood. We define the technical terms used in this presentation as follows:
Driftwood depository: that part of the marine supralittoral, often associated with salt marshes or small streams discharging to the sea, where significant amounts of driftwood accumulate (
Driftwood talitrids: an ecological grouping of talitrids specialized for obligately living in and feeding on rotting, damp driftwood as the primary ecotope (
Ecotype: a locally adapted population within a species which is characteristic of a particular ecotope (
Interspecific squatting: the relationship between two genetically different species, in which the builder inadvertently assists the occupier to find shelter. E.g. the use by a hermit crab of an un-occupied gastropod shell (
Primary ecotope: the particular habitat in which a species is commonly found and to which it has evolved characteristic adaptations (
Saproxylobios: those organisms living in, or on, rotting wood (
Secondary ecotope: the particular habitat in which a species is less commonly found and to which it lacks characteristic adaptations (
Wrack: dis-lodged marine macroalgae, either floating at the sea surface, or after stranding on a beach following tidal and wind action.
Xylophagous (= lignivorous, dendrophagous): said of an organism feeding on wood. (
Xylotomous. Used of an organism able to cut or bore directly into wood. (
Presently known driftwood talitrids (Table
Driftwood talitrid taxonomy suffers from all of the problems mentioned in the Introduction, with the key one being an inadequate sampling effort. In fact only two geographic areas: the north-eastern Atlantic and Mediterranean coasts have been examined in a preliminary way for driftwood specialist talitrids. Further intensive geographic sampling within this area would be expected to yield more species. Intensive sampling of stranded driftwood in other parts of the world, particularly in southern temperate regions, is predicted to yield many more species.
A talitrid, Macarorchestia roffensis, from the Medway estuary, U.K., specialized for living permanently in driftwood was originally recognized by
List of the known species of driftwood specialist talitrids recognized by 2017. N1 is the number of individuals reported as type material, N2 is the number of references which include each named species in the biological study reported. See Supplementary List for the full list of references.
Taxa | N1 | N2 | Zoogeographic area |
M. microphtalma (Amanieu & Salvat, 1963) | ?20 | 8 | North- East Atlantic |
M. roffensis (Wildish, 1969) | 418 | 24 | |
M. martini Stock, 1989 | 9 | 11 | |
M. remyi (Schellenberg, 1950) | 7 | 29 | Mediterranean Sea |
M. pavesiae Wildish, 2014 | 15 | 2 | |
Neotenorchestia kenwildishi Wildish, 2014 | 10 | 2 | NE Atlantic |
“Platorchestia” chatamensis Bousfield, 1984 | 1 | 9 | NW Pacific |
The primary ecotype for Platorchestia platensis is as a wrack generalist (Bock 1967;
Woody wastes from both monocotyledonous and dicotyledenous plants have existed for over 120 MYA (
Other invertebrates which are xylophagous, secondary colonizers besides talitrids and which spend at least part of their lifecycle within decomposing driftwood include: termites, ants, isopods, centipedes, a variety of insect larvae, inclusive of beetles (
At some stage towards the end of the decay succession (Fig.
Preliminary geographic sampling, using the special methods needed to sample stranded marine driftwood (see above) have only been completed in northeast Atlantic and Mediterranean coastal regions. As far as is known, no concerted efforts have been employed to sample floating driftwood at sea. Because of the lack of geographic coverage in sampling driftwood it is probably premature to begin a discussion on the zoogeography of driftwood talitirids. However, for the genus Macarorchestia, some preliminary findings are available for the five, probably incompletely, known species (
Northeast Atlantic: microphthalma – roffensis – martini, and
Mediterrananean: remyi – pavesiae
In both series there is a trend towards dwarfism (see total body length data for males and females in Pavesii et al. 2014).
Characteristic adaptations of driftwood specialist talitrids are contrasted with the other ecotypes shown in Table
Most of the adaptations associated with currently known talitrids from a primary driftwood ecotope (Table
Characteristic adaptations of talitrids (mainly from the Northeast Atlantic and Mediterranean Sea regions).
Morphological characteristic | Wrack generalist | Sand burrowing specialist | Driftwood specialist | Cave-living specialist | Rainforest leaf litter generalist |
Body length(TBL), mm | >15 | >15 | <15 | <15 | <15, >15 |
TBL, sexual dimorphism | M>F | M>F | F>M | F>M | F>M, M>F |
Male gnathopod 2 subchelation | Strongly subchelate | Mitten-like | Subchelate | Mitten-like | Subchelate or mitten-like |
Peraeopod length of 6 and 7 | Long | Medium | Short | Very long | Short/long |
Eye size | Medium | Large | Small | Vestigial/absent | Small/medium |
Pleopod size | Large | Large | Medium/small | Small | Small/large |
Oostegite size | Large | Large | Medium/small | Small | Small/large |
Dorsal pigment patterns | Present | Present, or reduced | Absent | Absent | Present/absent |
The evolution of Macarorchestia and Neotenorchestia has involved dwarfism from a presumed larger wrack generalist ancestor. The underlying physiological changes involved in dwarfism are reduced growth rates in M. roffensis (
The reduction in body length within Macarorchestia varies between the sexes (data in Table
Driftwood specialist talitrids spend much of their life in small confined spaces: typically in empty gribble (Isopoda, Limnoridae) burrows. The diameter of these burrows in driftwood ranges from 0.6 to 5 mm in diameter (
A further result of spending much of their life cryptozoically within driftwood is the lack of need for vision for foraging, astronomical-mediated locomotion and predator avoidance (
Both pleopods and oostegites are reduced relative to body length (compared to Orchestia). Pleopods are functional in swimming and when stationary in seawater of drawing a current across the ventral body groove, for respiratory exchanges. Because of dwarfism and lower basal metabolic rate the ventilator current need not be as energetic as in larger, more active wrack generalist talitrids. Consequently the evolutionary process supports a reduction in pleopod size. For oostegites the small body size of driftwood talitrids dictates a smaller reproductive output, as fewer ova per brood (
The function of dorsal pigment patterns in wrack generalist talitrids has been hypothesized to be as camouflage from shorebird predators (Wildish and Martell 2012;
Three hypotheses were experimentally investigated as environmental triggers for dwarfism in driftwood specialist talitrids (
Small size reduced the absolute quantity of dissolved oxygen needed during dispersal at sea. The environmental trigger was low availability of dissolved oxygen within driftwood.
That driftwood was a poor diet and forced slower growth
That smaller size allowed driftwood living talitrids to occupy many more of the available empty gribble burrows, which are commonly present in driftwood.
The first hypothesis was discarded because model calculations showed that all talitrid sizes would be limited by oxygen availability in static conditions. In fact such conditions would not occur within gribble burrows because talitrid pleopod beating would induce a ventilatory current across the ventral respiratory surfaces, thus preventing oxygen starvation.
Both of the next two hypotheses were supported by physiological and behavioural experiments. Culture experiments in which a driftwood specialist, M. remyi, and a wrack generalist, P. platensis, were fed driftwood, resulted in a reduced basal metabolic rate and consequent reduction in growth rate in comparison with wrack fed P. platensis. Recent experiments (
The above results do not explain how serial dwarfism observed in Macarorchestia species could occur. Perhaps the behavioural experiments, designed to examine the last hypothesis listed above can do so. In these experiments it was shown that talitrids were limited by body size to the gribble hole diameter that they could negotiate. The measurement of body depth (BD) proved to be the best indicator of body size which could negotiate a particular diameter of gribble burrow, in behavioural experiments on this point (
(BD) = 0.1298(TBL) + 0.1115, N = 24, R2 = 0.89. TBL size range from 7.0 to 20.5 mm.
If we assume that the same relationship applies to all species within Macarorchestia, we can predict from this equation what burrow diameter each species can occupy. The results are shown in Table
Maximum total body length, mm (TBL) of adult female Macarorchestia from
Taxa | TBL, mm | BD, mm | % of gribble burrows each taxon can occupy |
M. microphtalma (Amanieu & Salvat, 1963) | 13.94 | 1.92 | 58.2 |
M. remyi (Schellenberg, 1950) | 11.47 | 1.60 | 76.1 |
M. pavesiae Wildish, 2014 | 9.36 | 1.33 | 87.1 |
M. roffensis (Wildish, 1969) | 8.30 | 1.19 | 91.9 |
M. martini Stock, 1989 | 6.22 | 0.92 | 98.0 |
In an earlier review which included land colonization by talitrids (
The finding by
Because each driftwood specimen in which talitrids are residing has a finite life during the decay process, we know that there must be exchange from one driftwood specimen to another. When, and at what talitrid life history stage this re-colonization occurs remains a mystery. The transfer may be different depending on whether the talitrid involved is occupying a primary or secondary ecotope.
For primary, driftwood, ecotypical talitrids who appear to spend all their life history within damp, rotting driftwood, the most plausible location for exchange between driftwood specimens would be within a driftwood depository. Here driftwood specimens at all stages of the decay cycle are present and may be closely aligned to each other. As the old driftwood disintegrates nearby driftwood specimens are available to accept emigrating driftwood talitrids. However, no field observations are available which have observed driftwood transfer. Clearly further field and laboratory behavioural observations are needed to answer this question. This would include measuring the periodicity of locomotory activity in a representative driftwood specialist talitrid, as is already available for wrack generalists (
In secondary ecotypical talitrids the nature of the driftwood transfer is subtly different. Here the adoption of driftwood as a shelter and source of food occurs as part of the contiguous habitat hypothesis. Thus if wrack is scarce or unavailable wrack generalist talitrids, such as P. platensis, are able to acclimate to driftwood, but can return to feeding on wrack if it becomes available again (Wildish & Robinson, In preparation).
It is obvious from this review that all aspects of the biology of driftwood talitrids – either those occupying it as a primary or secondary ecotope – are at a very early stage of development. Thus all aspects of the biology of driftwood talitrids are in need of further work. To emphasize research projects of most use in advancing an understanding of the study of the evolutionary ecology of driftwood talitrids the following are listed:
World-wide search for talitrids in driftwood, particularly where driftwood depositories are present
More research on molecular genetic methods as it is applied to talitrid phylogeny and taxonomy
Develop experimental methods that can be used to distinguish primary from secondary ecotypes
Ecological studies on the decay succession of driftwood: inclusive of mass balances of driftwood to the oceans; microbial organisms, pathways and products involved in wood decay; secondary driftwood colonizers and how talitrids interact ecologically with the other invertebrates present in driftwood
Physiological studies of digestion to resolve the role of microbes and how they are utilized in the talitrid gut
Molecular genetic studies of driftwood talitrids and their potential ancestors, to provide a phylogeny of the driftwood talitrid ecological group
Identification of the genetic and hormonal system which initiates sexual development in talitrids
Experimental testing of the assumptions made in constructing the gribble burrow squatting hypothesis
Behavioural studies with driftwood talitrids in primary and secondary ecotopes.
Some of the above are suitable as post graduate research projects and their completion will establish the evolutionary ecological study of driftwood talitrids on a much firmer scientific foundation.
Thanks to Dr. Laura Pavesi and an un-named reviewer for improving an earlier version of this manuscript.
List of References to Driftwood Talitrid species compiled by David Sheppard, Fisheries and Oceans Library, Bedford Institute of Oceanography
Data type: additional reference list